Question

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Replacements for switch statement in python?

Suppose I have a list in Python:

list = (‘ADD’, ‘SUB’, ‘PUSH’, ‘POP’)

I want to run a function depending on input, and that input can be any value in the list.

Instead of writing a switch case statement for each element in list, is there a more compact way of writing it?

My reasoning is for the case of the list growing in the future.

Answer 1

Well, there is no switch/case statement in Python.

For a small list, you want to use if/elif:

def do_stuff(x, *args):
    if x == 'ADD':
        return do_add(*args)
    elif x == 'SUB':
        return do_sub(*args)
    # …
    else:
        raise RuntimeError('Never heard of {}'.format(x))

For a larger list, you want to make sure each case is a function (I already assumed that above, but if you had code like return args[0] + args[1], you’d have to change that into a do_add function), and create a dict mapping names to functions:

func_map = {'ADD': do_add, 'SUB': do_sub, … }

def do_stuff(x, *args):
    try:
        return func_map[x](*args)
    except KeyError:
        raise RuntimeError('Never heard of {}'.format(x))

This works because in Python, functions are normal objects that you can pass around like any other objects. So, you can store them in a dict, retrieve them from the dict, and still call them.

By the way, this is all explained in the FAQ, along with a bit of extra fanciness.

If you have some default function you’d like to call instead of raising an error, it’s obvious how to do that with the if/elif/else chain, but how do you do it with the dict map? You could do it by putting the default function into the except block, but there’s an easier way: just use the dict.get method:

def do_stuff(x, *args):
    return func_map.get(x, do_default)(*args)