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max(), sum(), next()

Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>sum([])
0

Because that [] may be an empty sequence of someobject:
>>sum(s for s in ["a", "b"] if len(s) 2)
0

In a statically typed language in that situation you may answer the
initializer value of the type of the items of the list, as I do in the
sum() in D.

This sounds like a more correct/clean thing to do:
>>max([])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: max() arg is an empty sequence

So it may be better to make the sum([]) too raise a ValueError, in
Python 3/3.1 (if this isn't already true). On the other hand often
enough I have code like this:
>>max(fun(x) for x in iterable if predicate(x))
This may raise the ValueError both if iterable is empty of if the
predicate on its items is always false, so instead of catching
exceptions, that I try to avoid, I usually end with a normal loop,
that's readable and fast:

max_value = smallvalue
for x in iterable:
if predicate(x):
max_value = max(max_value, fun(x))

Where running speed matters, I may even replace that max(max_value,
fun(x)) with a more normal if/else.

A possible alternative is to add a default to max(), like the next()
built-in of Python 2.6:
>>max((fun(x) for x in iterable if predicate(x)), default=smallvalue)
This returns smallvalue if there are no items to compute the max of.

Bye,
bearophile
Sep 3 '08 #1
  • viewed: 7443
Share:
54 Replies
<be************@lycos.comwrote:
>Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>>sum([])
0
>>help(sum)
sum(...)
sum(sequence, start=0) -value
>>sum(range(x) for x in range(5))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'list'
>>sum((range(x) for x in range(5)), [])
[0, 0, 1, 0, 1, 2, 0, 1, 2, 3]

.... so the list might not know what type it contains, but sum
does. And if you don't tell it, it makes a sensible guess. And
it *is* a case where refusing the temptation to guess is the
wrong thing: how many times would you use sum to do anything
other than sum numeric values? And how tedious would it be to
have to write sum(..., 0) for every other case? Particularly
bearing in mind:
>>sum(["a", "b"], "")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]

--
\S -- si***@chiark.greenend.org.uk -- http://www.chaos.org.uk/~sion/
"Frankly I have no feelings towards penguins one way or the other"
-- Arthur C. Clarke
her nu become se bera eadward ofdun hlddre heafdes bce bump bump bump
Sep 3 '08 #2
be************@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:

>>>sum([])
0

Because that [] may be an empty sequence of someobject:
You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values.
When adding real numbers, the neutral element is zero. ( X + 0 = X) It
is very logical to return zero for empty sequences.

Same way, if we would have a prod() function, it should return one for
empty sequences because X*1 = X. The neutral element for this operation
is one.

Of course this is not good for summing other types of objects. But how
clumsy would it be to use

sum( L +[0] )

or

if L:
value = sum(L)
else:
value = 0

instead of sum(L).

Once again, this is what sum() is used for in most cases, so this
behavior is the "expected" one.

Another argument to convince you: the sum() function in SQL for empty
row sets returns zero in most relational databases.

But of course it could have been implemented in a different way... I
believe that there have been excessive discussions about this decision,
and the current implementation is very good, if not the best.

Best,

Laszlo

Sep 3 '08 #3
On Sep 3, 8:18*pm, Laszlo Nagy <gand...@shopzeus.comwrote:
bearophileH...@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>sum([])
0
Because that [] may be an empty sequence of someobject:

You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values.
When adding real numbers, the neutral element is zero. ( X + 0 = X) It
is very logical to return zero for empty sequences.

Same way, if we would have a prod() function, it should return one for
empty sequences because X*1 = X. The neutral element for this operation
is one.

Of course this is not good for summing other types of objects. But how
clumsy would it be to use

sum( L +[0] )

or

if L:
value = sum(L)
else:
value = 0

instead of sum(L).

Once again, this is what sum() is used for in most cases, so this
behavior is the "expected" one.

Another argument to convince you: the sum() function in SQL for empty
row sets returns zero in most relational databases.

But of course it could have been implemented in a different way... I
believe that there have been excessive discussions about this decision,
and the current implementation is very good, if not the best.
An alternative would be for the start value to default to None, which
would mean no start value. At the moment it starts with the start
value and then 'adds' the items in the sequence to it, but it could
start with the first item and then 'add' the following items to it.
So:

sum([1, 2, 3]) =6
sum(["a", "b", "c"]) ="abc"

For backward compatibility, if the sequence is empty and the start
value is None then return 0.
Sep 3 '08 #4
Laszlo Nagy:
I believe that there have been excessive discussions about this
decision, and the current implementation is very good, if not the best.
I see. But note that my post is mostly about the max()/min()
functions :-)

Bye,
bearophile
Sep 3 '08 #5
On Sep 3, 2:18*pm, Laszlo Nagy <gand...@shopzeus.comwrote:
bearophileH...@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>sum([])
0
Because that [] may be an empty sequence of someobject:

You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values.
When adding real numbers, the neutral element is zero. ( X + 0 = X) It
is very logical to return zero for empty sequences.
No it isn't. Nothing is not 0, check with MS-Access, for instance:

Null + 1 returns Null. Any arithmetic expression involving a
Null evaluates to Null. Adding something to an unknown returns
an unknown, as it should.

It is a logical fallacy to equate unknown with 0.

For example, the water table elevation in ft above Mean Sea Level
is WTE = TopOfCasing - DepthToWater.

TopOfCasing is usually known and constant (until resurveyed).
But DepthToWater may or may not exist for a given event (well
may be covered with fire ants, for example).

Now, if you equate Null with 0, then the WTE calculation says
the water table elevation is flush with the top of the well,
falsely implying that the site is underwater.

And, since this particular site is on the Mississippi River,
it sometimes IS underwater, but this is NEVER determined by
water table elevations, which, due to the CORRECT treatment
of Nulls by Access, never returns FALSE calculations.
>>sum([])
0

is a bug, just as it's a bug in Excel to evaluate blank cells
as 0. It should return None or throw an exception like sum([None,1])
does.
>
Same way, if we would have a prod() function, it should return one for
empty sequences because X*1 = X. The neutral element for this operation
is one.

Of course this is not good for summing other types of objects. But how
clumsy would it be to use

sum( L +[0] )

or

if L:
value = sum(L)
else:
value = 0

instead of sum(L).

Once again, this is what sum() is used for in most cases, so this
behavior is the "expected" one.

Another argument to convince you: the sum() function in SQL for empty
row sets returns zero in most relational databases.

But of course it could have been implemented in a different way... I
believe that there have been excessive discussions about this decision,
and the current implementation is very good, if not the best.

Best,

Laszlo
Sep 3 '08 #6
On Sep 3, 7:48*am, bearophileH...@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>sum([])

0

Because that [] may be an empty sequence of someobject:
>sum(s for s in ["a", "b"] if len(s) 2)

0

In a statically typed language in that situation you may answer the
initializer value of the type of the items of the list, as I do in the
sum() in D.

This sounds like a more correct/clean thing to do:
>max([])

Traceback (most recent call last):
* File "<stdin>", line 1, in <module>
ValueError: max() arg is an empty sequence

So it may be better to make the sum([]) too raise a ValueError, in
Python 3/3.1 (if this isn't already true). On the other hand often
enough I have code like this:
>max(fun(x) for x in iterable if predicate(x))

This may raise the ValueError both if iterable is empty of if the
predicate on its items is always false, so instead of catching
exceptions, that I try to avoid, I usually end with a normal loop,
that's readable and fast:

max_value = smallvalue
for x in iterable:
* * if predicate(x):
* * * * max_value = max(max_value, fun(x))

Where running speed matters, I may even replace that max(max_value,
fun(x)) with a more normal if/else.

A possible alternative is to add a default to max(), like the next()
built-in of Python 2.6:
>max((fun(x) for x in iterable if predicate(x)), default=smallvalue)

This returns smallvalue if there are no items to compute the max of.

Bye,
bearophile
Two thoughts:
1/ 'Reduce' has a 'default' argument-- they call it 'initial'.
>>reduce( max, [ 0, 1, 2, 3 ] )
3
>>reduce( max, [ 0, 1, 2, 'a' ] )
'a'
>>reduce( max, [ 0, 1, 2, 'a', 'b' ] )
'b'

2/ Introduce a 'max' class object that takes a default type or default
argument. Query the default for an 'additive' identity, or query for
a 'comparitive' identity, comparisons to which always return true; or
call the constructor with no arguments to construct one.
Sep 3 '08 #7
On Wed, 03 Sep 2008 16:20:39 -0700, Mensanator wrote:
>>>sum([])
0

is a bug, just as it's a bug in Excel to evaluate blank cells as 0. It
should return None or throw an exception like sum([None,1]) does.
You're wrong, because 99.9% of the time when users leave a blank cell in
Excel, they want it to be treated as zero. Spreadsheet sum() is not the
same as mathematician's sum, which doesn't have a concept of "blank
cells". (But if it did, it would treat them as zero, since that's the
only useful thing and mathematicians are just as much pragmatists as
spreadsheet users.) The Excel code does the right thing, and your "pure"
solution would do the unwanted and unexpected thing and is therefore
buggy.

Bugs are defined by "does the code do what the user wants it to do?", not
"is it mathematically pure?". The current behaviour of sum([]) does the
right thing for the 99% of the time when users expect an integer. And the
rest of the time, they have to specify a starting value for the sum
anyway, and so sum([], initial_value) does the right thing *always*.

