[issue40897] Inheriting from class that defines __new__ causes inspect.signature to always return (*args, **kwargs) for constructor

[Jonathan Slenders]

Jonathan Slenders <jonat...@slenders.be> added the comment:

The following patch to inspect.py solves the issue that inspect.signature() 
returns the wrong signature on classes that inherit from Generic. Not 100% sure 
though if this implementation is the cleanest way possible. I've been looking 
into attaching a __wrapped__ to Generic as well, without success. I'm not very 
familiar with the inspect code.

To me, this fix is pretty important. ptpython, a Python REPL, has the ability 
to show the function signature of what the user is currently typing, and with 
codebases that have lots of generics, there's nothing really useful we can show.


 $ diff inspect.old.py  inspect.py  -p
*** inspect.old.py      2021-02-17 11:35:50.787234264 +0100
--- inspect.py  2021-02-17 11:35:10.131407202 +0100
*************** import sys
*** 44,49 ****
--- 44,50 ----
  import tokenize
  import token
  import types
+ import typing
  import warnings
  import functools
  import builtins
*************** def _signature_get_user_defined_method(c
*** 1715,1720 ****
--- 1716,1725 ----
      except AttributeError:
          return
      else:
+         if meth in (typing.Generic.__new__, typing.Protocol.__new__):
+             # Exclude methods from the typing module.
+             return
+
          if not isinstance(meth, _NonUserDefinedCallables):
              # Once '__signature__' will be added to 'C'-level
              # callables, this check won't be necessary


***

For those interested, the following monkey-patch has the same effect:

def monkey_patch_typing() -> None:
    import inspect, typing
    def _signature_get_user_defined_method(cls, method_name):
        try:
            meth = getattr(cls, method_name)
        except AttributeError:
            return
        else:
            if meth in (typing.Generic.__new__, typing.Protocol.__new__):
                # Exclude methods from the typing module.
                return

            if not isinstance(meth, inspect._NonUserDefinedCallables):
                # Once '__signature__' will be added to 'C'-level
                # callables, this check won't be necessary
                return meth

    inspect._signature_get_user_defined_method = 
_signature_get_user_defined_method

monkey_patch_typing()

----------
nosy: +jonathan.slenders

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<https://bugs.python.org/issue40897>
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