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?

Try `flake8 --disable-noqa` or `flake8 --ignore=F821` should work. — MaNKuR, May 15 '21 at 13:21
@gold_cy flake8 3.9.2 (mccabe: 0.6.1, pycodestyle: 2.7.0, pyflakes: 2.3.1) CPython 3.9.5 on Windows — Paul Lemarchand, May 15 '21 at 13:24
@MaNKuR as I put in my last paragraph, I'd rather not ignore all F821 warnings. — Paul Lemarchand, May 15 '21 at 13:25
@Paul My gut feeling says the flake8 has no ability to recognize the new variable assignment introduced in `Python3.8`. So either you have to wait until the feature introduced or change the code to use old-style or manually ignore those statements. — MaNKuR, May 15 '21 at 13:29
it says that newest versions should support the walrus operator, and you have the newest versions of these packages — gold_cy, May 15 '21 at 13:50
@gold_cy Can you try to reproduce the issue? Maybe it's only on my side. — Paul Lemarchand, May 15 '21 at 13:52
have you tried adding an inline comment `noqa: F821` next to the line? `word = match.group(0) # noqa: F821` ? like so — python_user, May 15 '21 at 13:59
@python_user If the condition fails, then match.group(0) is never called, so that shouldn't cause a warning. The inline comment removes the warning, thank you for that. — Paul Lemarchand, May 15 '21 at 14:09

score 13 · Accepted Answer · answered May 15 '21 at 14:58

13

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

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

answered May 15 '21 at 14:58

anthony sottile

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I did report it, thank you for the explanation and workaround. – Paul Lemarchand May 15 '21 at 18:26
1

For future readers: the fix for this has been merged with this PR --> https://github.com/PyCQA/pyflakes/pull/698 – Kurt Bourbaki Jun 02 '22 at 13:41

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

1 Answers1