The only time it does the wrong thing[1] is when you forget to pass an
initial value but expect a non-numeric result. And that's the
programmer's error, not a function bug.

[1] I believe it also does the wrong thing by refusing to sum strings,
but that's another story.
--
Steven
Sep 4 '08 #8

Quoting Laszlo Nagy <ga*****@shopzeus.com>:
be************@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:

>>sum([])
0

Because that [] may be an empty sequence of someobject:
You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values.
When adding real numbers, the neutral element is zero. ( X + 0 = X) It
is very logical to return zero for empty sequences.
Even better:

help(sum) shows

===
sum(...)
sum(sequence, start=0) -value

Returns the sum of a sequence of numbers (NOT strings) plus the value
of parameter 'start'. When the sequence is empty, returns start.
===

so the fact that sum([]) returns zero is just because the start value is zero...
sum([],object()) would return an object().

BTW, the original code:
>>sum(s for s in ["a", "b"] if len(s) 2)
wouldn't work anyway... it seems that sum doesn't like to sum strings:
>>sum(['a','b'],'')
<type 'exceptions.TypeError'>: sum() can't sum strings [use ''.join(seq) instead]

Cheers,

--
Luis Zarrabeitia
Facultad de Matemtica y Computacin, UH
http://profesores.matcom.uh.cu/~kyrie
Sep 4 '08 #9
Mensanator wrote:
No it isn't. Nothing is not 0, check with MS-Access, for instance:

Null + 1 returns Null. Any arithmetic expression involving a
Null evaluates to Null. Adding something to an unknown returns
an unknown, as it should.

It is a logical fallacy to equate unknown with 0.
http://en.wikipedia.org/wiki/Empty_sum

"In mathematics, the empty sum, or nullary sum, is the result of adding
no numbers, in summation for example. Its numerical value is zero."

</F>

Sep 4 '08 #10
On Wed, 03 Sep 2008 22:20:43 -0700, Mensanator wrote:
On Sep 3, 8:30�pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:
>On Wed, 03 Sep 2008 16:20:39 -0700, Mensanator wrote:
>>>sum([])
0
is a bug, just as it's a bug in Excel to evaluate blank cells as 0.
It should return None or throw an exception like sum([None,1]) does.

You're wrong, because 99.9% of the time when users leave a blank cell
in Excel, they want it to be treated as zero.

Then 99.9% of users want the wrong thing.
It is to laugh.
Microsoft knows that this is a bug
Says you.

but refuses to fix it to prevent breaking legacy documents (probably
dating back to VisiCalc). When graphimg data, a missing value should be
interpreted as a hole in the graph
"Graphing data" is not sum(). I don't expect graphing data to result in
the same result as sum(), why would I expect them to interpret input the
same way?

+------+ +--+------+------+-----+
Why should the graphing application ignore blanks ("missing data"), but
sum() treat missing data as an error? That makes no sense at all.
and not evaluated as 0

And Microsoft provides a workaround for graphs to make 0's appear as
holes. Of course, this will cause legitimate 0 values to disappear, so
the workaround is inconsistent.
I'm not aware of any spreadsheet that treats empty cells as zero for the
purpose of graphing, and I find your claim that Excel can't draw graphs
with zero in them implausible, but I don't have a copy of Excel to test
it.
>Spreadsheet sum() is not the
same as mathematician's sum, which doesn't have a concept of "blank
cells". (But if it did, it would treat them as zero, since that's the
only useful thing and mathematicians are just as much pragmatists as
spreadsheet users.) The Excel code does the right thing, and your
"pure" solution would do the unwanted and unexpected thing and is
therefore buggy.

Apparently, you don't use databases or make surface contours.
Neither databases nor surface contours are sum(). What possible relevance
are they to the question of what sum() should do?

Do you perhaps imagine that there is only "ONE POSSIBLE CORRECT WAY" to
deal with missing data, and every function and program must deal with it
the same way?

Contour programs REQUIRE that blanks are null, not 0
Lucky for them that null is not 0 then.

so that the Kriging
algorithm interpolates around the holes rather than return false
calculations. Excel's treatment of blank cells is inconsistent with
Access' treatment of Nulls and therefore wrong, anyway you slice it.
No no no, you messed that sentence up. What you *really* meant was:

"Access' treatment of Nulls is inconsistent with Excel's treatment of
blank cells and therefore wrong, anyway you slice it."

No of course not. That would be stupid, just as stupid as your sentence.
Excel is not Access. They do different things. Why should they
necessarily interpret data the same way?

Maybe you want to say a bug is when it doesn't do what the author
intended, but I say if what the intention was is wrong, then a perfect
implentation is still a bug because it doesn't do what it's supposed to
do.
Who decides what it is supposed to do if not the author? You, in your
ivory tower who doesn't care a fig for what people want the software to
do?

Bug report: "Software does what users want it to do."
Fix: "Make the software do something that users don't want."

Great.

>Bugs are defined by "does the code do what the user wants it to do?",
not "is it mathematically pure?".

ReallY? So you think math IS a democracy? There is no reason to violate
mathematical purity.
You've given a good example yourself: the Kriging algorithm needs a Null
value which is not zero. There is no mathematical "null" which is
distinct from zero, so there's an excellent violation of mathematical
purity right there.
If I am given the job of adding up the number of widgets inside a box,
and the box is empty, I answer that there are 0 widgets inside it. If I
were to follow your advice and declare that "An error occurred, can't
determine the number of widgets inside an empty box!" people would treat
me as an idiot, and rightly so.
If I don't get EXACTLY the same answer from Excel,
Access, Mathematica and Python, then SOMEBODY is wrong. It would be a
shame if that somebody was Python.
Well Excel, Python agree that the sum of an empty list is 0. What do
Access and Mathematica do?
>The current behaviour of sum([]) does the right thing for the 99% of
the time when users expect an integer.

Why shouldn't the users expect an exception? Isn't that why we have
try:except? Maybr 99% of users expect sum([])==0, but _I_ expect to be
able to distinguish an empty list from [4,-4].
The way to distinguish lists is NOT to add them up and compare the sums:
>>sum([4, -4]) == sum([0]) == sum([1, 2, 3, -6]) == sum([-1, 2, -1])
True

The correct way is by comparing the lists themselves:
>>[] == [4, -4]
False
>And the
rest of the time, they have to specify a starting value for the sum
anyway, and so sum([], initial_value) does the right thing *always*.

So if you really want [] to be 0, why not say sum([],0)?
I don't want [] == 0. That's foolish. I want the sum of an empty list to
be 0, which is a very different thing.

And I don't need to say sum([],0) because the default value for the
second argument is 0.
Why shouldn't nothing added to nothing return nothing? Having it
evaluate to 0 is wrong 99.9% of the time.
It is to laugh.

What's the difference between having 0 widgets in a box and having an
empty box with, er, no widgets in it?

--
Steven
Sep 4 '08 #11
On Sep 4, 1:26 am, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:
On Wed, 03 Sep 2008 22:20:43 -0700, Mensanator wrote:
On Sep 3, 8:30 pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:
On Wed, 03 Sep 2008 16:20:39 -0700, Mensanator wrote:
sum([])
0
is a bug, just as it's a bug in Excel to evaluate blank cells as 0.
It should return None or throw an exception like sum([None,1]) does.
You're wrong, because 99.9% of the time when users leave a blank cell
in Excel, they want it to be treated as zero.
Then 99.9% of users want the wrong thing.

It is to laugh.
Microsoft knows that this is a bug

Says you.
but refuses to fix it to prevent breaking legacy documents (probably
dating back to VisiCalc). When graphimg data, a missing value should be
interpreted as a hole in the graph

"Graphing data" is not sum(). I don't expect graphing data to result in
the same result as sum(), why would I expect them to interpret input the
same way?
+------+ +--+------+------+-----+

Why should the graphing application ignore blanks ("missing data"), but
sum() treat missing data as an error? That makes no sense at all.
Maybe it's important to know data is missing. You can see
the holes in a graph. You can't see the holes in a sum.
>
and not evaluated as 0
And Microsoft provides a workaround for graphs to make 0's appear as
holes. Of course, this will cause legitimate 0 values to disappear, so
the workaround is inconsistent.

I'm not aware of any spreadsheet that treats empty cells as zero for the
purpose of graphing, and I find your claim that Excel can't draw graphs
with zero in them implausible, but I don't have a copy of Excel to test
it.
That was a mistake. I made a followup correction, but
you probably didn't see it.
>
Spreadsheet sum() is not the
same as mathematician's sum, which doesn't have a concept of "blank
cells". (But if it did, it would treat them as zero, since that's the
only useful thing and mathematicians are just as much pragmatists as
spreadsheet users.) The Excel code does the right thing, and your
"pure" solution would do the unwanted and unexpected thing and is
therefore buggy.
Apparently, you don't use databases or make surface contours.

