Python Pocket Reference, Second Edition

Name

The yield Statement

Synopsis

yield expression

Optional in 2.2; enable with from _ _future_ _ import generators. Suspend function state and return expression. On the next iteration, the function’s prior state is restored, and control resumes immediately after the yield statement. Use a return statement with no value to end the iteration, or simply fall off the end of the function:

def generate_ints(N):
    for i in xrange(N):
        yield i

Generators and iterators

In 2.2, functions containing a yield statement are compiled as generators; when called, they return a generator object that supports the iterator interface (i.e., a next( ) method).

Iterators are objects returned by the iter(X) built-in function; they define a next( ) method, which returns the next item in the iteration or raises a StopIteration exception to end the iteration. Classes may provide an _ _iter_ _ method to overload the iter(X) built-in function call; if defined, the result is used to step through objects in for loops, rather than the traditional _ _getitem_ _ indexing overload scheme.