24.2. Iterators: Values that can produce a sequence of values

Closely related to Python's concept of sequences is the concept of an iterator:

For a given sequence S, an iterator I is essentially a set of instructions for producing the elements of S as a sequence of zero or more values.

To produce an iterator over some sequence S, use this function:

iter(S)

The result of this function is an “iterator object” that can be used in a for statement.

>>> continents = ('AF', 'AS', 'EU', 'AU', 'AN', 'SA', 'NA')
>>> worldWalker = iter(continents)
>>> type(worldWalker)
<type 'tupleiterator'>
>>> for landMass in worldWalker:
...     print "Visit {0}.".format(landMass,)
... 
Visit AF. Visit AS. Visit EU. Visit AU. Visit AN. Visit SA. Visit NA.

All iterators have a .next() method that you can call to get the next element in the sequence. This method takes no arguments. It returns the next element in the sequence, if any. When there are no more elements, it raises a StopIteration exception.
```
>>> trafficSignal = [ 'green', 'yellow', 'red' ]
>>> signalCycle = iter(trafficSignal)
>>> type(signalCycle)
<type 'listiterator'>
>>> signalCycle.next()
'green'
>>> signalCycle.next()
'yellow'
>>> signalCycle.next()
'red'
>>> signalCycle.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration
```
Once an iterator is exhausted, it will continue to raise StopIteration indefinitely.
You can also use an iterator as the right-hand operand of the “in” operator.
```
>>> signalCycle = iter(trafficSignal)
>>> 'red' in signalCycle
True
```

Python 2.7 quick reference

24.2. Iterators: Values that can produce a sequence of values