Infer attributes from __new__

[JukkaL]

In this example (from #982) we define an attribute via assignment in __new__:

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

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

Currently mypy doesn't recognize attribute foo and complains about x.foo. To implement this, we could do these things:

• Infer the fact that obj is something similar to self because of the way is created via object.__new__. This should happen during semantic analysis so that this will work in unannotated method as well.
• Infer attributes from assignments via obj because it is classified as similar to self.
• Also recognize object.__new__ calls via super(...).

[JukkaL]

This is follow-up to #982.

[JukkaL]

Decreased priority since this doesn't seem like users often struggle with this -- it's easy enough to work around by inserting type annotations.

[toolforger]

One use case is NamedTuple. It's turning mypy into a noise generator for me.
Not sure how this data point affects priorization.

[JelleZijlstra]

@toolforger how are you using NamedTuple? I don't think the example from the original post in this issue is applicable to NamedTuple.

[toolforger]

Using a NamedTuple subclass, subclass it with a __new__ override to provide standard parameters.
Details in #5194, which is a duplicate of #1279.

[ilevkivskyi]

Using a NamedTuple subclass, subclass it with a __new__ override to provide standard parameters.

Use instead:

class Foo(NamedTuple):
    x: int
    y: int = 0

Foo(1)  # OK, same as Foo(1, 0)
Foo(1, 2)  # OK

works on Python 3.6+. If you are on Python 2, then wait for this issue to be solved.

[toolforger]

@ilevkivskyi your advice works for direct subclasses. My use case is for an indirect subclass. (I already had to move all fields up into the root class because NamedTuples don't properly support subclassing, but I cannot use frozen dataclasses until 3.7 comes out for my distro. I'm on the verge of ripping out mypy again, or switching from Python to Kotlin.)