Neither databases nor surface contours are sum(). What possible relevance
are they to the question of what sum() should do?
Because a sum that includes Nulls isn't valid. If you treated
Nulls as 0, then not only would your sum be wrong, but so
would your count and the average based on those. Now you
can EXPLICITLY tell the database to only consider non-Null
values, which doesn't change the total, but DOES change
the count.
>
Do you perhaps imagine that there is only "ONE POSSIBLE CORRECT WAY" to
deal with missing data, and every function and program must deal with it
the same way?
But that's what sum() is doing now, treating sum([]) the same
as sum([],0). Why isn't sum() defined such that "...if list
is empty, return start, IF SPECIFIED, otherwise raise exception."
Then, instead of "ONE POSSIBLE CORRECT WAY", the user could
specify whether he wants Excel compatible behaviour or
Access compatible behaviour.
>
Contour programs REQUIRE that blanks are null, not 0

Lucky for them that null is not 0 then.
No, but blank cells are 0 as far as Excel is concerned.
That behaviour causes nothing but trouble and I am
saddened to see Python emulate such nonsense.
>
so that the Kriging
algorithm interpolates around the holes rather than return false
calculations. Excel's treatment of blank cells is inconsistent with
Access' treatment of Nulls and therefore wrong, anyway you slice it.

No no no, you messed that sentence up. What you *really* meant was:

"Access' treatment of Nulls is inconsistent with Excel's treatment of
blank cells and therefore wrong, anyway you slice it."

No of course not. That would be stupid, just as stupid as your sentence.
Excel is not Access. They do different things. Why should they
necessarily interpret data the same way?
Because you want consistent results?
>
Maybe you want to say a bug is when it doesn't do what the author
intended, but I say if what the intention was is wrong, then a perfect
implentation is still a bug because it doesn't do what it's supposed to
do.

Who decides what it is supposed to do if not the author?
The author can't change math on a whim.
You, in your ivory tower who doesn't care a fig for
what people want the software to do?
True, I could care less what peole want to do...

....as long as they do it consistently.
>
Bug report: "Software does what users want it to do."
Fix: "Make the software do something that users don't want."
What the users want doesn't carry any weight with respect
to what the database wants. The user must conform to the
needs of the database because the other way ain't ever gonna
happen.
>
Great.
If only. But then, I probably wouldn't have a job.
>
Bugs are defined by "does the code do what the user wants it to do?",
not "is it mathematically pure?".
ReallY? So you think math IS a democracy? There is no reason to violate
mathematical purity.

You've given a good example yourself: the Kriging algorithm needs a Null
value which is not zero. There is no mathematical "null" which is
distinct from zero, so there's an excellent violation of mathematical
purity right there.
Hey, I was talking databases, you brought up mathematical purity.
>
If I am given the job of adding up the number of widgets inside a box,
and the box is empty, I answer that there are 0 widgets inside it.
Right. it has a known quantity and that quantity is 0.
Just because the box is empty doesn't mean the quantity
is Null.
If I
were to follow your advice and declare that "An error occurred, can't
determine the number of widgets inside an empty box!" people would treat
me as an idiot, and rightly so.
Right. But a better analogy is when a new shipment is due
but hasn't arrived yet so the quantity is unknown. Now the
boss comes up and says he needs to ship 5 widgets tomorrow
and asks how many you have. You say 0. Now the boss runs
out to Joe's Widget Emporium and pays retail only to discover
when he gets back that the shipment has arrived containing
12 widgets. Because you didn't say "I don't know, today's
shipment isn't here yet", the boss not only thinks you're
an idiot, but he fires you as well.
>
If I don't get EXACTLY the same answer from Excel,
Access, Mathematica and Python, then SOMEBODY is wrong. It would be a
shame if that somebody was Python.

Well Excel, Python agree that the sum of an empty list is 0. What do
Access and Mathematica do?
I don't know abaout Mathmatica, but if you EXPLICITLY
tell Access to sum only the non-Null values, you'll get the
same answer Excel does. Otherwise, any expression that
includes a Null evaluates to Null, which certainly isn't
the same answer Excel gives.
>
The current behaviour of sum([]) does the right thing for the 99% of
the time when users expect an integer.
Why shouldn't the users expect an exception? Isn't that why we have
try:except? Maybr 99% of users expect sum([])==0, but _I_ expect to be
able to distinguish an empty list from [4,-4].

The way to distinguish lists is NOT to add them up and compare the sums:
>sum([4, -4]) == sum([0]) == sum([1, 2, 3, -6]) == sum([-1, 2, -1])

True

The correct way is by comparing the lists themselves:
>[] == [4, -4]

False
And the
rest of the time, they have to specify a starting value for the sum
anyway, and so sum([], initial_value) does the right thing *always*.
So if you really want [] to be 0, why not say sum([],0)?

I don't want [] == 0. That's foolish. I want the sum of an empty list to
be 0, which is a very different thing.
In certain circumstances. In others, an empty list summing
to 0 is just as foolish. That's why sum([]) should be an
error, so you can have it either way.

Isn't one of Python's slogans "Explicit is better than implicit"?
>
And I don't need to say sum([],0) because the default value for the
second argument is 0.
That's the problem. There is no justification for assuming
that unknown quantities are 0.
>
Why shouldn't nothing added to nothing return nothing? Having it
evaluate to 0 is wrong 99.9% of the time.

It is to laugh.

What's the difference between having 0 widgets in a box and having an
empty box with, er, no widgets in it?
There are no "empty" boxes. There are only boxes with
known quantities and those with unknown quantities.
I hope that's not too ivory tower.
>
--
Steven
Sep 4 '08 #12
In article
<71**********************************@25g2000prz.g ooglegroups.com>,
Mensanator <me********@aol.comwrote:
On Sep 3, 2:18*pm, Laszlo Nagy <gand...@shopzeus.comwrote:
bearophileH...@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>>sum([])
0
Because that [] may be an empty sequence of someobject:
You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values.
When adding real numbers, the neutral element is zero. ( X + 0 = X) It
is very logical to return zero for empty sequences.

No it isn't. Nothing is not 0, check with MS-Access, for instance:

Null + 1 returns Null. Any arithmetic expression involving a
Null evaluates to Null. Adding something to an unknown returns
an unknown, as it should.

It is a logical fallacy to equate unknown with 0.
Which has nothing to do with the "right" value for an
empty sum. If they hear about what you said here in
sci.math they're gonna kick you out - what do you
imagine the universally accepted value of \sum_{j=1}^0
is?

For example, the water table elevation in ft above Mean Sea Level
is WTE = TopOfCasing - DepthToWater.

TopOfCasing is usually known and constant (until resurveyed).
But DepthToWater may or may not exist for a given event (well
may be covered with fire ants, for example).

Now, if you equate Null with 0, then the WTE calculation says
the water table elevation is flush with the top of the well,
falsely implying that the site is underwater.

And, since this particular site is on the Mississippi River,
it sometimes IS underwater, but this is NEVER determined by
water table elevations, which, due to the CORRECT treatment
of Nulls by Access, never returns FALSE calculations.
>sum([])
0

is a bug, just as it's a bug in Excel to evaluate blank cells
as 0. It should return None or throw an exception like sum([None,1])
does.

Same way, if we would have a prod() function, it should return one for
empty sequences because X*1 = X. The neutral element for this operation
is one.

Of course this is not good for summing other types of objects. But how
clumsy would it be to use

sum( L +[0] )

or

if L:
value = sum(L)
else:
value = 0

instead of sum(L).

Once again, this is what sum() is used for in most cases, so this
behavior is the "expected" one.

Another argument to convince you: the sum() function in SQL for empty
row sets returns zero in most relational databases.

But of course it could have been implemented in a different way... I
believe that there have been excessive discussions about this decision,
and the current implementation is very good, if not the best.

Best,

Laszlo
--
David C. Ullrich
Sep 4 '08 #13
In article
<24**********************************@34g2000hsh.g ooglegroups.com>,
be************@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>sum([])
0

Because that [] may be an empty sequence of someobject:
>sum(s for s in ["a", "b"] if len(s) 2)
0

In a statically typed language in that situation you may answer the
initializer value of the type of the items of the list, as I do in the
sum() in D.

This sounds like a more correct/clean thing to do:
>max([])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: max() arg is an empty sequence

So it may be better to make the sum([]) too raise a ValueError,
I don't see why you feel the two should act the same.
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

And both for good reason:

(i) If A and B are disjoint sets we certainly want to
have sum(A union B) = sum(A) + sum(B). This requires
sum(empty set) = 0.

(ii) If A is a subset of B then we should have
max(A) <= max(B). This requires that max(empty set)
be something that's smaller than everything else.
So we give up on that.
in
Python 3/3.1 (if this isn't already true). On the other hand often
enough I have code like this:
>max(fun(x) for x in iterable if predicate(x))

This may raise the ValueError both if iterable is empty of if the
predicate on its items is always false, so instead of catching
exceptions, that I try to avoid, I usually end with a normal loop,
that's readable and fast:

max_value = smallvalue
for x in iterable:
if predicate(x):
max_value = max(max_value, fun(x))

Where running speed matters, I may even replace that max(max_value,
fun(x)) with a more normal if/else.

A possible alternative is to add a default to max(), like the next()
built-in of Python 2.6:
>max((fun(x) for x in iterable if predicate(x)), default=smallvalue)

This returns smallvalue if there are no items to compute the max of.

