Doing both regex match and assignment within a If loop?

[Chris Rebert]

On Thu, Mar 28, 2013 at 9:00 PM, Victor Hooi <victorhooi at gmail.com> wrote:
> Hi,
>
> I have logline that I need to test against multiple regexes. E.g.:
>
>     import re
>
>     expression1 = re.compile(r'....')
>     expression2 = re.compile(r'....')
>
>     with open('log.txt') as f:
>         for line in f:
>             if expression1.match(line):
>                 # Do something - extract fields from line.
>             elif expression2.match(line):
>                 # Do something else - extract fields from line.
>             else:
>                 # Oh noes! Raise exception.
>
> However, in the "Do something" section - I need access to the match object itself, so that I can strip out certain fields from the line.
>
> Is it possible to somehow test for a match, as well as do assignment of the re match object to a variable?
>
>     if expression1.match(line) = results:
>         results.groupsdict()...

AFAIK, not without hacks and/or being unidiomatic.

> Obviously the above won't work - however, is there a Pythonic way to tackle this?
>
> What I'm trying to avoid is this:
>
>     if expression1.match(line):
>         results = expression1.match(line)
>
> which I assume would call the regex match against the line twice - and when I'm dealing with a huge amount of log lines, slow things down.

def process(line):
    match = expr1.match(line)
    if match:
        # ...extract fields…
        return something
    match = expr2.match(line)
    if match:
        # ...extract fields…
        return something
    # etc…
    raise SomeError()  # Oh noes!

with open('log.txt') as f:
    for line in f:
        results = process(line)


If you choose to further move the extractor snippets into their own
functions, then you can do:


# these could be lambdas if they're simple enough
def case1(match):
    # ...
def case2(match):
    # …
# etc...

REGEX_EXTRACTOR_PAIRS = [
    (re.compile(r'....'), case1),
    (re.compile(r'....'), case2),
    # etc...
]

def process(line):
    for regex, extractor in REGEX_EXTRACTOR_PAIRS:
        match = regex.match(line)
        if match:
            return extractor(match)
    raise SomeError()

Although this second option is likely somewhat less performant, but it
definitely saves on repetition.

Cheers,
Chris