This seems like it ought to work, according to the
description of reduce(), but it doesn't. Is this
a bug, or am I missing something?
Python 2.3.2 (#1, Oct 20 2003, 01:04:35)
[GCC 3.2.2 20030222 (Red Hat Linux 3.2.2-5)] on linux2
Type "help", "copyright" , "credits" or "license" for more information. d1 = {'a':1}
d2 = {'b':2}
d3 = {'c':3}
l = [d1, d2, d3]
d4 = reduce(lambda x, y: x.update(y), l)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 1, in <lambda>
AttributeError: 'NoneType' object has no attribute 'update' d4 = reduce(lambda x, y: x.update(y), l, {})
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 1, in <lambda>
AttributeError: 'NoneType' object has no attribute 'update'
- Steve.
Jul 18 '05
226  12729 
"Alex Martelli" <al***@aleax.it > wrote in message
news:p1******** ************@ne ws1.tin.it... Terry Reedy wrote:
Hmmm, nobody else seemed to have any problem understanding my quip,
Among those who dared respond ...
including the poster at which it was directed. Let me spell it out
for you:
Since, as I said, your target was obscure to me, thank you...
a few people are attacking 'sum' and defending 'reduce'
because the latter is "more general".
Oh.... so that was your target.
Since reduce with initializer is completely general, equivalent to an
initialized for loop (which is why I might instead call it 'induce' or
'repeat'), and hence as or more general than max, min, range,
enumerate, map, filter, list.count, and man other functions and
methods, and since none of these other 'less general than reduce'
functions have been similarly attacked that I have noticed, the
special-case, ad-hominen-like attacks on 'sum' had mostly passed
beneath my notice as devoid of memorable content.
A consistent preference for either or both of 'more general' and 'only
one way to do it' might lead us toward a very lean Python with exactly
one general repetition-with-variation function -- either 'for'
statements or something equivalent to reduce with required
initializer. But I have not noticed anyone proposing that.
So, no, I don't think my idea of "more general" is different from
yours: e.g., a function that, given a sequence, returns its length
AND the number of times 'None' occurs as an item, is more general
than one which just returns the length. That does not make it in
any intrinsic way "necessaril y preferable" -- and THAT is my point.
Agreed. Generic versus specific involves tradeoff and balance.
In my garage-workshop tool set, I have perhaps 10 different general
(adjustable) wrenches, pliers, and grips, 5 specialize (fixed-size)
box-end wenches, and perhaps 30 sockets (both metric and 'English').
I have used most of them except for the large sockets meant for auto
and truck work I don't do. So I am glad to have all of them.
Terry J. Reedy
"Alex Martelli" <al***@aleax.it > wrote in message
news:P7******** ************@ne ws2.tin.it... Terry Reedy wrote: Would the hypothetical
results = sequence.map(fu nc x: x+23)
be any better?
Just replacing the keyword 'lambda' with 'func'? If you were
designing a green-field language, and couldn't find any other way
to express callable literals -- so it only came down to a 2-way
choice between lambda and func as keywords with the same semantics,
I guess I would suggest func as the lesser evil.
If such a change were plausibly within the boundaries changes allowed
for the as-yet hypothetical 3.0 (or 3000), and there were not a more
radical change that I preferred, I might seriously propose this.
Upgrading would be simple than most other code-break changes. How about a very hypothetical (post ``==repr deprecation)
results = sequence..map(` x+23`)
How would this notation imply that x is an argument rather than,
say, a global?
The above was a minimal 'concept' proposal to test the aesthetics of
something structurally different from current 'lambda's. I think I
would make all identifiers params by default, since I believe this to
be more common, and 'tag' non-locals, perhaps with one of the
reserved, as yet unused symbols. Example: lambda x: x + y == `x + @y`
or `x+y@`. Since expressions cannot assign, no global declaration is
needed.
