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
      
    
  

Python Generators vs Iterators