Dennis Sweeney <sweeney.dennis...@gmail.com> added the comment:
> `Mapping.__reversed__` exists
While ``'__reversed__' in dir(Mapping)`` is true, that unfortunately does not
mean that it is a real callable method:
from collections.abc import Mapping
class Map(Mapping):
def __getitem__(self, x):
if x == 42:
return 17
raise KeyError
def __len__(self, x):
return 1
def __iter__(self):
yield 42
>>> m = Map()
>>> reversed(m)
Traceback (most recent call last):
...
TypeError: 'Map' object is not reversible
In the code for Mapping, ``__reversed__`` is explicitly defined as None [1] so
that calling ``reversed(custom_mapping)`` doesn't accidentally fall back on the
sequence protocol, which would mean starting at len(custom_mapping)-1 and
calling __getitem__ on each index [2], which is certainly not desirable for
arbitrary mappings.
I don't think there is a reasonable way, given arbitrary implementations of
__len__, __iter__, and __getitem__, to have a mixin reversed iterator. If
someone wants their mapping to have a __reversed__ method, they should define
it themselves.
[1] https://github.com/python/cpython/blob/master/Lib/_collections_abc.py#L707
[2]
https://docs.python.org/3/reference/datamodel.html?highlight=__reversed__#object.__reversed__
----------
nosy: +Dennis Sweeney
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue40374>
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