New submission from Kirk Hansen:
I tried subclassing dict, and overriding its __setitem__ and __getitem__ and
got some interesting results. See
http://stackoverflow.com/questions/38362420/subclassing-dict-dict-update-returns-incorrrect-value-python-bug?noredirect=1#comment64142710_38362420
for an example.
I tried sub-classing UserDict.UserDict and experienced some of the same
problems. Eventually, I discovered that subclassing MutableMapping was my best
bet.
After an hour or two of searching and reading I discovered that CPython will
not call overridden built-in methods of the same object
(https://pypy.readthedocs.io/en/latest/cpython_differences.html#subclasses-of-built-in-types).
This behaviour could (and did) cause some hard to track down bugs in code.
I briefly looked at the later versions of python documentation and didn't see a
mention of this (sorry if it's there), but python2.7 definitely does not
mention this. In fact, python2.7 seems to __encourage__ users to subclass the
built-ins (see the note
https://docs.python.org/2.7/library/stdtypes.html?highlight=dict#built-in-types).
Subclassing dict to __extend__ functionality is great, but there should be a
big bad warning when trying to __override__ built-ins like __setitem__ and
__getitem__.
--
assignee: docs@python
components: Documentation
messages: 270754
nosy: Kirk Hansen, docs@python
priority: normal
severity: normal
status: open
title: Warn against subclassing builtins, and overriding their methods
type: enhancement
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6
___
Python tracker
<http://bugs.python.org/issue27561>
___
___
Python-bugs-list mailing list
Unsubscribe:
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
