Regression since 0.620: Argument 1 to "__div__" of "int" has incompatible type "float"; expected "int"

[gvanrossum]

Consider:

def f(a):
    # type: (Union[int, float]) -> None
    a /= 1.1  # <-- Line 10

This looks fine to me, and gives no errors in 0.620 in either Python 2 or Python 3 mode. In the current master it gives this error in Python 2 mode:

_.py:10: error: Argument 1 to "__div__" of "int" has incompatible type "float"; expected "int"
_.py:10: note: __rdiv__ will not be called when evaluating 'float / float': must define __div__

In Python 3 it gives this error:

_.py:10: error: Unsupported operand types for / (likely involving Union)
_.py:10: note: __rtruediv__ will not be called when evaluating 'float / float': must define __truediv__

@Michael0x2a IIRC you did something with operators recently?

[ilevkivskyi]

@Michael0x2a Another point about this is that ideally a user should not see any __X__ in error messages unless this will give some insight, for example this

Argument 1 to "__div__" of "int" has incompatible type "float"; expected "int"

Should be formulated as Unsupported operand types for / "int" and "float"

Also I am not sure we should show the note about some subtleties of Python internals unless these methods are defined in user code (not in typeshed stubs).

[ilevkivskyi]

(The comment above is in addition to the fact that it would be great to fix the false positive.)

[ilevkivskyi]

Btw the problem looks specific to unions so there is a chance PR #5545 will also fix this.

[ilevkivskyi]

@Michael0x2a If this is the case, could you please add few tests of this kind to #5545? (In both Python 2 and Python 3 mode.)

[gvanrossum]

Confirmed that #5545 fixes this.

[Michael0x2a]

Ah, whoops -- I probably should have been more vocal about what I was planning on doing here. But yes, this is fixed by #5545 -- I'll make sure to add some tests.

I also agree that we should get rid of the __rdiv__ will not be called when evaluating 'float / float': must define __div__ note -- I was actually planning on submitting a PR for that sometime this weekend. It's a useful error message if you're creating a library with custom numeric types + are trying to debug some code using said numeric types, but as I discovered earlier while debugging, isn't very useful for anybody else.

(Fun story: I ran into that error message while I was debugging some code the other day, and was severely confused for a good half-minute, even though I was originally the one who added that error message...)

Another point about this is that ideally a user should not see any X in error messages unless this will give some insight

I also agree -- I think the fact that we do is an unfortunate consequence of how mypy doesn't cleanly handle division in Python 2 in general. Mypy ought to have detected that __div__ is an operator method in Python 2, but doesn't. Basically, this is the comment I left here.

[mvaled]

I was thinking to add a bug, but I believe this takes cares of it. The PR #5545 also solves the case below.

The problematic code was having an 'Undefined' number that returns itself in most arithmetic operations:

import numbers
from typing import Any, Union


class _Undefined(numbers.Number):
    def idem(self, other: Any) -> 'Result':
        return self

    def selfy(self) -> 'Result':
        return self

    __add__ = __radd__ = __iadd__ = idem
    __sub__ = __rsub__ = __isub__ = idem
    __mul__ = __rmul__ = __imul__ = idem
    __div__ = __rdiv__ = __idiv__ = idem
    __truediv__ = __rtruediv__ = __itruediv__ = idem
    __floordiv__ = __rfloordiv__ = __ifloordiv__ = idem
    __mod__ = __rmod__ = __imod__ = idem
    __pow__ = __rpow__ = __ipow__ = idem
    __trunc__ = selfy
    __float__ = lambda s: 0.0
    __int__ = lambda s: 0
    __abs__ = selfy
    __neg__ = selfy
    __invert__ = selfy

    def __bool__(self):
        return False

    def __str__(self):
        return 'Undefined'
    __repr__ = __str__

    def __hash__(self):
        return id(self)

    def __eq__(self, other):
        return other is self

    def __ne__(self, other):
        return self is not other

    def nope(self, other):
        if isinstance(other, numbers.Number):
            return False
        else:
            return NotImplemented

    __lt__ = __le__ = __gt__ = __ge__ = nope

    del idem, selfy, nope


Undefined = _Undefined()
Result = Union[float, int, _Undefined]
BuiltinNumber = Union[float, int]


def divide(n: Result, d: Result) -> Result:
    return n / d


def divide2(n: BuiltinNumber, d: BuiltinNumber) -> BuiltinNumber:
    return n / d

However, mypy 0.600+ do complained in both 'divide' and 'divide2'.

[ilevkivskyi]

This is fixed by #5545