Mapping None. Why?

[Paddy]

Iam wondering why the peculiar behavior of map when the function in
given as None:

Help on built-in function map in module __builtin__:

map(...)
map(function, sequence[, sequence, ...]) -list

Return a list of the results of applying the function to the items
of
the argument sequence(s). If more than one sequence is given, the
function is called with an argument list consisting of the
corresponding
item of each sequence, substituting None for missing values when
not all
sequences have the same length. If the function is None, return a
list of
the items of the sequence (or a list of tuples if more than one
sequence).
It seems as the action whith none is the same as using a function of
lambda *x: x
As in the following example:

>>l1 = 'asdf'
l2 = 'qwertyuip'
l3 = range(3)
l1,l2,l3

('asdf', 'qwertyuip', [0, 1, 2])

>>map(lambda *x: x, l1,l2,l3) == map(None, l1,l2,l3)

True

>>>

On looking up map on Wikipedia there is no mention of this special
behaviour,
So my question is why?

Thanks, Paddy.

[Paddy]

On Jun 12, 9:36 pm, "Terry Reedy" <tjre...@udel.e duwrote:

"Paddy" <paddy3...@goog lemail.comwrote in message

news:a8******** *************** ***********@27g 2000hsf.googleg roups.com...
|
| Iam wondering why the peculiar behavior of map when the function in
| given as None:

The 'peculiar behavior' is the same as zip (except for padding short
iterators versus truncating long iterators. Map was added years before
zip. After that, map(None,...) was kept for back compatibility.

In 3.0, the doc for map is
"Return an iterator that applies function to every item of iterable,
yielding the results. If additional iterable arguments are passed, function
must take that many arguments and is applied to the items from all
iterables in parallel. With multiple iterables, the iterator stops when the
shortest iterable is exhausted."

Using a map defined with None raises
TypeError: 'NoneType' object is not callable

tjr

I really should get into the habit of reading the 3.0 docs before
asking questions :-)

My original question came about after answering this query:
http://gmcnaughton.livejournal.com/2...d=70451#t70451
- Paddy.

[David C. Ullrich]

On Thu, 12 Jun 2008 12:05:02 -0700 (PDT), Paddy
<pa*******@goog lemail.comwrote :

>
Iam wondering why the peculiar behavior of map when the function in
given as None:

If you start with a value x and then apply no function
at all to it, what results is x.

>Help on built-in function map in module __builtin__:

map(...)
map(function, sequence[, sequence, ...]) -list

Return a list of the results of applying the function to the items
of
the argument sequence(s). If more than one sequence is given, the
function is called with an argument list consisting of the
correspondin g
item of each sequence, substituting None for missing values when
not all
sequences have the same length. If the function is None, return a
list of
the items of the sequence (or a list of tuples if more than one
sequence).
It seems as the action whith none is the same as using a function of
lambda *x: x
As in the following example:

>>>l1 = 'asdf'
l2 = 'qwertyuip'
l3 = range(3)
l1,l2,l3

('asdf', 'qwertyuip', [0, 1, 2])

>>>map(lambda *x: x, l1,l2,l3) == map(None, l1,l2,l3)

True

>>>>

On looking up map on Wikipedia there is no mention of this special
behaviour,
So my question is why?

Thanks, Paddy.

David C. Ullrich

[Paddy]

On Jun 13, 12:49 pm, David C. Ullrich <dullr...@spryn et.comwrote:

On Thu, 12 Jun 2008 12:05:02 -0700 (PDT), Paddy

<paddy3...@goog lemail.comwrote :

Iam wondering why the peculiar behavior of map when the function in
given as None:

If you start with a value x and then apply no function
at all to it, what results is x.

David C. Ullrich

True, but None is not a function. It's a sentinel value to turn on the
functionality.

- Paddy.

[David C. Ullrich]

In article
<d5************ *************** *******@j22g200 0hsf.googlegrou ps.com>,
Paddy <pa*******@goog lemail.comwrote :

On Jun 13, 12:49 pm, David C. Ullrich <dullr...@spryn et.comwrote:

On Thu, 12 Jun 2008 12:05:02 -0700 (PDT), Paddy

<paddy3...@goog lemail.comwrote :

>Iam wondering why the peculiar behavior of map when the function in
>given as None:

If you start with a value x and then apply no function
at all to it, what results is x.

David C. Ullrich

True, but None is not a function. It's a sentinel value to turn on the
functionality.

Uh, thanks. I think I knew that - I was just suggesting why
the way map works makes sense.

- Paddy.

--
David C. Ullrich

[Terry Reedy]

"David C. Ullrich" <du******@spryn et.comwrote in message
news:du******** *************** *****@text.giga news.com...
| In article
| <d5************ *************** *******@j22g200 0hsf.googlegrou ps.com>,
| Paddy <pa*******@goog lemail.comwrote :
|
| True, but None is not a function. It's a sentinel value to turn on the
| functionality.
|
| Uh, thanks. I think I knew that - I was just suggesting why
| the way map works makes sense.

filter(None, iterable) works the same way: None-identity function,
The immediate reason is the Python has no builtin id().
But apparently there is also historical precedent in the functional
community for this convention.

[Ian Kelly]

On Fri, Jun 13, 2008 at 2:00 PM, Terry Reedy <tj*****@udel.e duwrote:

filter(None, iterable) works the same way: None-identity function,
The immediate reason is the Python has no builtin id().
But apparently there is also historical precedent in the functional
community for this convention.

Another way of viewing it is that filter(None, iterable) applies no
function at all before testing the truth values, which does make some
sense. With map, however, this is not strictly true.