Attached patch adds assertions to PyObject_GetItem() to ensure that an exception is raised if the function failed. _Py_CheckFunctionResult() could be used to implement such check, but it might adds a little overhead, whereas I really don't think that such bug occurs in the wild. The assertion only helps when you develop a new custom type implementing the mapping API.

The patch also simplifies Python/ceval.c: it now considers that an exception is raised if PyObject_GetItem() returns NULL.

Finally, the patch also fixes a bug in the LOAD_NAME bytecode, when globals are not a dict but a custom type and the name doesn't exist in globals: clear the exception before calling PyObject_GetItem() on builtins.

Where is a patch?

Oops, sorry. It's now attached to the issue :-)

LGTM. The fix for LOAD_GLOBAL should be applied for all versions.

New changeset 1e87bcf20707 by Victor Stinner in branch '3.5':
Issue #25556: Fix LOAD_GLOBAL bytecode when globals type is not dict and the
https://hg.python.org/cpython/rev/1e87bcf20707

New changeset c1414f80ebc9 by Victor Stinner in branch 'default':
Issue #25556: Add assertions to PyObject_GetItem() to ensure that an exception
https://hg.python.org/cpython/rev/c1414f80ebc9

> LGTM. The fix for LOAD_GLOBAL should be applied for all versions.

Thanks for the review. I applied the LOAD_GLOBAL fix to Python 3.5 and 3.6. Python 2.7 doesn't support globals not being dict.

What about 3.4?

> What about 3.4?

I consider it as almost dead (no more bugfixes), and this really is really a corner case. Except me, I don't think that anyone uses globals which are not dict :-D (It never worked...)

In issue25557 I mentioned that there is the same error for globals which are dict.

> In issue25557 I mentioned that there is the same error for globals which are dict.

This issue fixed a bug for LOAD_GLOBAL bytecode bytecode when globals are not dict. (Sorry, I wrote LOAD_NAME bytecode in my first message, it was a mistake.)

Please discuss the issue #25557 in the issue #issue25557. (I already replied there, I don't understand where you spotted a bug.)