Add support for jump statements in partially defined vars check

[ilinum]

Description

This builds on #13601 to add support for statements like continue, break, return, raise in partially defined variables check. The simplest example is:

def f1() -> int:
    if int():
        x = 1
    else:
        return 0
    return x

Previously, mypy would generate a false positive on the last line of example. See test cases for more details.

Adding this support was relatively simple, given all the already existing code.

Things that aren't supported yet: match, with, and detecting unreachable blocks.

After this PR, when enabling this check on mypy itself, it generates 18 errors, all of them are potential bugs.

Test Plan

Tests added.

[sobolevn]

Awesome feature!

[sobolevn]

Question: will it work well together with rechability check? For example:

if int():
   raise ValueError  # or `return` or whatever
   x = 1
else:
   x = 2

a = x  # ?

?

[ilinum]

The example you provided will work. But here's an example where it doesn't work very well yet:

if False:
    y = 1  # E: statement is unreachable
else:
    x = 2

a = x  # E: variable may be undefined

The plan is to check if a block is reachable, which I think should make the sample I posted return have only one error (the unreachable one).

[JukkaL]

Looks great! Left a few minor comments.

I also noticed that this case still doesn't work as expected:

def f() -> None:
    while True:
        x = 1
        if int():
            break
    print(x)  # should not be an error

If the while condition is always true, the body is always executed at least once. It's okay to fix this in a follow-up PR if the fix is non-trivial.

[JukkaL]

Test accessing variables defined in the body of a loop that uses break outside the loop?

[ilinum]

I'm not quite sure what you mean. Are you referring to the example you gave in the other message?

def f() -> None:
    while True:
        x = 1
        if int():
            break
    print(x)  # should not be an error

[ilinum]

I also noticed that this case still doesn't work as expected:

def f() -> None:
    while True:
        x = 1
        if int():
            break
    print(x)  # should not be an error

Added support for this. One question is that I have a check for if is_true_literal(expr) here, which means that something like while not False will not work the same way.

I noticed that there's a function called infer_condition_value, which would be exactly what I want but it, unfortunately, doesn't support bool literals. Is this something that we would want to fix in that function? Or is support for literals intentionally omitted there?

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JukkaL]

Thanks for the updates!

[JukkaL]

I noticed that there's a function called infer_condition_value, which would be exactly what I want but it, unfortunately, doesn't support bool literals. Is this something that we would want to fix in that function? Or is support for literals intentionally omitted there?

Hmm, I don't remember why it behaves like this. I think that using while True: / while 1: is a common idiom, and anything else that is always true is probably pretty marginal and not essential, at least initially. What about creating a follow-up issue about this, and we may get it to it later once we have tried this with more real-world code?