[New-bugs-announce] [issue44643] importing does not dispatch to __builtins__.__getitem__ to lookup __import__

[Patrick Reader]

New submission from Patrick Reader <pxeger at protonmail.com>:

When a frame's __builtins__ is a subclass of dict with an overridden __getitem__ method, this overriden method is not used by the IMPORT_NAME instruction to lookup __import__ in the dictionary; it uses the lookup function of normal dictionaries (via _PyDict_GetItemIdWithError). This is contrary to the behaviour of the similar LOAD_BUILD_CLASS, as well as the typical name lookup semantics of LOAD_GLOBAL/LOAD_NAME, which all use PyDict_CheckExact for a "fast path" before defaulting to PyObject_GetItem, which is not the behaviour I expected.

Perhaps more seriously, if __builtins__ is not a dict at all, then it gets erroneously passed to some internal dict functions resulting in a mysterious SystemError ("Objects/dictobject.c:1440: bad argument to internal function") which, to me, indicates fragile behaviour that isn't supposed to happen.

Could this be changed, so that __builtins__ is used dynamically? I understand this is a highly specific use case and changing it would probably cause a bit of a slow-down in module importing so is perhaps not worth doing, but I've posted this issue to track it anyway.

I cannot find evidence that this behaviour has changed at all in recent history and it seems to be the same on the main branch as in 3.9.6.

Per a brief discussion with Brett Cannon on python-dev (https://mail.python.org/archives/list/python-dev@python.org/thread/ZQMF6XC76J4APJPB3X6PGATG6CV5NN44/), I have labelled this a feature request because it has never really been expected behaviour.

A short demo of these things is attached.

Links to relevant CPython code in v3.9.6:

IMPORT_NAME: https://github.com/python/cpython/blob/v3.9.6/Python/ceval.c#L5179

BUILD_CLASS: https://github.com/python/cpython/blob/v3.9.6/Python/ceval.c#L2316

LOAD_NAME: https://github.com/python/cpython/blob/v3.9.6/Python/ceval.c#L2488

LOAD_GLOBAL: https://github.com/python/cpython/blob/v3.9.6/Python/ceval.c#L2546

----------
components: Interpreter Core
files: dict_lookup_demo.py
messages: 397531
nosy: brett.cannon, pxeger
priority: normal
severity: normal
status: open
title: importing does not dispatch to __builtins__.__getitem__ to lookup __import__
type: enhancement
versions: Python 3.10, Python 3.11, Python 3.9
Added file: https://bugs.python.org/file50151/dict_lookup_demo.py

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44643>
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