Mapping None. Why?

Paddy paddy3118 at googlemail.com
Thu Jun 12 21:05:02 CEST 2008


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.




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