Doing both regex match and assignment within a If loop?

[Steven D'Aprano]

On Thu, 28 Mar 2013 21:00:44 -0700, Victor Hooi wrote:


> Is it possible to somehow test for a match, as well as do assignment of
> the re match object to a variable?


mo = expression.match(line)
if mo:
    ...


Many problems become trivial when we stop trying to fit everything into a 
single line :-)


>     if expression1.match(line) = results:
>         results.groupsdict()...
> 
> Obviously the above won't work - however, is there a Pythonic way to
> tackle this?

Yes. Stop trying to fit everything into a single line :-)

I would approach the problem like this:


LOOKUP_TABLE = {expression1: do_something, 
                expression2: do_something_else, 
                expression3: function3, 
                expression4: function4, # etc.
                }

with open('log.txt') as f:
    for line in f:
        for expr, func in LOOKUP_TABLE.items():
            mo = expr.match(line)
            if mo:
                func(line, mo)
                break
        else:
            # If we get here, we never reached the break.
            raise SomeException


If you don't like having that many top level functions, you could make 
them methods of a class.


If you only have two or three expressions to test, and the body of each 
if clause is small, it's probably too much effort to write functions for 
each one. In that case, I'd stick to the slightly more verbose form:

with open('log.txt') as f:
    for line in f:
        mo = expression1.match(line)
        if mo:
            do_this()
            do_that()
        mo = expression2.match(line)
        if mo:
            do_something_else()
        mo = expression3.match(line)
        if mo:
            fe()
            fi()
            fo()
            fum()
        else:
            raise SomeException





> 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

Correct.



-- 
Steven