mypy --py2 does not respect division future statement

[li-dan]

When I use from __future__ import division, mypy looks for __div__ on objects instead of __truediv__. Example code:

from __future__ import division


class ClassWithDiv(object):
    def __div__(self, other):
        # type: (ClassWithDiv) -> None
        return None


class ClassWithTrueDiv(object):
    def __truediv__(self, other):
        # type: (ClassWithTrueDiv) -> None
        return None


ClassWithDiv() / ClassWithDiv()
ClassWithTrueDiv() / ClassWithTrueDiv()

The actual output of running mypy --py2:

$ mypy --py2 test_div.py
test_div.py:17: error: "ClassWithTrueDiv" has no attribute "__div__"

The expected output should be something like this:

test_div.py:16: error: "ClassWithDiv" has no attribute "__truediv__"

It would also be acceptable to give the same output as in Python 3 mode, which is:

$ mypy test_div.py
test_div.py:16: error: Unsupported left operand type for / ("ClassWithDiv")

I am using mypy from Git master. Version info:

$ mypy --version
mypy 0.640+dev-2244721df395ea7ccb986286a12594e2d7b097e8
$ python --version
Python 3.6.5

I am not using any mypy flags besides --py2.

[li-dan]

A more obvious reproducer:

from __future__ import division

reveal_type(1 / 2)

The output of this is:

$ mypy --py2 test_div2.py
test_div2.py:3: error: Revealed type is 'builtins.int'

But it should be:

test_div2.py:3: error: Revealed type is 'builtins.float'

[gvanrossum]

That's a good catch. Do you feel up to attempting to submit a fix as a PR?

[ilevkivskyi]

This is actually a duplicate of #2048, so I am going to close this one, since the original has more discussion. Feel free to submit a PR.