Sphinx throws errors when using function in __getattr__ override

[maehster]

Describe the bug
When using anonymous functions in getattr sphinx will raise an error:

Handler <function record_typehints at 0x7f3626d18790> for event 'autodoc-process-signature' threw an exception (exception: 'function' object has no attribute 'get')

To Reproduce

def fun():
    pass

class Tmp:

    def __call__(self, name, content):
        print(name, content)

    def __getattr__(self, item):
        return fun

a = Tmp()
"""asdfasdf"""

cd docs && \
make clean && \
sphinx-apidoc --force -e -o ./source ../<package> && \
make html

Expected behavior
No error.

Environment info

• Python version: [3.8.5]

Sphinx==3.4.3
sphinx-rtd-theme==0.5.1
sphinxcontrib-applehelp==1.0.2
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==1.0.3
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.4

extensions = ['sphinx.ext.todo', 'sphinx.ext.viewcode', 'sphinx.ext.autodoc', 'sphinx.ext.napoleon']

[tk0miya]

Thank you for reporting. I posted #8923 to fix the warning. But it could not build the document for the function because inspect.signature() raises an error for the function-like object:

$  python
Python 3.9.1 (default, Dec 12 2020, 22:52:49)
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import example
>>> dir(example)
['Tmp', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'a', 'fun']
>>> example.a
<example.Tmp object at 0x10d3e3ee0>
>>> import inspect
>>> inspect.signature(example.a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 3130, in signature
    return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
  File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2879, in from_callable
    return _signature_from_callable(obj, sigcls=cls,
  File "/Users/tkomiya/.pyenv/versions/3.9.1/lib/python3.9/inspect.py", line 2290, in _signature_from_callable
    raise TypeError(
TypeError: unexpected object <function fun at 0x10d3f71f0> in __signature__ attribute

To resolve the error, please set None to a.__signature__.

[tk0miya]

Note: Internally, the warning is emitted because the given function returns an invalid value from its __globals__ attribute. So you can avoid the error by setting an empty dictionary to the function-like object additionally.

[maehster]

Thank you very much for the fast response and support :) I will try this and let you know if I have further questions.