What is the difference between iterators and generators? Some examples for when you would use each case would be helpful.
iterator
is a more general concept: any object whose class has a __next__
method (next
in Python 2) 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 generator expression) 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 range(start, stop):
yield i * i
generator = squares(a, b)
or the equivalent generator expression (genexp)
generator = (i*i for i in range(a, b))
would take more code to build as a custom iterator:
class Squares(object):
def __init__(self, start, stop):
self.start = start
self.stop = stop
def __iter__(self): return self
def __next__(self): # next in Python 2
if self.start >= self.stop:
raise StopIteration
current = self.start * self.start
self.start += 1
return current
iterator = Squares(a, b)
But, of course, with class Squares
you could easily offer extra methods, i.e.
def current(self):
return self.start
if you have any actual need for such extra functionality in your application.