Augmented assignment leads to type complaints

[JdeH]

Hi,

I am using mypy 0.4.4. It seems augmented assignments aren't yet dealt with correctly.

weight0 = 65.5
weight0 = 65
weight0 *= 0.5                  # Gives error, unsup operand types for * ("int" and "float")

weight1 = 65.5
weight1 = 65
weight1 = weight1 * 0.5    # Gives no error

[gvanrossum]

Wow. Good catch. Seems there's something strange going on indeed! We'll investigate.

[elazarg]

The error is not directly related to augmented assignment, I think. Mypy assumes that the second assignment kills the first

x = 65.5  # type: float
reveal_type(x)  # E: Revealed type is 'builtins.float'
x = 65
reveal_type(x)  # E: Revealed type is 'builtins.int'

Just like any other case of compatible assignment. So how is that different from the simpler case

x = 65
x += 5.0  # E: Unsupported operand types for + ("int" and "float")

Allowing the latter (inferring float without any other hint) can easily miss real type errors.

[JdeH]

@elazarg:

If that's the cause, why then is

weight1 = 65.5
weight1 = 65
weight1 = weight1 * 0.5    # Gives no error

considered correct?

[elazarg]

I think that because x = x * 0.5 is an assignment, mypy considers it as introducing a "new variable", whereas x *= 0.5 is a method call, and it operates on a previously defined variable.

In other words, x = x * 0.5 contains two distinct variables with possibly distinct types, and x *= 0.5 contains a single variable.

[JukkaL]

It seems that *= doesn't rebind the variable type, unlike =. However, I think that it should.

= doesn't actually introduce new variable but it does introduce a new "type binding" for a variable. This means that the new binding must be compatible with the declared or inferred type of the variable -- mypy doesn't currently let you arbitrarily redefine a variable.

[JdeH]

Indeed from a 'user' point of view, the disparity between simple assignment and augmented assignment is hard to comprehend. At least for me it was... So if it could be avoided I think it would be an improvement.