Bye,
bearophile
--
David C. Ullrich
Sep 4 '08 #14
On Sep 4, 11:13*am, "David C. Ullrich" <dullr...@sprynet.comwrote:
In article
<719910b1-3776-4bf2-a0b6-236f3167e...@25g2000prz.googlegroups.com>,

*Mensanator <mensana...@aol.comwrote:
On Sep 3, 2:18*pm, Laszlo Nagy <gand...@shopzeus.comwrote:
bearophileH...@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>sum([])
0
Because that [] may be an empty sequence of someobject:
You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values..
When adding real numbers, the neutral element is zero. ( X + 0 = X)It
is very logical to return zero for empty sequences.
No it isn't. Nothing is not 0, check with MS-Access, for instance:
Null + 1 returns Null. Any arithmetic expression involving a
Null evaluates to Null. Adding something to an unknown returns
an unknown, as it should.
It is a logical fallacy to equate unknown with 0.

Which has nothing to do with the "right" value for an
empty sum.
I'm less concerned about the "right" value than a consistent
value. I'm fairly certain you can't get 0 from a query that
returns no records, so I don't like seeing empty being
treated as 0, even if it means that in set theory because
databases aren't sets.
If they hear about what you said here in
sci.math they're gonna kick you out
They usually don't kick me out, just kick me.
- what do you
imagine the universally accepted value of \sum_{j=1}^0
is?
I can't follow your banter, so I'm not sure what it should be.
>

For example, the water table elevation in ft above Mean Sea Level
is WTE = TopOfCasing - DepthToWater.
TopOfCasing is usually known and constant (until resurveyed).
But DepthToWater may or may not exist for a given event (well
may be covered with fire ants, for example).
Now, if you equate Null with 0, then the WTE calculation says
the water table elevation is flush with the top of the well,
falsely implying that the site is underwater.
And, since this particular site is on the Mississippi River,
it sometimes IS underwater, but this is NEVER determined by
water table elevations, which, due to the CORRECT treatment
of Nulls by Access, never returns FALSE calculations.
>>sum([])
0
is a bug, just as it's a bug in Excel to evaluate blank cells
as 0. It should return None or throw an exception like sum([None,1])
does.
Same way, if we would have a prod() function, it should return one for
empty sequences because X*1 = X. The neutral element for this operation
is one.
Of course this is not good for summing other types of objects. But how
clumsy would it be to use
sum( L +[0] )
or
if L:
value = sum(L)
else:
value = 0
instead of sum(L).
Once again, this is what sum() is used for in most cases, so this
behavior is the "expected" one.
Another argument to convince you: the sum() function in SQL for empty
row sets returns zero in most relational databases.
But of course it could have been implemented in a different way... I
believe that there have been excessive discussions about this decision,
and the current implementation is very good, if not the best.
Best,
Laszlo

--
David C. Ullrich
Sep 4 '08 #15
On Thu, 4 Sep 2008 10:57:35 -0700 (PDT), Mensanator wrote:
Why then, doesn't
>>>sum([A for A in [None, None, None, None, None, None] if A != None])
0

give me an error?
Because "[A for A in [None, None, None, None, None, None] if A != None]"
returns an empty list, and sum([]) doesn't return an error. What did you
expect?
--
Regards,
Wojtek Walczak,
http://tosh.pl/gminick/
Sep 4 '08 #16
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.
What do you think about my idea of adding that 'default' argument to
the max()/min() functions?

Bye,
bearophile
Sep 4 '08 #17
On Sep 4, 2:42*pm, bearophileH...@lycos.com wrote:
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

What do you think about my idea of adding that 'default' argument to
the max()/min() functions?

Bye,
bearophile
For max and min, why can't you just add your argument to the set
itself?

The reason max([]) is undefined is that max( S ) is in S. The reason
sum([]) is 0 is that sum( [ x ] ) - x = 0.
Sep 4 '08 #18
castironpi:
For max and min, why can't you just add your argument to the set
itself?
Sometimes that can be done, but in many other situations it's less
easy, like in the example I have shown in my first post:

max((fun(x) for x in iterable if predicate(x)))

There are some ways to add the max there, for example using an
itertools.chain to chan the default value to the end of the iterable,
but most of the time I just write a for loop.

Bye,
bearophile
Sep 4 '08 #19
On Thu, Sep 4, 2008 at 4:25 PM, castironpi <ca********@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
>David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

What do you think about my idea of adding that 'default' argument to
the max()/min() functions?

Bye,
bearophile

For max and min, why can't you just add your argument to the set
itself?

The reason max([]) is undefined is that max( S ) is in S.
It makes sense.
>The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.
It doesn't make sense to me. What do you set x to?
Sep 5 '08 #20
David C. Ullrich wrote:
In article
<71**********************************@25g2000prz.g ooglegroups.com>,
Mensanator <me********@aol.comwrote:
>On Sep 3, 2:18 pm, Laszlo Nagy <gand...@shopzeus.comwrote:
>>bearophileH...@lycos.com wrote:
Empty Python lists [] don't know the type of the items it will
contain, so this sounds strange:
>>sum([])
0
Because that [] may be an empty sequence of someobject:
You are right in that sum could be used to sum arbitrary objects.
However, in 99.99% of the cases, you will be summing numerical values.
When adding real numbers, the neutral element is zero. ( X + 0 = X) It
is very logical to return zero for empty sequences.
No it isn't. Nothing is not 0, check with MS-Access, for instance:

Null + 1 returns Null. Any arithmetic expression involving a
Null evaluates to Null. Adding something to an unknown returns
an unknown, as it should.

It is a logical fallacy to equate unknown with 0.

Which has nothing to do with the "right" value for an
empty sum. If they hear about what you said here in
sci.math they're gonna kick you out - what do you
imagine the universally accepted value of \sum_{j=1}^0
is?

>For example, the water table elevation in ft above Mean Sea Level
is WTE = TopOfCasing - DepthToWater.

TopOfCasing is usually known and constant (until resurveyed).
But DepthToWater may or may not exist for a given event (well
may be covered with fire ants, for example).

Now, if you equate Null with 0, then the WTE calculation says
the water table elevation is flush with the top of the well,
falsely implying that the site is underwater.

And, since this particular site is on the Mississippi River,
it sometimes IS underwater, but this is NEVER determined by
water table elevations, which, due to the CORRECT treatment
of Nulls by Access, never returns FALSE calculations.
>>>>sum([])
0

is a bug, just as it's a bug in Excel to evaluate blank cells
as 0. It should return None or throw an exception like sum([None,1])
does.
>>Same way, if we would have a prod() function, it should return one for
empty sequences because X*1 = X. The neutral element for this operation
is one.

Of course this is not good for summing other types of objects. But how
clumsy would it be to use

sum( L +[0] )

or

if L:
value = sum(L)
else:
value = 0

instead of sum(L).

Once again, this is what sum() is used for in most cases, so this
behavior is the "expected" one.

Another argument to convince you: the sum() function in SQL for empty
row sets returns zero in most relational databases.

But of course it could have been implemented in a different way... I
believe that there have been excessive discussions about this decision,
and the current implementation is very good, if not the best.

Best,

Laszlo
I suppose the following is accepted by statisticians. Here,
for reference, here is the what the 'R' statistic package
says on the subject 9if you type 'help(sum)'
<quote>
Sum of Vector Elements
Description
sum returns the sum of all the values present in its arguments.

Usage
sum(..., na.rm = FALSE)

Arguments
.... numeric or complex or logical vectors.
na.rm logical. Should missing values be removed?

Details
This is a generic function: methods can be defined for it directly or
via the Summary group generic. For this to work properly, the arguments
.... should be unnamed, and dispatch is on the first argument.

If na.rm is FALSE an NA value in any of the arguments will cause a value
of NA to be returned, otherwise NA values are ignored.

Logical true values are regarded as one, false values as zero. For
historical reasons, NULL is accepted and treated as if it were integer(0).

Value
The sum. If all of ... are of type integer or logical, then the sum is
integer, and in that case the result will be NA (with a warning) if
integer overflow occurs. Otherwise it is a length-one numeric or complex
vector.
NB: the sum of an empty set is zero, by definition.

References
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S
Language. Wadsworth & Brooks/Cole.
Sep 5 '08 #21
David C. Ullrich wrote:
>
I don't see why you feel the two should act the same.
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

And both for good reason:

(i) If A and B are disjoint sets we certainly want to
have sum(A union B) = sum(A) + sum(B). This requires
sum(empty set) = 0.

(ii) If A is a subset of B then we should have
max(A) <= max(B). This requires that max(empty set)
be something that's smaller than everything else.
So we give up on that.
Do we give up? Really ?

From wikipedia: http://en.wikipedia.org/wiki/Empty_set
(Uses wikipedia's LaTeX notation -- I hope those interested
are OK with that )

<quote>
Mathematics

[edit] Extended real numbers

Since the empty set has no members, when it is considered as a subset of
any ordered set, then any member of that set will be an upper bound and
lower bound for the empty set. For example, when considered as a subset
of the real numbers, with its usual ordering, represented by the real
number line, every real number is both an upper and lower bound for the
empty set.[3] When considered as a subset of the extended reals formed
by adding two "numbers" or "points" to the real numbers, namely negative
infinity, denoted -\infty\!\,, which is defined to be less than every
other extended real number, and positive infinity, denoted +\infty\!\,,
which is defined to be greater than every other extended real number, then:

\sup\varnothing=\min(\{-\infty, +\infty \} \cup \mathbb{R})=-\infty,

and

\inf\varnothing=\max(\{-\infty, +\infty \} \cup \mathbb{R})=+\infty.

