On Wed, Jul 14, 2021 at 10:17 AM Patrick Reader <python-dev@pxeger.com> wrote:

Hi there,

I've noticed that, 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 unexpected.

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.

I'm not sure if this intended, so I didn't want to open an issue yet. It also seems 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 fixing. I just wanted to ask here in case this issue had been documented anywhere before, and to check if it might actually be supposed to happen before opening a bug report.

At this point it's basically a feature request since it's been like that for so long.

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.

I believe it's been like that for as long as I have worked on importlib which is over a decade. The lookup also skips over any global definition of __import__ and goes straight to __builtins__.

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


Patrick Reader

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