Control Flow - Page 1

This chapter describes statements related to the control flow of a program. Topics include conditionals, loops, and exceptions.

Conditionals

The if, else, and elif statements control conditional code execution. The general format of a conditional statement is as follows:

if expression:
  statements
elif expression:
  statements
elif expression:
  statements
. . .
else:
  statements

If no action is to be taken, you can omit both the else and elif clauses of a conditional. Use the pass statement if no statements exist for a particular clause:

if expression:
  pass # Do nothing
else:
  statements

Loops

You implement loops using the for and while statements. For example:

while expression :
  statements

for i in s:
  statements

The while statement executes statements until the associated expression evaluates to false. The for statement iterates over all the elements in a sequence until no more elements are available. If the elements of the sequence are tuples of identical size, you can use the following variation of the for statement:

for x , y , z in s :
  statements 1

In this case, s must be a sequence of tuples, each with three elements. On each iteration, the contents of the variables x, y, and z are assigned the contents of the corresponding tuple.

To break out of a loop, use the break statement. For example, the following function reads lines of text from the user until an empty line of text is entered:

while 1:
  cmd = raw_ input('Enter command > ')
  if not cmd:
    break   # No input, stop loop
  # process the command
  . . .

To jump to the next iteration of a loop (skipping the remainder of the loop body), use the continue statement. This statement tends to be used less often, but is sometimes useful when the process of reversing a test and indenting another level would make the program too deeply nested or unnecessarily complicated. As an example, the following loop prints only the non-negative elements of a list:

for a in s:
  if a < 0:
    continue   # Skip negative elements
  print a

The break and continue statements apply only to the innermost loop being executed. If it's necessary to break out of a deeply nested loop structure, you can use an exception. Python doesn't provide a goto statement.

You can also attach the else statement to loop constructs, as in the following example:

# while-else
while i < 10:
  do something
  i = i + 1
else:
  print Done

# for-else
for a in s:
  if a = = Foo :
    break
else:
  print Not found!

The else clause of a loop executes only if the loop runs to completion. This either occurs immediately (if the loop wouldn't execute at all) or after the last iteration. On the other hand, if the loop is terminated early using the break statement, the else clause is skipped.

Python Essential Reference, Second Edition
Exceptions - Page 2