That is, the least upper bound (sup or supremum) of the empty set is
negative infinity, while the greatest lower bound (inf or infimum) is
positive infinity. By analogy with the above, in the domain of the
extended reals, negative infinity is the identity element for the
maximum and supremum operators, while positive infinity is the identity
element for minimum and infimum.
Sep 5 '08 #22
In article <g9*******************@news.demon.co.uk>,
Ken Starks <st*****@lampsacos.demon.co.ukwrote:
David C. Ullrich wrote:

I don't see why you feel the two should act the same.
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

And both for good reason:

(i) If A and B are disjoint sets we certainly want to
have sum(A union B) = sum(A) + sum(B). This requires
sum(empty set) = 0.

(ii) If A is a subset of B then we should have
max(A) <= max(B). This requires that max(empty set)
be something that's smaller than everything else.
So we give up on that.

Do we give up? Really ?
Erm, thanks. I was aware of all that below. If we're
being technical what's below is talking about the sup
and inf, which are not the same as max and min. More
relevant to the present context, I didn't mention what's
below because it doesn't seem likely that saying max([])
= -infinity and min([]) = +infinity is going to make the
OP happy...
From wikipedia: http://en.wikipedia.org/wiki/Empty_set
(Uses wikipedia's LaTeX notation -- I hope those interested
are OK with that )

<quote>
Mathematics

[edit] Extended real numbers

Since the empty set has no members, when it is considered as a subset of
any ordered set, then any member of that set will be an upper bound and
lower bound for the empty set. For example, when considered as a subset
of the real numbers, with its usual ordering, represented by the real
number line, every real number is both an upper and lower bound for the
empty set.[3] When considered as a subset of the extended reals formed
by adding two "numbers" or "points" to the real numbers, namely negative
infinity, denoted -\infty\!\,, which is defined to be less than every
other extended real number, and positive infinity, denoted +\infty\!\,,
which is defined to be greater than every other extended real number, then:

\sup\varnothing=\min(\{-\infty, +\infty \} \cup \mathbb{R})=-\infty,

and

\inf\varnothing=\max(\{-\infty, +\infty \} \cup \mathbb{R})=+\infty.

That is, the least upper bound (sup or supremum) of the empty set is
negative infinity, while the greatest lower bound (inf or infimum) is
positive infinity. By analogy with the above, in the domain of the
extended reals, negative infinity is the identity element for the
maximum and supremum operators, while positive infinity is the identity
element for minimum and infimum.
--
David C. Ullrich
Sep 5 '08 #23
In article
<18**********************************@n38g2000prl. googlegroups.com>,
be************@lycos.com wrote:
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

What do you think about my idea of adding that 'default' argument to
the max()/min() functions?
How the Python max and min functions should work has to
do with how people want them to work and how people expect
them to work. I wouldn't know about most people, but I
would have been surprised if min([]) was not an error,
and I would have been disappointed if sum([]) was not 0.

From a mathematical point of view, not that that's directly
relevant, it doesn't make much sense to me to add that default
argument. The max of a set is supposed to be the largest
element of that set. If the set is empty there's no such
thing.

In Python you'd better make sure that S is nonempty before
asking for max(S). That's not just Python - in math you need
to make certain that S is nonempty and also other conditions
before you're allowed to talk about max(S). That's just the
way it is.

Think about all the previously elected female or black
presidents of the US. Which one was the tallest?
Bye,
bearophile
--
David C. Ullrich
Sep 5 '08 #24
David C. Ullrich:
I didn't mention what's below because it doesn't seem
likely that saying max([]) = -infinity and
min([]) = +infinity is going to make the OP happy...
Well, it sounds cute having Neginfinite and Infinite as built-int
objects that can be compared to any other type and are < of or of
everything else but themselves. Probably they can be useful as
sentinels, but in Python I nearly never use sentinels anymore, and
they can probably give some other problems...

Bye,
bearophile

Sep 5 '08 #25
On Sep 5, 3:28*am, "Manu Hack" <manuh...@gmail.comwrote:
On Thu, Sep 4, 2008 at 4:25 PM, castironpi <castiro...@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.
What do you think about my idea of adding that 'default' argument to
the max()/min() functions?
Bye,
bearophile
For max and min, why can't you just add your argument to the set
itself?
The reason max([]) is undefined is that max( S ) is in S.

It makes sense.
The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.

It doesn't make sense to me. *What do you set x to?
For all x.
Sep 5 '08 #26
On Sep 5, 1:08*am, Dennis Lee Bieber <wlfr...@ix.netcom.comwrote:
On Thu, 4 Sep 2008 18:09:49 -0700 (PDT), Mensanator <mensana...@aol.com>
declaimed the following in comp.lang.python:
Too bad. I brought this up because I use Python a lot with
database work and rarely for proving theorms in ZFC.

* * * * As a by-stander... let the DBMS do its work, don't try tomake
Python do what DBMS SQL does...
Sure, and in most cases I use Visual Basic for Applications
when I need functionality I can't get directly from SQL.

But anybody who's used VBA with Access must know what a PITA
it is. And even when you get it working, you sometimes wish you
hadn't. I have a Mann-Kendall trend analysis that must be done
quarterly on over 150 combinations of well:analyte. It takes
over 6 hours to process this (and I don't know how much is due to
VBA, Access, server, network, etc.). It's something I'd love to
try in Python (if I can find the time to translate it).

But I'm wary of things that Python might do (such as return 0
when summing an empty list) that SQL/VBA does not.

--
* * * * Wulfraed * * * *Dennis Lee Bieber * * * ** * * KD6MOG
* * * * wlfr...@ix.netcom.com * * * * * * wulfr...@bestiaria.com
* * * * * * * * HTTP://wlfraed.home.netcom.com/
* * * * (Bestiaria Support Staff: * * * * * * * web-a...@bestiaria.com)
* * * * * * * * HTTP://www.bestiaria.com/
Sep 5 '08 #27
David C. Ullrich wrote:
In article <g9*******************@news.demon.co.uk>,
Ken Starks <st*****@lampsacos.demon.co.ukwrote:
>David C. Ullrich wrote:
>>I don't see why you feel the two should act the same.
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.

And both for good reason:

(i) If A and B are disjoint sets we certainly want to
have sum(A union B) = sum(A) + sum(B). This requires
sum(empty set) = 0.

(ii) If A is a subset of B then we should have
max(A) <= max(B). This requires that max(empty set)
be something that's smaller than everything else.
So we give up on that.
Do we give up? Really ?

Erm, thanks. I was aware of all that below. If we're
being technical what's below is talking about the sup
and inf, which are not the same as max and min. More
relevant to the present context, I didn't mention what's
below because it doesn't seem likely that saying max([])
= -infinity and min([]) = +infinity is going to make the
OP happy...
Of course you were aware, I have seen enough of your posts
to know that. And I agree that, whatever Wikipedia seems to
imply, max and supremum should be distiguished.

It was your prelude, "At least in mathematics ..." that
made me prick up my ears. So I couldn't resist responding,
without _any_ malice I assure you.

Cheers,
Ken.

Sep 5 '08 #28
On Fri, 05 Sep 2008 10:22:22 -0500, David C. Ullrich wrote about why max
and min shouldn't accept a default argument:
Think about all the previously elected female or black presidents of the
US. Which one was the tallest?
I know the answer to that one:

All of them!

--
Steven
Sep 6 '08 #29
On Fri, Sep 5, 2008 at 1:04 PM, castironpi <ca********@gmail.comwrote:
On Sep 5, 3:28 am, "Manu Hack" <manuh...@gmail.comwrote:
>On Thu, Sep 4, 2008 at 4:25 PM, castironpi <castiro...@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.
>What do you think about my idea of adding that 'default' argument to
the max()/min() functions?
>Bye,
bearophile
For max and min, why can't you just add your argument to the set
itself?
The reason max([]) is undefined is that max( S ) is in S.

It makes sense.
>The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.

It doesn't make sense to me. What do you set x to?

For all x.
But then how can you conclude sum([]) = 0 from there? It's way far
from obvious.
Sep 6 '08 #30
On Fri, 05 Sep 2008 22:20:06 -0400, Manu Hack wrote:
On Fri, Sep 5, 2008 at 1:04 PM, castironpi <ca********@gmail.comwrote:
....
>>The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.

It doesn't make sense to me. What do you set x to?

For all x.

But then how can you conclude sum([]) = 0 from there? It's way far from
obvious.
I think Castironpi's reasoning is to imagine taking sum([x])-x for *any*
possible x (where subtraction and addition is defined). Naturally you
always get 0.

Now replace x by *nothing at all* and you get:

sum([]) "subtract nothing at all" = 0

I think that this is a reasonable way to *informally* think about the
question, but it's not mathematically sound, because if you replace x
with "nothing at all" you either get:

sum([]) - = 0

which is invalid (only one operand to the subtraction operator), or you
get:

sum([0]) - 0 = 0

which doesn't involve an empty list. What castironpi seems to be doing is
replacing "nothing at all" with, er, nothing at all in one place, and
zero in the other. And that's what makes it unsound and only suitable as
an informal argument.

[The rest of this is (mostly) aimed at Mensanator, so others can stop
reading if they like.]

