for ... else in Python

Python has an interesting for statement (reference) which lets you specify an else suite.

In a construct like this one:

for i in foo:
  if bar(i):
    break
else:
  baz()

the else suite is executed after the for, but only if the for terminates normally (not by a break).

Here's some code written without for...else:

def contains_even_number(l):
  "Prints whether or not the list l contains an even number."
  has_even_number = False
  for elt in l:
    if elt % 2 == 0:
      has_even_number = True
      break
  if has_even_number:
    print "list contains an even number"
  else:
    print "list does not contain an even number"

The equivalent code snippet below illustrates how the use of for...else lets you remove an extraneous flag variable from that loop:

def contains_even_number(l):
  "Prints whether or not the list l contains an even number."
  for elt in l:
    if elt % 2 == 0:
      print "list contains an even number"
      break
  else:
    print "list does not contain an even number"

Use your good judgment when deciding whether to use the for...else construct. It's not unequivocally better, but when there's an asymmetry between the two possibilities, you can make your code more readable by using for...else to keep the "happy path" logic at the top and the exceptional/error case at the bottom.