There’s Something About Code

Sometimes, when you try to explain something using an example, people will pick on the details of your example instead of seeing the greater picture. Scott Hanselman got some flack for daring to write something == true. Whether or not he was justified in writing his code that way, there certainly are good reasons to check for explicit truth values, especially in Python.

When using keyword arguments with a default value of None, it’s easy to check if the argument was supplied or not:

def do_expensive_thing(times):
    for n in range(times + 2):
        print "Index: ", n
 
def do_it(argument_for_expensive_operation=None):
    if argument_for_expensive_operation:
        do_expensive_thing(argument_for_expensive_operation)
    print "It's done"

Let’s try it out with some different parameters:

In [3]: do_it()
It's done

In [4]: do_it(2)
Index:  0
Index:  1
Index:  2
Index:  3
It's done

Works nicely. Let’s try a third time:

In [5]: do_it(0)
It's done

Whooah! What happened this time? Conveniently, not only None gives a truth value of False. Other examples are the integer value zero and the empty list. This explains why the above example fails. The correct way is to explicitly check if the supplied argument is None:

def do_it_right(argument_for_expensive_operation=None):
    if argument_for_expensive_operation is not None:
        do_expensive_thing(argument_for_expensive_operation)
    print "It's done"

Now it prints what we would expect:

In [7]: do_it_right(0)
Index:  0
Index:  1
It's done

The Python documentation covers truth value testing in detail.