Fundamentally, the abstract function "sum" and the concrete Python
implementation of sum() are both human constructs. It's not like there is
some pure Platonic[1] "Ideal Sum" floating in space that we can refer to.
Somewhere, sometime, some mathematician had to *define* sum(), and other
mathematicians had to agree to use the same definition.

They could have decided that sum must take at least two arguments,
because addition requires two arguments and it's meaningless to talk
about adding a single number without talking about adding it to something
else. But they didn't. Similarly, they might have decided that sum must
take at least one argument, and therefore prohibit sum([]), but they
didn't: it's more useful for sum of the empty list to give zero than it
is for it to be an error. As I mentioned earlier, mathematicians are
nothing if not pragmatists.


[1] Or was it Aristotle who believed in Ideal Forms? No, I'm sure it was
Plato.

--
Steven
Sep 6 '08 #31
On Fri, Sep 5, 2008 at 11:45 PM, Steven D'Aprano
<st***@remove-this-cybersource.com.auwrote:
On Fri, 05 Sep 2008 22:20:06 -0400, Manu Hack wrote:
>On Fri, Sep 5, 2008 at 1:04 PM, castironpi <ca********@gmail.comwrote:
...
>>>The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.

It doesn't make sense to me. What do you set x to?

For all x.

But then how can you conclude sum([]) = 0 from there? It's way far from
obvious.

I think Castironpi's reasoning is to imagine taking sum([x])-x for *any*
possible x (where subtraction and addition is defined). Naturally you
always get 0.

Now replace x by *nothing at all* and you get:

sum([]) "subtract nothing at all" = 0

I think that this is a reasonable way to *informally* think about the
question, but it's not mathematically sound, because if you replace x
with "nothing at all" you either get:

sum([]) - = 0

which is invalid (only one operand to the subtraction operator), or you
get:

sum([0]) - 0 = 0

which doesn't involve an empty list. What castironpi seems to be doing is
replacing "nothing at all" with, er, nothing at all in one place, and
zero in the other. And that's what makes it unsound and only suitable as
an informal argument.
Actually it's even more natural to state sum([x]) = x, and this way
you can never conclude that sum([]) = 0 from there.
Sep 6 '08 #32
On Sep 5, 9:20*pm, "Manu Hack" <manuh...@gmail.comwrote:
On Fri, Sep 5, 2008 at 1:04 PM, castironpi <castiro...@gmail.comwrote:
On Sep 5, 3:28 am, "Manu Hack" <manuh...@gmail.comwrote:
On Thu, Sep 4, 2008 at 4:25 PM, castironpi <castiro...@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.
What do you think about my idea of adding that 'default' argument to
the max()/min() functions?
Bye,
bearophile
For max and min, why can't you just add your argument to the set
itself?
The reason max([]) is undefined is that max( S ) is in S.
It makes sense.
The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.
It doesn't make sense to me. *What do you set x to?
For all x.

But then how can you conclude sum([]) = 0 from there? *It's way far
from obvious.
You can define sum([a1,a2,...,aN]) recursively as
sum([a1,a2,...a(N-1)])+aN. Call the sum sum([a1,a2,...,aN]) "X", then
subtract aN.

sum([a1,a2,...a(N-1)])+aN=X
sum([a1,a2,...a(N-1)])+aN-aN=X-aN

For N=2, we have:

sum([a1,a2])=X
sum([a1,a2])-a2=X-a2
sum([a1,a2])-a2-a1=X-a2-a1

Since X= a1+ a2, replace X.

sum([a1,a2])-a2-a1=(a1+a2)-a2-a1

Or,

sum([a1,a2])-a2-a1=0

Apply the recursive definition:

sum([a1])+a2-a2-a1=0

And again:

sum([])+a1+a2-a2-a1=0

And we have:

sum([])=0.

Sep 6 '08 #33
On Sat, 06 Sep 2008 00:33:25 -0400, Manu Hack wrote:
Actually it's even more natural to state sum([x]) = x, and this way you
can never conclude that sum([]) = 0 from there.
But what you can say is that for any list L, sum(L) = sum(L + [0]).

Therefore sum([]) = sum([] +[0]) = 0

--
Steven
Sep 6 '08 #34
On Sat, Sep 6, 2008 at 12:57 AM, castironpi <ca********@gmail.comwrote:
On Sep 5, 9:20 pm, "Manu Hack" <manuh...@gmail.comwrote:
>On Fri, Sep 5, 2008 at 1:04 PM, castironpi <castiro...@gmail.comwrote:
On Sep 5, 3:28 am, "Manu Hack" <manuh...@gmail.comwrote:
On Thu, Sep 4, 2008 at 4:25 PM, castironpi <castiro...@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
David C. Ullrich:
At least in mathematics, the sum of the elements of
the empty set _is_ 0, while the maximum element of the
empty set is undefined.
>What do you think about my idea of adding that 'default' argument to
the max()/min() functions?
>Bye,
bearophile
For max and min, why can't you just add your argument to the set
itself?
The reason max([]) is undefined is that max( S ) is in S.
>It makes sense.
>The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.
>It doesn't make sense to me. What do you set x to?
For all x.

But then how can you conclude sum([]) = 0 from there? It's way far
from obvious.

You can define sum([a1,a2,...,aN]) recursively as
sum([a1,a2,...a(N-1)])+aN. Call the sum sum([a1,a2,...,aN]) "X", then
subtract aN.

sum([a1,a2,...a(N-1)])+aN=X
sum([a1,a2,...a(N-1)])+aN-aN=X-aN

For N=2, we have:

sum([a1,a2])=X
sum([a1,a2])-a2=X-a2
sum([a1,a2])-a2-a1=X-a2-a1

Since X= a1+ a2, replace X.

sum([a1,a2])-a2-a1=(a1+a2)-a2-a1

Or,

sum([a1,a2])-a2-a1=0

Apply the recursive definition:

sum([a1])+a2-a2-a1=0

And again:

sum([])+a1+a2-a2-a1=0

And we have:

sum([])=0.
It makes more sense now, I just wanted to point out that only with
sum([x]) = x, you can't get sum([]) = 0.
Sep 6 '08 #35
castironpi wrote:
On Sep 5, 9:20 pm, "Manu Hack" <manuh...@gmail.comwrote:
>On Fri, Sep 5, 2008 at 1:04 PM, castironpi <castiro...@gmail.comwrote:
>>On Sep 5, 3:28 am, "Manu Hack" <manuh...@gmail.comwrote:
On Thu, Sep 4, 2008 at 4:25 PM, castironpi <castiro...@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
>David C. Ullrich:
>>At least in mathematics, the sum of the elements of
>>the empty set _is_ 0, while the maximum element of the
>>empty set is undefined.
>What do you think about my idea of adding that 'default' argument to
>the max()/min() functions?
>Bye,
>bearophile
For max and min, why can't you just add your argument to the set
itself?
The reason max([]) is undefined is that max( S ) is in S.
It makes sense.
The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.
It doesn't make sense to me. What do you set x to?
For all x.
But then how can you conclude sum([]) = 0 from there? It's way far
from obvious.

You can define sum([a1,a2,...,aN]) recursively as
sum([a1,a2,...a(N-1)])+aN. Call the sum sum([a1,a2,...,aN]) "X", then
subtract aN.

sum([a1,a2,...a(N-1)])+aN=X
sum([a1,a2,...a(N-1)])+aN-aN=X-aN

For N=2, we have:

sum([a1,a2])=X
sum([a1,a2])-a2=X-a2
sum([a1,a2])-a2-a1=X-a2-a1

Since X= a1+ a2, replace X.

sum([a1,a2])-a2-a1=(a1+a2)-a2-a1

Or,

sum([a1,a2])-a2-a1=0

Apply the recursive definition:

sum([a1])+a2-a2-a1=0

And again:

sum([])+a1+a2-a2-a1=0

And we have:

sum([])=0.
This is not necessarily so.

The flaw is that you provide a recursive definition with no start value,
which is to say it is not a recursive definition at all.

A recursive definition should be (for lists where elements
can be added, and ignoring pythonic negative indexing):

Define 'sum(L)' by
a. sum(L[0]) = L[0]
b. sum(L[0:i]) = sum(L[0:i-1]) + L[i] ... if i 0

From this you can prove the reverse recursion
sum{L[0:k]) = sum(L[0:k+1]) - L[k+1]
__only__ if k >= 0

It says nothing about the empty list.

You could add, as part of the definition, that sum{[]) = 0, or any other
value.

A rather different approach, not quite simple recursion, would be to
start with

A. a slicing axiom, something like:

for all non-negative integers, a,b,c with a <=b <= c:

sum(L[a:c]) = sum(L[a:b]) + sum(L[b:c])

B. a singleton axiom:

for all integers a where L[a] exists:
sum(L[a:a]) = L[a]


2a. sum{
Sep 6 '08 #36
castironpi wrote:
On Sep 5, 9:20 pm, "Manu Hack" <manuh...@gmail.comwrote:
>On Fri, Sep 5, 2008 at 1:04 PM, castironpi <castiro...@gmail.comwrote:
>>On Sep 5, 3:28 am, "Manu Hack" <manuh...@gmail.comwrote:
On Thu, Sep 4, 2008 at 4:25 PM, castironpi <castiro...@gmail.comwrote:
On Sep 4, 2:42 pm, bearophileH...@lycos.com wrote:
>David C. Ullrich:
>>At least in mathematics, the sum of the elements of
>>the empty set _is_ 0, while the maximum element of the
>>empty set is undefined.
>What do you think about my idea of adding that 'default' argument to
>the max()/min() functions?
>Bye,
>bearophile
For max and min, why can't you just add your argument to the set
itself?
The reason max([]) is undefined is that max( S ) is in S.
It makes sense.
The reason sum([]) is 0 is that sum( [ x ] ) - x = 0.
It doesn't make sense to me. What do you set x to?
For all x.
But then how can you conclude sum([]) = 0 from there? It's way far
from obvious.

