[New-bugs-announce] [issue39960] Using typename.__setattr__ in extension type with Py_TPFLAGS_HEAPTYPE is broken (hackcheck too eager?)

[Matthias Braun]

New submission from Matthias Braun <matthiasb at fb.com>:

This is about an extension type created via `PyType_FromSpec` that overrides `tp_setattro` (minimal example attached). In this case cpython does not let me grab and use the `__setattr__` function "manually". Example:

```
>>> import demo
>>> mytype_setattr = demo.MyType.__setattr__
>>> i = demo.MyType()
>>> mytype_setattr(i, "foo", "bar")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't apply this __setattr__ to object object
```

I suspect this is related to the loop at the beginning of typeobject.c / hackcheck() that skips over types with Py_TPFLAGS_HEAPOBJECT. (Though removing the loop breaks things like the enum module).

----------
components: C API
files: demomodule.zip
messages: 364123
nosy: Matthias Braun
priority: normal
severity: normal
status: open
title: Using typename.__setattr__ in extension type with Py_TPFLAGS_HEAPTYPE is broken (hackcheck too eager?)
versions: Python 3.7
Added file: https://bugs.python.org/file48974/demomodule.zip

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39960>
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