[FIXED] Python 2 vs Python 3 - Difference in map behavior with three arguments?

Issue

The following code behaves differently in Python 2 vs Python 3:

all(map(lambda x,y: x, [1, 2], [1, 2, 3]))

Python 2 gives False whereas Python 3 gives True. The documentation for Python 2 says that it will supply None if the shorter list is exhausted but Python 3 doesn't do that.

I am working on a code that really needs the length to be maintained for some reason. What is the cleanest way to get the old behavior? I know I can use from past.builtin import map as old_map, but is there a more elegant solution that would work in both versions?

Solution

Essentially, map with multiple iterables for the arguments will zip the iterables, and then call the function with the tuples from the zip as var-args. So, you can get the same behaviour using itertools.starmap and zip:

>>> a = [10, 20]
>>> b = [1, 2, 3]
>>> f = lambda x, y: x
>>> list(map(f, a, b))
[10, 20]
>>> from itertools import starmap
>>> list(starmap(f, zip(a, b)))
[10, 20]

Then the behaviour you want can be achieved by replacing zip with itertools.zip_longest:

>>> from itertools import starmap, zip_longest
>>> list(starmap(f, zip_longest(a, b)))
[10, 20, None]

Both functions from itertools also exist in Python 2, except the second one is named izip_longest instead. You can just import ... as ... to get around that.

Answered By - kaya3