You can define sum([a1,a2,...,aN]) recursively as
sum([a1,a2,...a(N-1)])+aN. Call the sum sum([a1,a2,...,aN]) "X", then
subtract aN.

sum([a1,a2,...a(N-1)])+aN=X
sum([a1,a2,...a(N-1)])+aN-aN=X-aN

For N=2, we have:

sum([a1,a2])=X
sum([a1,a2])-a2=X-a2
sum([a1,a2])-a2-a1=X-a2-a1

Since X= a1+ a2, replace X.

sum([a1,a2])-a2-a1=(a1+a2)-a2-a1

Or,

sum([a1,a2])-a2-a1=0

Apply the recursive definition:

sum([a1])+a2-a2-a1=0

And again:

sum([])+a1+a2-a2-a1=0

And we have:

sum([])=0.
This is not necessarily so.

The flaw is that you provide a recursive definition with no start value,
which is to say it is not a recursive definition at all.

A recursive definition should be (for lists where elements
can be added, and ignoring pythonic negative indexing):

Define 'sum(L)' by
a. sum(L[0:1]) = L[0]
b. sum(L[0:i]) = sum(L[0:i-1]) + L[i] ... if i 1

From this you can prove the reverse recursion
sum{L[0:k]) = sum(L[0:k+1]) - L[k+1]
__only__ if k >= 0

It says nothing about the empty list.

You could add, as part of the definition, that sum{[]) = 0, or any other
value.

A rather different approach, not quite simple recursion, would be to
start with

A. a slicing axiom, something like:

for all non-negative integers, a,b,c with a <=b <= c:

sum(L[a:c]) = sum(L[a:b]) + sum(L[b:c])

B. a singleton axiom:

for all integers a where L[a] exists:
sum(L[a:a]) = L[a]


2a. sum{
Sep 6 '08 #37
Mel
Steven D'Aprano wrote:
On Sat, 06 Sep 2008 00:33:25 -0400, Manu Hack wrote:
>Actually it's even more natural to state sum([x]) = x, and this way you
can never conclude that sum([]) = 0 from there.

But what you can say is that for any list L, sum(L) = sum(L + [0]).

Therefore sum([]) = sum([] +[0]) = 0
Yep. The way it is preserves the distributive property

sum(a+b) = sum(a) + sum(b)

This would matter in cases like (untested code..)

suvsales = sum (sum (s.price for s in d.sales if s.class='suv') for d in
districts)
Mel.
Sep 6 '08 #38
On Sep 5, 10:45�pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:
On Fri, 05 Sep 2008 22:20:06 -0400, Manu Hack wrote:
On Fri, Sep 5, 2008 at 1:04 PM, castironpi <castiro...@gmail.comwrote:
<snip>
[The rest of this is (mostly) aimed at Mensanator,
Ok, I see where you're coming from.
Fundamentally, the abstract function "sum" and the concrete Python
implementation of sum() are both human constructs. It's not like there is
some pure Platonic[1] "Ideal Sum" floating in space that we can refer to.
Somewhere, sometime, some mathematician had to *define* sum(), and other
mathematicians had to agree to use the same definition.

They could have decided that sum must take at least two arguments,
because addition requires two arguments and it's meaningless to talk
about adding a single number without talking about adding it to something
else. But they didn't.
Ok. But the problem is they DID in SQL: x + Null = Null.

Earlier, you said that an empty box contains 0 widgets.
Fine, empty means 0. But Null doesn't mean empty. Say
your widget supplier just delivers a box and you haven't
opened it yet. Is the box likely to be empty? Probably
not, or they wouldn't have shipped it. In this case,
Null means "unknown", not 0. The number of widgets you
have on hand is Null (unknown) because inventory + Null = Null.

SQL will correctly tell you that the amount on hand is unknown,
whereas Python will tell you the amount on hand is inventory,
which is incorrect.
Similarly, they might have decided that sum must
take at least one argument, and therefore prohibit sum([]), but they
didn't: it's more useful for sum of the empty list to give zero than it
is for it to be an error. As I mentioned earlier, mathematicians are
nothing if not pragmatists.
Here's a real world example (no ivory tower stuff):

An oil refinery client has just excavated a big pile of
dirt to lay a new pipeline. Due to the volume of the
pipe, there's dirt left over. Ideally, the client
would like to use that dirt as landfill (free), but it
must be tested for HAPS (by summing the concentrations of
organic constituents) to see whether it is considered
hazardous waste, it which cas it must be taken off site
and incinerated (costly).

In MOST cases, a HAPS sum of 0 would be illegal because
0's generally cannot be reported in analytical tests,
you can't report a result less than it's legal reporting
limit. If ALL the consituents were undetected, the sum
should be that of the sum of the reporting limits, thus,
it cannot be 0.

Can't I just use a sum of 0 to tell me when data is missing?
No, because in some cases the reporting limit of undetected
compounds is set to 0.

In which case, a 0 HAPS score means we can confidently
reccomend that the dirt is clean and can be freely reused.

But if the analysis information is missing (hasn'r arrived
yet or still pending validation) we WANT the result to be
UNKNOWN so that we don't reccomend to the client that he take
an illegal course of action.

In this case, SQL does the correct thing and Python would
return a false result.
--
Steven
Sep 6 '08 #39
On Sat, 06 Sep 2008 11:22:07 -0700, Mensanator wrote:

[...]
>They could have decided that sum must take at least two arguments,
because addition requires two arguments and it's meaningless to talk
about adding a single number without talking about adding it to
something else. But they didn't.

Ok. But the problem is they DID in SQL: x + Null = Null.
Sheesh. That's not a problem, because Python is not trying to be a
dialect of SQL.

If you want a NULL object, then there are recipes on the web that will
give you one. Then all you need to do is call sum(alist or [NULL]) and it
will give you the behaviour you want.
[...]
Here's a real world example (no ivory tower stuff):

An oil refinery client has just excavated a big pile of dirt to lay a
new pipeline.
[snip details]
Can't I just use a sum of 0 to tell me when data is missing? No, because
in some cases the reporting limit of undetected compounds is set to 0.
You can't use a sum of 0 to indicate when data is missing, full stop. The
data may require 15 tests when only 3 have actually been done:

sum([1.2e-7, 9.34e-6, 2.06e-8])

Missing data and a non-zero sum. How should sum() deal with that?

The answer is that sum() can't deal with that. You can't expect sum() to
read your mind, know that there should be 15 items instead of 3, and
raise an error. So why do you expect sum() to read your mind and
magically know that zero items is an error, especially when for many
applications it is NOT an error?

The behaviour you want for this specific application is unwanted,
unnecessary and even undesirable for many other applications. The
solution is for *you* to write application-specific code to do what your
application needs, instead of relying on a general purpose function
magically knowing what you want.

--
Steven
Sep 7 '08 #40
On Sep 6, 11:05�pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:
On Sat, 06 Sep 2008 11:22:07 -0700, Mensanator wrote:

[...]
They could have decided that sum must take at least two arguments,
because addition requires two arguments and it's meaningless to talk
about adding a single number without talking about adding it to
something else. But they didn't.
Ok. But the problem is they DID in SQL: x + Null = Null.

Sheesh. That's not a problem, because Python is not trying to be a
dialect of SQL.
And yet, they added a Sqlite3 module.
>
If you want a NULL object, then there are recipes on the web that will
give you one. Then all you need to do is call sum(alist or [NULL]) and it
will give you the behaviour you want.
Actualy, I already get the behaviour I want. sum([1,None])
throws an exception. I don't see why sum([]) doesn't throw
an exception also (I understand that behaviour is by design,
I'm merely pointing out that the design doesn't cover every
situation).
>
[...]
Here's a real world example (no ivory tower stuff):
An oil refinery client has just excavated a big pile of dirt to lay a
new pipeline.
[snip details]
Can't I just use a sum of 0 to tell me when data is missing? No, because
in some cases the reporting limit of undetected compounds is set to 0.

