New submission from Thibaut Horel <thibaut.ho...@gmail.com>: In cases where multiple iterables are passed to the built-in function map(), the documentation is ambiguous about the order in which they are consumed [1]. Although the order of evaluation of function arguments is documented to be left-to-right in general [2], this does not necessarily imply that the __next__() functions of the underlying iterators are called from left to right *before* passing the returned elements to the function being mapped. This is particularly relevant when the same iterator is passed multiple times, or when there are side effects to consuming the iterables.
I suggest adding the sentence “The iterables are consumed in left-to-right order at each iteration.”, similar to how it is done for the function zip() [3]. Furthermore, I think providing the following (roughly) equivalent implementation in pure Python might be illuminating: def map(function, *iterables): iterators = tuple(iter(it) for it in iterables) while True: try: args = [next(it) for it in iterators] except StopIteration: break yield func(*args) Finally, the following example could be added. “This makes it possible to apply a function to consecutive groups of elements from the same iterator by passing it multiple times to `map`: from itertools import count ctr = count() # map(func, ctr, ctr) -> func(0, 1), func(2, 3), ... ” I am happy to submit a pull request once we reach a consensus on the formulation. [1] https://docs.python.org/3/library/functions.html#map [2] https://docs.python.org/3/reference/expressions.html#evaluation-order [3] https://docs.python.org/3/library/functions.html#zip ---------- assignee: docs@python components: Documentation messages: 406693 nosy: docs@python, eric.araujo, ezio.melotti, mdk, thibaut.horel, willingc priority: normal severity: normal status: open title: [doc] map() documentation ambiguous about consumption order type: enhancement versions: Python 3.10, Python 3.11, Python 3.6, Python 3.7, Python 3.8, Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue45856> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com