Unsupported operand types using += with floats initialized to 0 (not 0.0)

[timabbott]

mypy complains with:
/home/tabbott/foo.py:4: error: Unsupported operand types for + ("int" and "float") with the following code:

foo = 0
foo += 0.5

While one can work around this by initializing to 0.0, it seems worth thinking about if mypy should accept this.

[gvanrossum]

I agree that we should do something about this. The question is, what should the scope of this type adjustment be? What if I have a foo that initializes a class variable foo to 0 and then a method executes `self.foo += 0.5` ? What if that method is in a subclass? The fix we have in place for variables initialized to empty lists only works for locals -- but it is often insufficient, so I think we need a more general architecture for these types of type adjustments.

[JukkaL]

We could probably tweak partial types to work for the original example, but as Guido pointed out, it would not work in general. However, it might be good enough. I'd like to see more real-world examples before deciding what to do with this.

Note that you can also add an annotation (though changing the initializer is usually easier):

foo = 0  # type: float
foo += 0.5

[timabbott]

Yep makes sense; for a data point I had to do this 6 times in zulip.git.

    -Tim Abbott

On Fri, Jan 29, 2016 at 12:02 PM, Jukka Lehtosalo notifications@github.com
wrote:

We could probably tweak partial types to work for the original example,
but as Guido pointed out, it would not work in general. However, it might
be good enough. I'd like to see more real-world examples before deciding
what to do with this.

Note that you can also add an annotation (though changing the initializer
is usually easier):

foo = 0 # type: float
foo += 0.5

—
Reply to this email directly or view it on GitHub
#1193 (comment).

[JukkaL]

@timabbott Could you give more information about the 6 cases in zulip.git -- or were they all similar to x = 0 / x += 0.5?

[timabbott]

See timabbott/zulip@bf82ac3

5/6 are like that and the other is like that except x is in a dict.