else in loops

In Python, an else block could be presented not only after if, but after for and while as well. The code inside else is executed unless the loop was interrupted by break.

The common usage for this is to search something in a loop and use break when found:

In : first_odd = None
In : for x in [2,3,4,5]:
...:     if x % 2 == 1:
...:         first_odd = x
...:         break
...: else:
...:     raise ValueError('No odd elements in list')
...:
In : first_odd
Out: 3
In : for x in [2,4,6]:
...:     if x % 2 == 1:
...:         first_odd = x
...:         break
...: else:
...:     raise ValueError('No odd elements in list')
...:
...
ValueError: No odd elements in list