http://stackoverflow.com/questions/2776829/difference-between-python-generators-vs-iterators
iterator
is a more general concept: any object whose class has a next
method (__next__
in Python 3) and an __iter__
method that does return self
.
Every generator is an iterator, but not vice versa. A generator is built by calling a function that has one or more yield
expressions (yield
statements, in Python 2.5 and earlier), and is an object that meets the previous paragraph's definition of an iterator
.
You may want to use a custom iterator, rather than a generator, when you need a class with somewhat complex state-maintaining behavior, or want to expose other methods besides next
(and __iter__
and __init__
). Most often, a generator (sometimes, for sufficiently simple needs, a generatorexpression) is sufficient, and it's simpler to code because state maintenance (within reasonable limits) is basically "done for you" by the frame getting suspended and resumed.
For example, a generator such as:
def squares(start, stop):for i in xrange(start, stop):yield i * i
or the equivalent generator expression (genexp)
(i*i for i in xrange(start, stop))
would take more code to build as a custom iterator:
classSquares(object):def __init__(self, start, stop):
self.start = start
self.stop = stop
def __iter__(self):return self
def next(self):if self.start >= self.stop:raiseStopIteration
current = self.start * self.start
self.start +=1return current
But, of course, with class Squares
you could easily offer extra methods, i.e.
def current(self):return self.start