在python中反转dict并访问其值的正确方法是什么？

Question

While the following code perfectly prints out c , b , a mypy complains about it.

main.py:10: error: No overload variant of "reversed" matches argument types [typing.ValuesView[builtins.str*]]

What is the correct way to traverse x in reverse order and get the values?

from collections import OrderedDict

def main() -> None:
    x = OrderedDict([
        (1, 'a'),
        (2, 'b'),
        (3, 'c'),
    ])

    for y in reversed(x.values()):
        print(y)


if __name__ == '__main__':
    main()

Answer 1

This is because the OrderedDict dictionary views do implement __reversed__ but do not inherit from collections.abc.Reversible (the values view does inherit from collections.abc.ValuesView ).

This could be fixed both by adding that base class in the Python standard library, and by updating the Python Typeshed definition . I've filed an issue with the latter , as that's the faster method to get you a fix.

You can clone my pull-request branch to get the new definitions locally, then use the --custom-typeshed-dir switch to mypy to use it over the one compiled into mypy itself:

git clone https://github.com/mjpieters/typeshed.git \
    --branch ordereddict_views_reversible \
    ~/typeshed_ordereddict_views_reversible   # or a different location
mypy --custom-typeshed-dir ~/typeshed_ordereddict_views_reversible <yourproject>

Answer 2

While it works on Python 3.6, on previous versions of Python 3, your code doesn't work. on python 3.4:

TypeError: argument to reversed() must be a sequence

(has to do with the type of dict.values() )

One workaround is to convert to list first but that's wasteful.

for y in reversed(list(x.values())):
    print(y)

mypy isn't aware of that novelty (yet) and issues an error.

You should ignore that error. I didn't try it but maybe commenting with # type: ignore works (discussed in https://github.com/python/mypy/issues/500 ):

for y in reversed(list(x.values())):  # type: ignore