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 <rep...@bugs.python.org> <http://bugs.python.org/issue27561> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
