OK, I want to scan a file for lines matching a certain regexp. I'd
like to use an assignment expression, like
for line in file:
if (g := re.match(pat, line)):
croggle(g.group(1))
Since there are no assignment expressions in Python, I have to use a
temp var. That's a little more messy, but bearable:
for line in file:
g = re.match(pat, line)
if g:
croggle(g.group(1))
It gets annoying when there are 4 different regexps that the line
might match, and I want to do something different depending on which
one matches. That's not that uncommon a text scanning situation.
With assignment expressions, it's a very natural if/elif chain:
for line in file:
if g := re.match(pat1, line):
croggle(g.group(1), 17)
elif g := re.match(pat2, line):
x = mugwump(g.group(3))
y = wumpus(g.group(2))
return defenestrate(x, y+3)
elif g := re.match(pat3, line):
# do something completely different with groups of g
elif g := re.match(pat4, line):
# more of the same
Without assigment expressions, it gets unspeakably ugly. You have to
use a deeply nested if/else if sequence where you match the regexp and
test the result on 2 separate lines at each branch, or reorganize the
code to use some kind of dispatch table (good if there's a lot more
than 4 regexps, but overkill for just 4), or whatever. I ended up
creating a special class instance just to match a regexp and remember
the result, so I could write in the if/elif style.
This kind of regexp matching is a common pattern and I keep wanting
assignment expressions whenever I code it, and end up crocking up some
silly workaround.