Python Cookbook

Dispatching Using a Dictionary

Credit: Dick Wall

Problem

You need to execute appropriate pieces of code in correspondence with the value of some control variable—the kind of problem that in some other languages you might approach with a case, switch, or select statement.

Solution

Object-oriented programming, thanks to its elegant concept of dispatching, does away with many (but not all) such needs. But dictionaries, and the fact that in Python functions are first-class values (in particular, they can be values in a dictionary), conspire to make the problem quite easy to solve:

animals = []
number_of_felines = 0

def deal_with_a_cat(  ):
    global number_of_felines
    print "meow"
    animals.append('feline')
    number_of_felines += 1

def deal_with_a_dog(  ):
    print "bark"
    animals.append('canine')

def deal_with_a_bear(  ):
    print "watch out for the *HUG*!"
    animals.append('ursine')

tokenDict = {
    "cat": deal_with_a_cat,
    "dog": deal_with_a_dog,
    "bear": deal_with_a_bear,
    }

# Simulate, say, some words read from a file
words = ["cat", "bear", "cat", "dog"]

for word in words:
    # Look up the function to call for each word, then call it
    functionToCall = tokenDict[word]
    functionToCall(  )
    # You could also do it in one step, tokenDict[word](  )

Discussion

The basic idea behind this recipe is to construct a dictionary with string (or other) keys and with bound methods, functions, or other callables as values. During execution, at each step, use the string keys to select which method or function to execute. This can be used, for example, for simple parsing of tokens from a file through a kind of generalized case statement.

It’s embarrassingly simple, but I use this technique often. Instead of functions, you can also use bound methods (such as self.method1) or other callables. If you use unbound methods (such as class.method), you need to pass an appropriate object as the first actual argument when you do call them. More generally, you can also store tuples, including both callables and arguments, as the dictionary’s values, with diverse possibilities.

I primarily use this in places where in other languages I might want a case, switch, or select statement. I also use it to provide a poor man’s way to parse command files (e.g., an X10 macro control file).

See Also

The Library Reference section on mapping types; the Reference Manual section on bound and unbound methods.