Comparing a string to multiple items in Python

23
votes

I'm trying to compare a string called facility to multiple possible strings to test if it is valid. The valid strings are:

auth, authpriv, daemon, cron, ftp, lpr, kern, mail, news, syslog, user, uucp, local0, ... , local7

Is there an efficient way of doing this other than:

if facility == "auth" or facility == "authpriv" ...
3
pythonstringstring-comparison
For looking for a substring, try stackoverflow.com/questions/3389574/… – tripleee
Many newcomers are tripped by the fact that if facility == "auth" or "authpriv" doesn't do what they want (it checks if facility == "auth" is true or if "authpriv" is not an empty string). – tripleee

3 Answers

45
votes

If, OTOH, your list of strings is indeed hideously long, use a set:

accepted_strings = {'auth', 'authpriv', 'daemon'}

if facility in accepted_strings:
    do_stuff()

Testing for containment in a set is O(1) on average.

10
votes

Unless your list of strings gets hideously long, something like this is probably best:

accepted_strings = ['auth', 'authpriv', 'daemon'] # etc etc 

if facility in accepted_strings:
    do_stuff()
3
votes

To efficiently check if a string matches one of many, use this:

allowed = set(('a', 'b', 'c'))
if foo in allowed:
    bar()

set()s are hashed, unordered collections of items optimized for determining whether a given item is in them.