map, filter, lambda, list comprehensions (was RE: parameterundefined in procedure)

Tim Peters tim_one at
Sun Feb 27 22:08:17 CET 2000

[Greg Ewing's original list comprehension example, from '98]
> def zip(L1, L2):
>     return [(x, y) for x in L1, y in L2]

[Michal Wallace]
> is this the same as:
> [code that acts much like Python's current map(None, L1, L2), except
>  builds a list of two-lists instead of two-tuples]

Close.  It was long ago agreed that map's None-padding of "short" sequences
is at best a dubious idea, and to drop that for (the not yet implemented)
"parallel iteration" (whether in "for" or list comprehension context).  So
it would actually act like:

def zip(L1, L2):
    result = []
    i = 0
    while 1:
            next = L1[i], L2[i]
        except IndexError:
        i = i+1  # ignoring a subtlety here on OverflowError
    return result

Note that Python's iteration protocol terminates via exception, not via
reaching a precomputed length.  This is so you can iterator over, e.g.,
lines of a file, where you don't know len() in advance.

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