UserDict __getitem__ behavior change between 3.11.1 and 3.12.0b2

[stefanv]

I define the following UserDict, that implements nested lookups (d['key.subkey']):

import collections

class DotDict(collections.UserDict):
    def __getitem__(self, key):
        subitem = self.data
        for subkey in key.split("."):
            try:
                subitem = subitem[subkey]
            except KeyError:
                raise KeyError(f"`{key}` not found in configuration") from None
        return subitem

I expect the following to work as it did in Python 3.11, but it does not:

d = DotDict({'one': {'two': 'buckle my shoe'}})
d['one.two']   # works
d.get('one.two')  # does not work

I do not see any release notes indicating that __getitem__ is no longer the correct method to override.

[Eclips4]

This also reproduced on the current main.
Seems that in 3.12 and newer versions __getitem__ isn't called in .get

[Eclips4]

Also, if you define a __contains__ with the same behavior, it will work on 3.12.

Introduced in #17910
After some research of this issue, I came to the conclusion that the best solution here would be to add documentation of that.

[stefanv]

IUUC, the proposed fix is to change the implementation to the following:

import collections

class DotDict(collections.UserDict):
    def __getitem__(self, key):
        subitem = self.data
        for subkey in key.split("."):
            try:
                subitem = subitem[subkey]
            except KeyError:
                raise KeyError(f"`{key}` not found in configuration") from None
        return subitem

    def __contains__(self, key):
        subitem = self.data
        for subkey in key.split("."):
            try:
                subitem = subitem[subkey]
            except KeyError:
                return False
        return True

Not ideal, but it works.

[dholth]

This also affects me.