On Wed, Jul 14, 2021 at 10:17 AM Patrick Reader <python-...@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
>
> Thanks,
>
> Patrick Reader
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