Terry
Quoth Ville Vainio <vi************ ********@spamtu t.fi>:
....
| I was merely arguing that 'reduce' is not more readable or intuitive
| than 'sum', which was the argument of OP.
My point is that readability and intuition depend on acquired
experience. Your non-programmer girlfriend lacks enough experience
to be able to understand all kinds of stuff in Python, so she's not
a useful gauge, because at that level you can't use Python anyway.
It isn't that simple, and I don't think there really is any good
evidence for what is readable. We could talk about why someone might
think reduce is about readability, but it can only be speculation.
| I wouldn't mind Python getting more influence from functional realm,
| as Python seems to me to be *the* hybrid language that can pull the FP
| thing while still remaining practical and intuitive (and delightfully
| non-academic).
Python may be a hybrid, but certainly none of its parents were FPLs.
Trying to make it one gives both Python and FP a bad name. If you
want a language that really supports both functional and procedural
styles, I think you need Lisp. Look at Dylan, I haven't tried it but
it may be quite a bit more comfortable for Python and C programmers.
Donn Cave, do**@drizzle.co m
Maybe, it's worth to have str(x,8) and str(x,16) instead of oct(x) and hex(x)
and make str() a true inverse function to int()?
G-:
--
a='a=;print a[:2]+chr(39)+a+chr( 39)+a[2:]';print a[:2]+chr(39)+a+chr( 39)+a[2:]
"Steve Lamb" <gr**@despair.d miyu.org> wrote in message news:sl******** *********@dmiyu .org...
| <..>
| Because as much as it helps when reading the code to only have to learn a
| minimal set of controls the same applies to writing code as well. When I
| approach a problem I don't have to agonize over "well, should I do a
| do...until(), a for(;;), a while(), or something else?" It breaks down to
| this.
I can't agree here. I NEVER wondered which of constructions should I
use in any particular case. I just solved the problem and wrote its solution
in that language and the language HELPED me to express myself clearly.
| Is it a sequence? For. Is it a condition to be met? While. There,
| done, move along.
Aha. Half of all the conditions in real Python programs are 1 or True. :-)
| --
| Steve C. Lamb | I'm your priest, I'm your shrink, I'm your
| PGP Key: 8B6E99C5 | main connection to the switchboard of souls.
| -------------------------------+---------------------------------------------
--
Georgy Pruss
E^mail: 'ZDAwMTEyMHQwMz MwQGhvdG1haWwuY 29t\n'.decode(' base64')
Paul Rubin: The change was that native lists will implement stable sort. My gripe
is that if native lists are required to sort stably and list-alikes
can sort unstably, then list.sort and listalike.sort act differently
in a way that can lead to subtle bugs.
... That's a statement only that list.sort shall be stable and
not that all .sort() methods must be stable.
It would be icky if some .sort() methods are required to be stable but
others are not.
I've been thinking about this. I'm of two minds about it, both
for and against.
The 'for' side, which dominates, says that the only extra work is
for those people who implement a version of Python. There will
only be few of these, and the work needed to implement at least
a rough cut at a stable sort is pretty minimal. Any user of a list-alike
sort who is concerned about its stability or wants the code to work
for pre-2.3 will use the DSU idiom. People who don't know about
differences in how sort algorithms handle ties will have their sorts
work "as expected".
The 'against' side says, as you do, that having list.sort required
to be stable now places an extra barrier to anyone else who
implements a list-alike sort, because that sort should now also
be stable.
What mollifies the latter view is that some user-defined sorts
won't have ties, so there will never be a problem. And some
user-defined sorts will want a stable sort -- anything which
sorts rows based on a column in a grid display must be stable.
So there are only going to be a relatively few cases where
this will be a problem.
Besides, what I really want is a set of algorithms implemented
independent of the container so that I can have a generic
'sort' function work on a user-defined container. At that
point it would be trivial for a user-defined list-alike to
implement its .sort() method as a call to the stable-sort
algorithm, which further reduces the number of times when
the expectation of a stable sort causes a problem.
