Nicholas Musolino <n.musol...@gmail.com> added the comment:
Before seeing this issue and its closed status, I created PR 14432, which adds
`__length_hint__()` to the iterator returned by builtin `map`.
This PR differs from the original 2017 PR by MSeifert in that the code can
distinguish between the cases where a length hint (or length) function is not
available, versus a case in which the `__length_hint__()` method of the
user-supplied iterable raises an exception.
That is, the new PR propagates exceptions (other than TypeError, per PEP 424)
raised in the `__length_hint__()` functions of the user-supplied iterators.
I've read the comments in this issue, and the notes in `test_iterlen.py`:
> [Other] iterators [...], such as enumerate and the other itertools,
> are not length transparent because they have no way to distinguish between
> iterables that report static length and iterators whose length changes with
> each call (i.e. the difference between enumerate('abc') and
> enumerate(iter('abc')).
Can anyone provide a concrete example that illustrates the inherent
difficulties of computing a length hint for `map`?
In ad-hoc testing, the current PR handles situations listed there, and I
haven't come across some fundamental show-stopper problem.
There are two limitations to the current PR:
1. The case where a single iterator is supplied as multiple arguments to
`map`, as pointed out by MSeifert:
it = iter([1,2,3,4])
map_it = map(f, it, it)
2. When a user-supplied `__length_hint__()` method returns a non-integer, it
results in a TypeError in an *inner* evaluation of `PyObject_LengthHint()` (per
PEP 424). When this exception reaches an *outer* evaluation of
`PyObject_LengthHint()`, it is cleared (per PEP 424), and leads to
operator.length_hint's default value being returned.
If we consider issue 2 to be incorrect, I think I could fix it by raising
RuntimeError from the original TypeError, or by somewhat invasive refactoring
of `PyObject_LengthHint()` (which I do not recommend).
----------
nosy: +nmusolino
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<https://bugs.python.org/issue26828>
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