Message329475
The collections.abc — Abstract Base Classes for Containers documentation says:
> This module provides abstract base classes that can be used to test whether a class provides a particular interface; for example, whether it is hashable or whether it is a mapping.
https://docs.python.org/3/library/collections.abc.html
However this is not true for Sequence.
When I implement a class that provides a particular interface (defined in the Collections Abstract Base Classes table in that very page), I cannot check whether it implements a Sequence.
See an example:
from collections import abc
class Box:
def __init__(self, wrapped):
self._w = wrapped
def __len__(self):
return len(self._w)
def __iter__(self):
yield from self._w
def __getitem__(self, i):
return self._w[i]
def __reversed__(self):
yield from reversed(self._w)
def __contains__(self, i):
return i in self._w
def index(self, value, start=0, stop=None):
return self._w.index(value, start, stop)
def count(self, value):
return self._w.count(value)
b = Box([1, 2, 3])
for t in 'Sized', 'Iterable', 'Reversible', 'Container', 'Collection', 'Sequence':
print(f'{t}: {isinstance(b, getattr(abc, t))}')
My class is Reversible.
My class is a Collection (as it is a Sized Iterable Container).
It implements __getitem__, __len__, __contains__, __iter__, __reversed__, index, and count.
Yet my class instance is not an instance of Sequence.
I suppose this behavior might be intentional, as discussed in issue16728 - or it might as well not be.
The main concern was that dict also provides these methods, but is not considered a Sequence,
however dict does not provide index() or count().
Regardless whether this is right or wrong behavior, as documented this should be a Sequence.
See also https://stackoverflow.com/questions/34927949/issubclass-of-abstract-base-class-sequence
As I see it, either:
collections.abc.Sequence needs a __subclasshook__ so it can be used as the documentation implies.
Or:
the documentation should not say that "abstract base classes (from abc module) can be used to test whether a class provides a particular interface" if it doesn't generally apply
Or:
the Sequence documentation should say: "this particular abstract base class cannot be used to test whether a class provides a particular interface because reasons" (yet I don't really get those reasons) |
|
Date |
User |
Action |
Args |
2018-11-08 14:41:26 | hroncok | set | recipients:
+ hroncok, vstinner, docs@python |
2018-11-08 14:41:26 | hroncok | set | messageid: <1541688086.42.0.788709270274.issue35190@psf.upfronthosting.co.za> |
2018-11-08 14:41:26 | hroncok | link | issue35190 messages |
2018-11-08 14:41:26 | hroncok | create | |
|