Inconsistency with unions and overloads in when checking overlapping op methods

[Michael0x2a]

As a part of my adventures in attempting to add semi-usable stubs for the fractions and statistics modules, I ran into an edge case involving the __add__ and __radd__ methods (or any pair of operator methods). Here is a simplified version of decimal.Decimal:

from typing import Union

class FakeDecimal1:
    def __add__(self, other: Union[int, FakeDecimal1]) -> FakeDecimal1:
        return FakeDecimal1()
    def __radd__(self, other: Union[int, FakeDecimal1]) -> FakeDecimal1:
        return FakeDecimal1()

When typechecking this, I got the following (and somewhat cryptic) error message:

experiments\test.py: note: In member "__radd__" of class "FakeDecimal":
experiments\test.py:6: error: Forward operator "__add__" is not callable

It seems mypy wanted me to do this instead:

from typing import Union

class FakeDecimal2:
    def __add__(self, other: Union[int, FakeDecimal2]) -> FakeDecimal2:
        return FakeDecimal2()
    def __radd__(self, other: int) -> FakeDecimal2:
        return FakeDecimal2()

However, the following code which uses overrides instead of Union does not result in any errors, which feels inconsistent:

class FakeDecimal3:
    @overload
    def __add__(self, other: int) -> FakeDecimal3:
        return FakeDecimal2()
    @overload
    def __add__(self, other: FakeDecimal3) -> FakeDecimal3:
        return FakeDecimal3()
    @overload
    def __radd__(self, other: int) -> FakeDecimal3:
        return FakeDecimal2()
    @overload
    def __radd__(self, other: FakeDecimal3) -> FakeDecimal3:
        return FakeDecimal2()

After some digging, it appeared to me that the reason for this behavior is because the check_overlapping_op_methods within checker.py handles the case where forward_type is either a CallableType, Overloaded, or AnyType, but not the case where it's a UnionType.

Is this intentional, or an oversight? I think this is a bug, mainly since if mypy accepts version 3, it ought to accept verison 1? Or perhaps the fact that __radd__ should never need to accept a value of the same type as its instance means that the version 2 is more correct and versions 1 and 3 ought to be rejected?

(For what it's worth, adding a new case to make UnionType do the same thing as the Overloaded case makes version 1 pass.)

This is perhaps related: #1442

[gvanrossum]

There's also #1264, and and perhaps #1987. Jukka has said a few times that
the checks are too strict and should be fixed. So... eat your heart out!

On Wed, Sep 14, 2016 at 12:19 AM, Michael Lee notifications@github.com
wrote:

As a part of my adventures in attempting to add semi-usable stubs for the
fractions and statistics modules, I ran into an edge case involving the
add and radd methods (or any pair of operator methods). Here is a
simplified version of decimal.Decimal:

from typing import Union
class FakeDecimal1:
def add(self, other: Union[int, FakeDecimal1]) -> FakeDecimal1:
return FakeDecimal1()
def radd(self, other: Union[int, FakeDecimal1]) -> FakeDecimal1:
return FakeDecimal1()

When typechecking this, I got the following (and somewhat cryptic) error
message:

experiments\test.py: note: In member "radd" of class "FakeDecimal":
experiments\test.py:6: error: Forward operator "add" is not callable

It seems mypy wanted me to do this instead:

from typing import Union
class FakeDecimal2:
def add(self, other: Union[int, FakeDecimal2]) -> FakeDecimal2:
return FakeDecimal2()
def radd(self, other: int) -> FakeDecimal2:
return FakeDecimal2()

However, the following code which uses overrides instead of Union does
not result in any errors, which feels inconsistent:

class FakeDecimal3:
@overload
def add(self, other: int) -> FakeDecimal3:
return FakeDecimal2()
@overload
def add(self, other: FakeDecimal3) -> FakeDecimal3:
return FakeDecimal3()
@overload
def radd(self, other: int) -> FakeDecimal3:
return FakeDecimal2()
@overload
def radd(self, other: FakeDecimal3) -> FakeDecimal3:
return FakeDecimal2()

After some digging, it appeared to me that the reason for this behavior is
because the check_overlapping_op_methods

mypy/mypy/checker.py

Line 664 in 5e822a8

def check_overlapping_op_methods(self,

within checker.py handles the case where forward_type is either a
CallableType, Overloaded, or AnyType, but not the case where it's a
UnionType.

Is this intentional, or an oversight? I think this is a bug, mainly
since if mypy accepts version 3, it ought to accept verison 1? Or perhaps
the fact that radd should never need to accept a value of the same
type as its instance means that the version 2 is more correct and versions
1 and 3 ought to be rejected?

(For what it's worth, adding a new case to make UnionType do the same
thing as the Overloaded case makes version 1 pass.)

This is perhaps related: #1442
#1442

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[Michael0x2a]

I tried re-running all three of the above examples on master, and they all seem to typecheck now... I don't think it was due to any of my overload changes though -- I suspect this was fixed a year ago by #3086?

(Also, apparently I was the one who originally submitted this issue? I completely forgot about this...)