bpo-31506: Improve the error message logic for object_new & object_init

[CuriousLearner]

With this patch applied, the classes without any method overrides behaves like this:

>>> class C:
...     pass
...
>>> C(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: C() takes no arguments
>>> C.__new__(C, 42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: C() takes no arguments
>>> C().__init__(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: C().__init__() takes no arguments
>>> object.__new__(C, 42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: C() takes no arguments
>>> object.__init__(C(), 42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: C().__init__() takes no arguments

And for classes having method overrides, it behaves like this:

>>> class D:
...     def __new__(cls, *args, **kwds):
...         super().__new__(cls, *args, **kwds)
...     def __init__(self, *args, **kwds):
...         super().__init__(*args, **kwds)
...
>>> D(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __new__
TypeError: object().__new__() takes no arguments
>>> D.__new__(D, 42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __new__
TypeError: object().__new__() takes no arguments
>>> object.__new__(D, 42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object().__new__() takes no arguments

cc @ncoghlan . I'm still trying to figure out the changes to be done for cases of object.__init__(D(), 42) and D().__init__(42) because they don't report error right now.

https://bugs.python.org/issue31506

[ncoghlan]

@CuriousLearner If I recall correctly, object.__init__() has a special case to make it permissive whenever __new__ is overridden. Rather than fiddling with that logic in this patch (since doing so would risk breaking backwards compatibility), I'd suggest defining a 3rd test class that only overrides __init__:

class E:
    def __init__(self, *args, **kwds):
        super().__init__(*args, **kwds)

Then E().__init__(42) and object.__init__(E(), 42) should both fail.

[CuriousLearner]

@ncoghlan Seems like you're right. That was a special case. I wanted to know what would be the best file to add these test cases in?

[ncoghlan]

@CuriousLearner I'd suggest adding a new def testConstructorErrorMessages(self): test case to https://github.com/python/cpython/blob/master/Lib/test/test_class.py, and then use self.assertRaisesRegex() to check for the desired error messages.

[CuriousLearner]

@ncoghlan I've updated the patch and added test cases. Can you please have a look?

[ncoghlan]

LGTM!