bool/int substitutions are valid only sometimes: x, y = False, True # type int, int

[cjwelborn]

Like the title says, this is okay in mypy:

mypy -c 'x, y = False, True  # type: int, int'

But the reverse is not:

mypy -c 'x, y = 1, 0  # type: bool, bool'
<string>:1: error: Incompatible types in assignment (expression has type "int", variable has type "bool")

I first found it checking to see if mypy cares about bool/int where Python itself doesn't.
Python is okay with this because bool sub-classes int, and mixed arithmetic is okay:

(False, 0.0, 0, 1).count(False)
>> 3
[2, 1].index(True)
>> 1

mypy doesn't seem to care about this:

mypy -c 'l = [False, True, 0, 1]  # type: List[int]'

..but it cares about the opposite:

mypy -c 'l = [False, True, 0, 1]  # type: List[bool]'
<string>:1: error: List item 2 has incompatible type "int"
<string>:1: error: List item 3 has incompatible type "int"

Is this an actual bug, or is this documented somewhere? I know why Python does it, I just thought that mypy would say something about it.

[refi64]

I don't think this is a bug. In Python, because bool subclasses int, but not the other way around. For instance:

x = True
print(isinstance(x, int)) # True
x = 0
print(isinstance(x, bool)) # False

[gvanrossum]

Indeed, I think the OP is confused by Python's accepting if 0: ... -- but that's not because ints and bools are interchangeable; it's because if doesn't require a bool at all, it takes any object and applies a "truth-finding" dunder method to it (__bool__ or __len__).

[cjwelborn]

I can see the reasoning, that because bool is technically an int it type checks. I just thought that mypy would be a little stricter where python isn't. I wasn't really talking about __bool__ or if statements.

I was talking about how in python, these examples are equivalent:

sum([False, True, True])
sum([0, 1, 1])

x = ['a', 'b'][False]
x = ['a', 'b'][0]

(0.0, 0, False).count(0.0)
(0.0, 0, False).count(0)
(0.0, 0, False).count(False)

I just thought mypy would be a little stricter, and say that x = False # type: int is an error. No big deal though, my question was answered.

[0ion9]

@cjwelborn people aren't notified of comments after an issue is closed. If you want them to notice you have to @mention them like I did in this message.

[cjwelborn]

@0ion9, thanks. I should have done that.