Can't cast from Fraction to int?

[jvdwetering]

In mypy 0.770 the following typechecks:

from fractions import Fraction

def f(a: Fraction) -> int:
	return int(a)

But it raises the following error in mypy 0.790: "error: No overload variant of "int" matches argument type "Fraction""

If I replace the last line with return int(float(a)), it does typecheck.

Is this intended behaviour?

EDIT: The code above actually also doesn't compile on 0.770, see below on modified code that does work on 0.770 but gives an error on 0.790

[hauntsaninja]

This fails for me with 0.770 as well. I assume this is similar to #3186

[jvdwetering]

My bad, I accidentally had it ignoring the error in my 0.770 installation. But similar code I had in a more complicated code base did parse with 0.770, but started to raise errors when I upgraded mypy to 0.790 (even though I did not change anything about the codebase).

And yeah, this definitely looks to be related to #3186.

[hauntsaninja]

Regressions are typically easier to fix, so if you find the failure that would be useful. Could be a problem in python/typeshed#4401 if that made it into the release

[jvdwetering]

Okay, I found a variant of my code above that does work in 0.770 but not in 0.790:

from fractions import Fraction

class Foo(object):
	def __init__(self,a: Fraction):
		self.a = a

def f(bar: Foo) -> int:
	return int(bar.a*2)

Removing the *2 also breaks it in 0.770

[hauntsaninja]

Thanks! Okay, this is triggered by python/typeshed#4401 (but isn't a bug). That PR adds a return type for Fraction's __mul__, where previously it returned Any (since the stubs were incomplete).
This causes the underlying error to surface, since mypy now knows that bar.a * 2 is a Fraction.

[jvdwetering]

That makes sense. So now the remaining problem is that int() should really know how to deal with a Fraction, right?

[hauntsaninja]

Yeah. From a glance, I'm not actually sure how this is even accomplished in CPython since Fraction doesn't have __int__, but will take a look later.

[hauntsaninja]

Here you go, this should fix things: python/typeshed#4665