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(python2.1-ref.info)for statement

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The `for' statement
===================

The `for' statement is used to iterate over the elements of a sequence
(string, tuple or list):

     for_stmt:       "for" target_list "in" expression_list ":" suite
                    ["else" ":" suite]

The expression list is evaluated once; it should yield a sequence.  The
suite is then executed once for each item in the sequence, in the order
of ascending indices.  Each item in turn is assigned to the target list
using the standard rules for assignments, and then the suite is
executed.  When the items are exhausted (which is immediately when the
sequence is empty), the suite in the `else' clause, if present, is
executed, and the loop terminates.

A `break' statement executed in the first suite terminates the loop
without executing the `else' clause's suite.  A `continue' statement
executed in the first suite skips the rest of the suite and continues
with the next item, or with the `else' clause if there was no next item.

The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.

The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.  Hint: the built-in function `range()' returns a sequence of
integers suitable to emulate the effect of Pascal's `for i := a to b
do'; e.g., `range(3)' returns the list `[0, 1, 2]'.

*Warning:* There is a subtlety when the sequence is being modified by
the loop (this can only occur for mutable sequences, i.e. lists).  An
internal counter is used to keep track of which item is used next, and
this is incremented on each iteration.  When this counter has reached
the length of the sequence the loop terminates.  This means that if the
suite deletes the current (or a previous) item from the sequence, the
next item will be skipped (since it gets the index of the current item
which has already been treated).  Likewise, if the suite inserts an
item in the sequence before the current item, the current item will be
treated again the next time through the loop.  This can lead to nasty
bugs that can be avoided by making a temporary copy using a slice of
the whole sequence, e.g.,

     for x in a[:]:
         if x < 0: a.remove(x)