Understanding generators in Python

Question

I am reading the Python cookbook at the moment and am currently looking at generators. I\'m finding it hard to get my head round.

As I come from a Java background, i

Answer 1

It helps to make a clear distinction between the function foo, and the generator foo(n):

def foo(n):
    yield n
    yield n+1

foo is a function. foo(6) is a generator object.

The typical way to use a generator object is in a loop:

for n in foo(6):
    print(n)

The loop prints

# 6
# 7

Think of a generator as a resumable function.

yield behaves like return in the sense that values that are yielded get "returned" by the generator. Unlike return, however, the next time the generator gets asked for a value, the generator's function, foo, resumes where it left off -- after the last yield statement -- and continues to run until it hits another yield statement.

Behind the scenes, when you call bar=foo(6) the generator object bar is defined for you to have a next attribute.

You can call it yourself to retrieve values yielded from foo:

next(bar)    # Works in Python 2.6 or Python 3.x
bar.next()   # Works in Python 2.5+, but is deprecated. Use next() if possible.

When foo ends (and there are no more yielded values), calling next(bar) throws a StopInteration error.