Using a walrus operator in if statement does not work

Question

I have a simple function that should output a prefix based on a pattern or None if it does not match. Trying to do a walrus it does not seem to work. Any idea?

import re

def get_prefix(name):
    if m := re.match(f'^.+(\d\d)-(\d\d)-(\d\d\d\d)$', name) is not None:
        return m.group(3) + m.group(2) + m.group(1)

get_prefix('abc 10-12-2020')

Traceback

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in get_prefix
AttributeError: 'bool' object has no attribute 'group'

Nofretete

You're setting m to re.match(f'^.+(\d\d)-(\d\d)-(\d\d\d\d)$', name) is not None, which is a boolean. Get rid of is not None. – Purely

is not None is redundant anyway, because re.match always returns either a (non-falsy) match object or None. – Gayton

@Gayton It's theoretically faster to check is None since it compares the identities directly instead of having to call __bool__(). Also it's more explicit and, according to PEP-8, more Pythonic. – Howlyn

@MarkusMeskanen PEP-8 refers to the cases when an accepted value can be Falsy. Which is not the case. – Gayton

Answer 1

You're setting m to re.match(f'^.+(\d\d)-(\d\d)-(\d\d\d\d)$', name) is not None, which is a boolean.

You probably mean

if (m := re.match(f'^.+(\d\d)-(\d\d)-(\d\d\d\d)$', name)) is not None:

But you don't need is not None here anyway. Matches are truthy and None is falsey. So you just need:

if m := re.match(f'^.+(\d\d)-(\d\d)-(\d\d\d\d)$', name):

(Arguably it's better practice to use () whenever you're using an assignment expression, to make clear what's being assigned.)

See PEP572#Relative precedence of :=

Purely