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 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
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:
class
Squares
(
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
:
raise
StopIteration
current
=
self
.
start
*
self
.
start self
.
start
+=
1
return
current
But, of course, with class
Squares
you could easily offer extra methods, i.e.
def
current
(
self
):
return
self
.
start
