Why does flake8 give the warning F821 -- undefined name 'match' when using match.group(0) with Python 3

Question

Please consider the following example, which finds the first String in a list that contains the Substring "OH":

list = ["STEVE", "JOHN", "YOANN"]
pattern = re.compile(".*%s.*" % "OH")
word = ""

if any((match := pattern.match(item)) for item in list):
    word = match.group(0)
print(word)

The code works as intended and outputs "JOHN", but I am getting the following warning from flake8 at the line word = match.group(0):

F821 -- undefined name 'match'

Why is this happening, and can I remove the warning without command line arguments or disabling all F821 errors?

Answer 1

This is a bug in pyflakes -- I'd suggest reporting it there

The subtlety is that assignment expressions break out of comprehension scopes, but pyflakes assumes that comprehension scopes enclose all assignments

I'd suggest reporting an issue here

as a workaround, you can place a # noqa comment on the line which produces the error, for example:

# this one ignores *all* errors on the line
word = match.group(0)  # noqa

# this one ignores specifically the F821 error
word = match.group(0)  # noqa: F821

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disclaimer: I'm the maintainer of flake8 and one of the maintainers of pyflakes