python - Using OR comparisons with IF statements

16
votes

When using IF statements in Python, you have to do the following to make the "cascade" work correctly.

if job == "mechanic" or job == "tech":
        print "awesome"
elif job == "tool" or job == "rock":
        print "dolt"

Is there a way to make Python accept multiple values when checking for "equals to"? For example,

if job == "mechanic" or "tech":
    print "awesome"
elif job == "tool" or "rock":
    print "dolt"
6
pythoncomparisonboolean

6 Answers

39
votes
if job in ("mechanic", "tech"):
    print "awesome"
elif job in ("tool", "rock"):
    print "dolt"

The values in parentheses are a tuple. The in operator checks to see whether the left hand side item occurs somewhere inside the right handle tuple.

Note that when Python searches a tuple or list using the in operator, it does a linear search. If you have a large number of items on the right hand side, this could be a performance bottleneck. A larger-scale way of doing this would be to use a frozenset:

AwesomeJobs = frozenset(["mechanic", "tech", ... lots of others ])
def func():
    if job in AwesomeJobs:
        print "awesome"

The use of frozenset over set is preferred if the list of awesome jobs does not need to be changed during the operation of your program.

4
votes

You can use in:

if job  in ["mechanic", "tech"]:
    print "awesome"

When checking very large numbers, it may also be worth storing off a set of the items to check, as this will be faster. Eg.

AwesomeJobs = set(["mechanic", "tech", ... lots of others ])
...

def func():
    if job in AwesomeJobs:
        print "awesome"
1
votes
if job in ("mechanic", "tech"):
    print "awesome"
elif job in ("tool", "rock"):
    print "dolt"
1
votes

While I don't think you can do what you want directly, one alternative is:

if job in [ "mechanic", "tech" ]:
    print "awesome"
elif job in [ "tool", "rock" ]:
    print "dolt"
1
votes

Tuples with constant items are stored themselves as constants in the compiled function. They can be loaded with a single instruction. Lists and sets on the other hand, are always constructed anew on each execution.

Both tuples and lists use linear search for the in-operator. Sets uses a hash-based look-up, so it will be faster for a larger number of options.

0
votes

In other languages I'd use a switch/select statement to get the job done. You can do that in python too.