Python Fundamentals Tutorial: A Bit More Iteration

An addition to both for and while loops in Python that is not common to all languages is the availability of an else clause. The else block after a loop is executed in the case where no break occurs inside the loop.

The most common paradigm for using this clause occurs when evaluating a dataset for the occurences of a certain condition and breaking as soon as it is found. Rather than setting a flag when found and checking after to see the result, the else clause simplifies the code.

In the following example, if a multiple of 5 is found, the break exits the for loop without executing the else clause.

>>> for x in range(1,5):
...     if x % 5 == 0:
...         print '%d is a multiple of 5' % x
...         break
... else:
...     print 'No multiples of 5'
...
No multiples of 5

>>> for x in range(11,20):
...     if x % 5 == 0:
...         print '%d is a multiple of 5' % x
...         break
... else:
...     print 'No multiples of 5'
...
15 is a multiple of 5

Without this feature, the code would look something like:

>>> found = False
>>> for x in range(1,5):
...     if x % 5 == 0:
...         print '{0} is a multiple of 5'.format(x)
...         found = True
...         break
...
>>> if not found:
...     print 'No multiples of 5'
...
No multiples of 5