No Python switch statement?

When I was relatively new to Python and writing my first real program, one of the first things I noticed was the lack of a switch statement, which had become commonplace when coding in Java. Since then I've found multiple ways to achieve the same effect (without the use of if-else's!); Firstly the concept of assigning a variable using a switch, which in Java may look something like this;

  int result;
  switch(condition)
   {
     case 'a': result = 1; 
       break;
     case 'b': result = 2; 
       break;
     case 'c': result = 3;  
       break
     default: result = 0;
   }

We can achieve the same in Python using a dictionary;

result = {
  'a': 1,
  'b': 2,
  'c': 3
}[condition]

And lets say we want to define functions as the result of our switch. Also you may have noticed that the above does not define a default action, which is present in the Java equivalent. We can include both like so;

# define the dict
values = { 
           'a': do_stuff, 
           'b': do_stuff2, 
           'c': do_stuff3,
 }
# get the function
values.get(condition, do_default)()

This will execute the do_stuff function defined by the condition, and if a value is not found then it will execute do_default. Much less verbose wouldn't you say? I have one more to add to the list, which I guess is really Reflection, but can be used for the same purpose and is pretty cool in Python.

method_name = 'do_' + condition
if not hasattr(self, method_name):
   do_default
else:
   method = getattr(self, method_name)
   result = method()

def do_a:
  return 1
def do_b:
  return 2
def do_c:
  return 3
def do_default:
  return 0

Some explanation may be in order if you're not familiar with Python. If you're a Java pro, then you may be used to fetching attributes or methods using the getClass() function. This is much easier and awesome in Python. To put it bluntly, the built in function hasattr checks the object's (in this case 'self') attributes for the given string. Then we use getattr to retrieve the attribute, in this case a function, which is then executed. There you have it. As you see, these examples of switch statements provide much more flexibility (mainly in the fact that you are not limited to primitive types!), and are in most cases much less verbose. Happy coding!

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Published

20 February 2011

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