You can't use a sum of 0 to indicate when data is missing, full stop.
Exactly. That's why I would prefer sum([]) to raise an
exception instead of giving a false positive.
The
data may require 15 tests when only 3 have actually been done:

sum([1.2e-7, 9.34e-6, 2.06e-8])
Biggest problem here is that it is often unknown just
how many records you're supposed to get from the query,
so we can't tell that a count of 3 is supposed to be 15.
>
Missing data and a non-zero sum. How should sum() deal with that?
That's a seperate issue and I'm not saying it should as
long as the list contains actual numbers to sum.
sum([1.2e-7, 9.34e-6, 2.06e-8, None]) will raise an
exception, as it should. But what types are contained
in []?
>
The answer is that sum() can't deal with that. You can't expect sum() to
read your mind, know that there should be 15 items instead of 3, and
raise an error. So why do you expect sum() to read your mind and
magically know that zero items is an error, especially when for many
applications it is NOT an error?
For the simple reason it doesn't have to read your mind,
a mechanism has already been built into the function: start
value. For those situations where an empty list is desired
to sum to 0, you could use sum(alist,0) and use sum(alist) for
those cases where summing an empty list is meaningless.
Shouldn't you have to explicitly tell sum() how deal with
situations like empty lists rather than have it implicitly
assume a starting value of 0 when you didn't ask for it?
>
The behaviour you want for this specific application is unwanted,
unnecessary and even undesirable for many other applications. The
solution is for *you* to write application-specific code to do what your
application needs, instead of relying on a general purpose function
magically knowing what you want.
Does division magically know what you want? No, it raises an
exception when you do something like divide by 0. Isn't it
Pythonic to not write a litany of tests to cover every
possible case, but instead use try:except?

But try:except only works if the errors are recognized.
And sum() says that summing an empty list is NEVER an error
under ANY circumstance. That may be true in MOST cases, but
it certainly isn't true in ALL cases.
>
--
Steven
Sep 7 '08 #41
On Sep 7, 12:30*pm, Mensanator <mensana...@aol.comwrote:
On Sep 6, 11:05 pm, Steven D'Aprano <st...@REMOVE-THIS-
Sheesh. That's not a problem, because Python is not trying to be a
dialect of SQL.

And yet, they added a Sqlite3 module.
Does that mean that, because there is an 'os' module, Python is trying
to compete with Linux and Windows?

This is starting to feel like a troll, but JUST IN CASE you are really
serious about wanting to get work done with Python, rather than
complaining about how it is not perfect, I offer the following snippet
which will show you how you can test the results of a sum() to see if
there were any items in the list:
>>class MyZero(int):
.... pass
....
>>zero = MyZero()
x=sum([], zero)
isinstance(x,MyZero)
True
>>x = sum([1,2,3], zero)
isinstance(x,MyZero)
False
>>>
Sep 7 '08 #42
En Sun, 07 Sep 2008 14:30:09 -0300, Mensanator <me********@aol.comescribi:
Actualy, I already get the behaviour I want. sum([1,None])
throws an exception. I don't see why sum([]) doesn't throw
an exception also (I understand that behaviour is by design,
I'm merely pointing out that the design doesn't cover every
situation).
[...]
Exactly. That's why I would prefer sum([]) to raise an
exception instead of giving a false positive.
The built in behavior can't be good for every usage. Nobody prevents you from defining yoru own function tailored to your own specs, like this:

def strict_sum(items):
items = iter(items)
try:
first = items.next()
except StopIteration:
raise ValueError, "strict_sum with empty argument"
return sum(items, first)

Tweak as needed. Based on other posts I believe your Python skills are enough to write it on your own, so I don't see why you're complaining so hard about the current behavior.

--
Gabriel Genellina

Sep 7 '08 #43
On Sep 7, 3:13�pm, "Gabriel Genellina" <gagsl-...@yahoo.com.arwrote:
En Sun, 07 Sep 2008 14:30:09 -0300, Mensanator <mensana...@aol.comescribi�:
Actualy, I already get the behaviour I want. sum([1,None])
throws an exception. I don't see why sum([]) doesn't throw
an exception also (I understand that behaviour is by design,
I'm merely pointing out that the design doesn't cover every
situation).
[...]
Exactly. That's why I would prefer sum([]) to raise an
exception instead of giving a false positive.

The built in behavior can't be good for every usage. Nobody prevents you from defining yoru own function tailored to your own specs, like this:

def strict_sum(items):
� � items = iter(items)
� � try:
� � � � first = items.next()
� � except StopIteration:
� � � � raise ValueError, "strict_sum with empty argument"
� � return sum(items, first)

Tweak as needed. Based on other posts I believe your Python skills are enough to write it on your own, so I don't see why you're complaining so hardabout the current behavior.
I'm not complaining about the behaviour anymore, I just don't like
being told I'm wrong when I'm not.

But I think I've made my point, so there's no point in harping on
this anymore.
>
--
Gabriel Genellina
Sep 8 '08 #44
On Sep 7, 1:17�pm, Patrick Maupin <pmau...@gmail.comwrote:
On Sep 7, 12:30�pm, Mensanator <mensana...@aol.comwrote:
On Sep 6, 11:05 pm, Steven D'Aprano <st...@REMOVE-THIS-
Sheesh. That's not a problem, because Python is not trying to be a
dialect of SQL.
And yet, they added a Sqlite3 module.

Does that mean that, because there is an 'os' module, Python is trying
to compete with Linux and Windows?
I wasn't thinking "compete", rather "complement". Python obviously
wants to be a player in the SQL market, so you would think it
would be in Python's interest to know how SQL behaves, just as it's in
Python's interest for the os module to know how BOTH Linnux and
Windows work.
>
This is starting to feel like a troll,
It wasn't intended to be.
but JUST IN CASE you are really
serious about wanting to get work done with Python, rather than
complaining about how it is not perfect,
Things never change if no one ever speaks up.
I offer the following snippet
which will show you how you can test the results of a sum() to see if
there were any items in the list:
Thanks. I'll drop this from this point on.
>
>class MyZero(int):

... � � pass
...
>zero = MyZero()
x=sum([], zero)
isinstance(x,MyZero)
True
>x = sum([1,2,3], zero)
isinstance(x,MyZero)
False- Hide quoted text -

- Show quoted text -
Sep 8 '08 #45
On Sep 7, 2:17�pm, Dennis Lee Bieber <wlfr...@ix.netcom.comwrote:
On Sun, 7 Sep 2008 10:30:09 -0700 (PDT), Mensanator <mensana...@aol.com>
declaimed the following in comp.lang.python:
On Sep 6, 11:05?pm, Steven D'Aprano <st...@REMOVE-THIS-
cybersource.com.auwrote:
Sheesh. That's not a problem, because Python is not trying to be a
dialect of SQL.
And yet, they added a Sqlite3 module.

� � � � Which is an interface TO an embedded/stand-alone SQL-based RDBM
engine; it does not turn Python into a dialect of SQL -- Python does not
process the SQL, it gets passed to the engine for SQL data processing.
But that's only half the story. The other half is data returned
as a result of SQL queries. And that's something Python DOES process.
And sometimes that processed data has to be inserted back into the
database. We certainly don't want Python to process the data in a way
that the database doesn't expect.

When I see a potential flaw (such as summing an empty list to 0),
should I just keep quiet about it, or let everyone know?

Well, now they know, so I'll shut up about this from now on, ok?
--
� � � � Wulfraed � � � �Dennis Lee Bieber � � � � � � � KD6MOG
� � � � wlfr...@ix.netcom.com � � � � � � wulfr...@bestiaria.com
� � � � � � � � HTTP://wlfraed.home.netcom.com/
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Sep 8 '08 #46
David C. Ullrich wrote:
>
(ii) If A is a subset of B then we should have
max(A) <= max(B). This requires that max(empty set)
be something that's smaller than everything else.
So we give up on that.
Er, what about instances of variations/elaborations on

class Smaller(object) : __cmp__ = lambda *_ : -1

?

Cheers, BB

Sep 8 '08 #47
On Sep 8, 8:54*am, Boris Borcic <bbor...@gmail.comwrote:
David C. Ullrich wrote:
(ii) If A is a subset of B then we should have
max(A) <= max(B). This requires that max(empty set)
be something that's smaller than everything else.
So we give up on that.

Er, what about instances of variations/elaborations on

class Smaller(object) : __cmp__ = lambda *_ : -1

?

Cheers, BB
You still don't have the property max(X) is in X.

And it's the equivalent of a special builtin constant for max on the
empty set.
Sep 8 '08 #48
In article
<b4**********************************@m73g2000hsh. googlegroups.com>,
be************@lycos.com wrote:
David C. Ullrich:
I didn't mention what's below because it doesn't seem
likely that saying max([]) = -infinity and
min([]) = +infinity is going to make the OP happy...

Well, it sounds cute having Neginfinite and Infinite as built-int
objects that can be compared to any other type and are < of or of
everything else but themselves.
Like I said, I'm not going to say anything about how Python
should be. If I were going to comment on that I'd say it would
be cute but possibly silly to actually add to the core.

But in the math library I made some time ago there was an
AbsoluteZero with the property that when you added it to
x you got x for any x whatever (got used as the default
additive identity for classes that didn't have an
add_id defined...)
Probably they can be useful as
sentinels, but in Python I nearly never use sentinels anymore, and
they can probably give some other problems...

Bye,
bearophile
--
David C. Ullrich
Sep 9 '08 #49
In article <00**********************@news.astraweb.com>,
Steven D'Aprano <st***@REMOVE-THIS-cybersource.com.auwrote:
On Fri, 05 Sep 2008 10:22:22 -0500, David C. Ullrich wrote about why max
and min shouldn't accept a default argument:
Think about all the previously elected female or black presidents of the
US. Which one was the tallest?

I know the answer to that one:

All of them!
Heh. Mysteries of the empty set.

--
David C. Ullrich
Sep 9 '08 #50
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