Incorrect `__init__` signature for classes inheriting from `Generic`

[dhirschfeld]

Correct __init__ signature:

In [21]: class C:
    ...:     """A docstring."""
    ...:     def __init__(self, a: float, b:int, c:str):
    ...:         pass
    ...:         

In [22]: C?
Init signature: C(a: float, b: int, c: str)
Docstring:      A docstring.
Type:           type
Subclasses:     

Incorrect signature when inheriting from Generic:

In [23]: from typing import Generic, TypeVar

In [24]: T = TypeVar('T')

In [25]: class C(Generic[T]):
    ...:     """A docstring."""
    ...:     def __init__(self, a: float, b:int, c:str):
    ...:         pass
    ...:         

In [26]: C?
Init signature: C(*args, **kwds)
Docstring:      A docstring.
Type:           type
Subclasses:     

help does work correctly in this case:

In [27]: help(C)
Help on class C in module __main__:

class C(typing.Generic)
 |  C(*args, **kwds)
 |  
 |  A docstring.
 |  
 |  Method resolution order:
 |      C
 |      typing.Generic
 |      builtins.object
 |  
 |  Methods defined here:
 |  
 |  __init__(self, a: float, b: int, c: str)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  ----------------------------------------------------------------------
<SNIP>

[dhirschfeld]

This is important because being able to introspect the signature is a fundamental tool in understanding the working of a particular class.

[dhirschfeld]

inspect.signature returns the incorrect signature when applied to the class:

In [28]: from inspect import signature

In [29]: signature(C)
Out[29]: <Signature (*args, **kwds)>

In [30]: signature(C.__init__)
Out[30]: <Signature (self, a: float, b: int, c: str)>

It seems IPython first tries the class-level signature and only tries the __init__ signature if that fails:

ipython/IPython/core/oinspect.py

Lines 820 to 838 in 2b4bc75

	# get the init signature:
	try:
	init_def = self._getdef(obj, oname)
	except AttributeError:
	init_def = None
	# get the __init__ docstring
	try:
	obj_init = obj.__init__
	except AttributeError:
	init_ds = None
	else:
	if init_def is None:
	# Get signature from init if top-level sig failed.
	# Can happen for built-in types (list, etc.).
	try:
	init_def = self._getdef(obj_init, oname)
	except AttributeError:
	pass

If it's supposed to be reporting the __init__ signature shouldn't it just call signature(C.__init__) directly instead of signature(C)?

[dhirschfeld]

Workaround:

In [35]: C.__signature__ = signature(C.__init__)

In [36]: C?
Init signature: C(self, a: float, b: int, c: str)
Docstring:      A docstring.
Type:           type
Subclasses: