Type guard is not working properly with walrus conditional

[erictraut]

In issue #474, @alexkoay reported the following bug:

import re
def foo(s: str) -> str:
	if m := re.fullmatch('(test).+', s):
		return m.group(1)
	return 'oops'

group reports a reportOptionalMemberAccess error under Pyright 1.1.38.

[erictraut]

Looking into this more, the behavior is correct. The bug is in the code, not in Pyright. It was not reported as an error in previous versions of Pyright, but a recent bug fix in 1.1.38 changed that.

It has nothing to do with type guards or the walrus operator. The same error is identified in this simplified version of the code:

def foo(s: str):
    m = re.fullmatch('(test).+', s)
    m.group(1)

The problem here is that re.fullmatch can return None. If it does, your program will crash when it executes m.group(1). Your code needs to deal with the None case. If you're confident that your usage of re.fullmatch will never produce a None result, you can add an assertion to the code, like this:

def foo(s: str):
    m = re.fullmatch('(test).+', s)
    assert m is not None
    m.group(1)

[decorator-factory]

These two are not equivalent at all (that's the point of the if).

import re
def foo(s: str) -> str:
	if m := re.fullmatch('(test).+', s):
		return m.group(1)
	return 'oops'

def foo(s: str):
    m = re.fullmatch('(test).+', s)
    m.group(1)

The first one will never execute return m.group(1) if m is None.

In [12]: re.Match??
Init signature: re.Match()
Docstring
The result of re.match() and re.search().
Match objects always have a boolean value of True.

I've had the same issue with:

        if rematch := re.match(rf'^\s*{escaped}', string):
            _, end = rematch.span() # <-- "span" is not a known member of "None"

and even

        if (rematch := re.match(rf'^\s*{escaped}', string)) is not None:
            _, end = rematch.span() # <-- "span" is not a known member of "None"

With an assert, it works correctly. But it's redundant here.

        if rematch := re.match(rf'^\s*{escaped}', string):
            assert rematch is not None
            _, end = rematch.span()

[erictraut]

You are correct. These aren't equivalent. Thanks for pointing that out. Re-opening the issue.

[decorator-factory]

Thank you!

[erictraut]

Type guards were already working for some assignment expressions, but it depended on the type of expression on the right-hand side. I've eliminated the restriction and added your example to the test suite.

The fix will be in the next version of Pyright.

[erictraut]

This is now fixed in Pyright 1.1.40, which I just published.