On Lack of Default __len__ in Python Abstract Base Classes #122410
Description
Thoughts about # NOTE [ Lack of Default __len__ in Python Abstract Base Classes ].
TL;DR:
- We don't have the
__len__("abstract") methods but seems like they could be (via aTypeErroror an abstract method implementations). - We don't have the
__len__docstrings but they could be (without the method itself). - We don't point in the documentation datasets have the size. (Or they don't?)
We have https://github.com/pytorch/pytorch/blob/v2.2.1/torch/utils/data/sampler.py#L70-L95 (introduced in #19228):
# NOTE [ Lack of Default `__len__` in Python Abstract Base Classes ]
#
# Many times we have an abstract class representing a collection/iterable of
# data, e.g., `torch.utils.data.Sampler`, with its subclasses optionally
# implementing a `__len__` method. In such cases, we must make sure to not
# provide a default implementation, because both straightforward default
# implementations have their issues:
## + `return NotImplemented`:
# Calling `len(subclass_instance)` raises:
# TypeError: 'NotImplementedType' object cannot be interpreted as an integerBut what should return len(subclass_instance) without an implemented length value?
No matter, on other hand, NotImplemented is for binary operators only.
Let's remove this part of the comment?
#
# + `raise NotImplementedError()`:
# This prevents triggering some fallback behavior. E.g., the built-in
# `list(X)` tries to call `len(X)` first, and executes a different code
# path if the method is not found or `NotImplemented` is returned, while
# raising a `NotImplementedError` will propagate and make the call fail
# where it could have used `__iter__` to complete the call.This comment says we should write list(iter(X)) instead of list(X). (See the list cconstructor documentation.)
#
# Thus, the only two sensible things to do are
#
# + **not** provide a default `__len__`.
## + raise a `TypeError` instead, which is what Python uses when users call
# a method that is not defined on an object.
# (@ssnl verifies that this works on at least Python 3.7.)It is a good idea, isn't it? This comment says it is the correct one.
More options:
This comment suggest to make an abstract method.
Are we able to add the documentation stub (without the method itself)?
Thanks!
cc @svekars @brycebortree @VitalyFedyunin @ejguan @dzhulgakov @ssnl