[Python-ideas] why has itertools.imap other semantics as the bultin map?

[Stefan Behnel]

Raymond Hettinger, 28.04.2010 19:19:
> On Apr 28, 2010, at 8:59 AM, Mathias Panzenböck wrote:
>> See:
>>>>> map(None,['a','b'],[])
>> [('a', None), ('b', None)]
>>>>> list(imap(None,['a','b'],[]))
>> []
>>
>> Why is that?
>
> Excerpt from the source file at http://svn.python.org/view/python/trunk/Modules/itertoolsmodule.c?view=markup :
>
> /*	
> imap() is an iterator version of __builtins__.map() except that it does
> not have the None fill-in feature.  That was intentionally left out for
> the following reasons:
>
>    1) Itertools are designed to be easily combined and chained together.
>       Having all tools stop with the shortest input is a unifying principle
>       that makes it easier to combine finite iterators (supplying data) with
>       infinite iterators like count() and repeat() (for supplying sequential
>       or constant arguments to a function).
>
>    2) In typical use cases for combining itertools, having one finite data
>       supplier run out before another is likely to be an error condition which
>       should not pass silently by automatically supplying None.
>
>    3) The use cases for automatic None fill-in are rare -- not many functions
>       do something useful when a parameter suddenly switches type and becomes
>       None.
>
>    4) If a need does arise, it can be met by __builtins__.map() or by
>       writing:  chain(iterable, repeat(None)).
>
>    5) Similar toolsets in Haskell and SML do not have automatic None fill-in.
> */

It's worth adding that itertools.imap() became map() in Python 3.

Stefan