Hmmm.. To do this nicely would seem to require a
'swap' method on lists as otherwise there's a lot of
movements when swapping two items in a list.
Note that the most obvious way to implement sort() in C is to call the
standard C library qsort function, which is unstable.
And again I note that I've seen commercial software which
made the expectation that C's qsort(3C) was ... well, not
stable, but it assumed it would give the same results on
different machines. That error was fixed by implementing
their own sort algorithm. (It wasn't caught for years because
there are very few ties in their data and even fewer people
moved data from one platform to another.)
My preference is for a stable sort. It works the right way
for most cases, and no sort algorithm is going to do the
right thing for all cases. (Hence the explosion of sort
variants you rightly pointed out.)
Andrew da***@dalkescie ntific.com
Terry Reedy: Perhaps my career in statistics and data reduction made reduce() more
immediately obvious to me than some other people.
I know when I first saw reduce, in Python BTW, I did not
intuit that meaning. I was in physics grad school and
with a bachelor's in math, with an emphasis in analysis, so
my math background was pretty strong. (Though nowadays
I look through my books and wonder that I actual knew
all that once upon a time... *sigh*)
Andrew da***@dalkescie ntific.com
"Donn Cave" <do**@drizzle.c om> writes: Python may be a hybrid, but certainly none of its parents were FPLs.
Trying to make it one gives both Python and FP a bad name. If you
I don't think there is much "trying to make Python a FPL" involved w/
making lots of tried-and-true FPL constructs available in Python. It's
just taking stuff that works and enabling us to use it w/o encumbering
the language (because it's in the modules). It's all good as long as
we don't go the way of doing everything w/ recutrsion.
want a language that really supports both functional and procedural
styles, I think you need Lisp. Look at Dylan, I haven't tried it but
it may be quite a bit more comfortable for Python and C programmers.
Lisp is too verbose for my tastes (I don't want to write 'let' ot
'setq'), doesn't have much in the way of libs and generally doesn't
feel as 'right' as Python (I do use Emacs Lisp occasionally,
though.. and will try out some CL one of these days). Dylan, OTOH,
doesn't seem to be all that active a project, at least the last time I
checked.
--
Ville Vainio http://www.students.tut.fi/~vainio24
Georgy Pruss: Maybe, it's worth to have str(x,8) and str(x,16) instead of oct(x) and
hex(x) and make str() a true inverse function to int()?
What then are the results of
str(30+44j, 16)
str(3.1415926, 8)
str([9, 8, 7], 8)
str("A", 16)
str({"A": 20}, 16)
?
Andrew da***@dalkescie ntific.com
"Terry Reedy" <tj*****@udel.e du> wrote in message news:sI******** ************@co mcast.com...
|
| "Alex Martelli" <al***@aleax.it > wrote in message
| news:P7******** ************@ne ws2.tin.it...
| <...>
|
| > > How about a very hypothetical (post ``==repr deprecation)
| > > results = sequence..map(` x+23`)
| >
| > How would this notation imply that x is an argument rather than,
| > say, a global?
|
| The above was a minimal 'concept' proposal to test the aesthetics of
| something structurally different from current 'lambda's. I think I
| would make all identifiers params by default, since I believe this to
| be more common, and 'tag' non-locals, perhaps with one of the
| reserved, as yet unused symbols. Example: lambda x: x + y == `x + @y`
| or `x+y@`. Since expressions cannot assign, no global declaration is
| needed.
|
| Terry
Or maybe
lambda x: x + y == `x: x + y`
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This must be because of implementation right? Shouldn't reduce be
faster since it iterates once over the list?
doesnt sum first construct the list then sum it?
-----------------------
reduce with named function: 37.9864357062
reduce with nested, named function: 39.4710288598
reduce with lambda: 39.2463927678
sum comprehension: 25.9530121845
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