Args to __new__ not recognized

[gvanrossum]

I have this code:

class C(object):
    def __new__(cls, foo=None):
        obj = object.__new__(cls)
        obj.foo = foo
        return obj

x = C(foo=12)
print(x.foo)
y = C(12)

This elicits an error complaining about the foo keyword to C, another about the foo attribute, and then one about the positional argument:

mypy_new.py:7: error: Unexpected keyword argument "foo" for "C"
mypy_new.py:8: error: "C" has no attribute "foo"
mypy_new.py:9: error: Too many arguments for "C"

This may be related to #794 -- feel free to close as a dupe if fixing that would automatically fix this. (There seem to be two separate issues here -- it doesn't turn the __new__ signature into the class signature, and it doesn't believe attributes set by __new__ become attributes of the object.)

[JukkaL]

#794 is a third issue -- classes should be able to override __new__ with a different signature.

Attribute assignment can be worked around by defining attributes in the class body. Inferring attributes in __new__ is hard to do in general, but we can probably support at least the most common idioms.

[JukkaL]

Marking as priority as this is a fundamental language feature that mypy doesn't support yet.

[JukkaL]

#794 actually covers an additional issue -- using super() with __new__.

[JukkaL]

Inferring signature from __new__ now works in the most common cases. I'm going to create separate issues for remaining limitations:

• Inferring attributes from __new__
• Honoring return type of __new__

[JukkaL]

Closing this as the immediate issue is resolved, but see also the follow-up issues linked to above and #794 -- they are needed for more complete __new__ support.