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To: python-list@python.org
From: Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de>
Subject: how to avoid checking the same condition repeatedly ?
Date: Mon, 28 Oct 2013 09:50:19 +0000 (UTC)
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Dear all,
this is a recurring programming problem that I'm just not sure how to solve
optimally, so I thought I'd ask for your advice:
imagine you have a flag set somewhere earlier in your code, e.g.,

needs_processing = True

then in a for loop you're processing the elements of an iterable, but the
kind of processing depends on the flag, e.g.,:

for elem in iterable:
    if needs_processing:
        pre_process(elem)  # reformat elem in place
    print(elem)

this checks the condition every time through the for loop, even though there
is no chance for needs_processing to change inside the loop, which does not
look very efficient. Of course, you could rewrite the above as:

if needs_processing:
    for elem in iterable:
        pre_process(elem)  # reformat elem in place
        print(elem)
else:
    for elem in iterable:
        print(elem)

but this means unnecessary code-duplication.

You could also define functions (or class methods):
def pre_process_and_print (item):
    pre_process(item)
    print(item)

def raw_print (item):
    print(item)

then:
process = pre_process_and_print if needs_processing else raw_print
for elem in iterable:
    process(elem)

but while this works for the simple example here, it becomes complicated if
pre_process requires more information to do its job because then you will
have to start passing around (potentially lots of) arguments.

So my question is: is there an agreed-upon generally best way of dealing
with this?

Thanks for your help,
Wolfgang