Chapter 12. Iterators, generators and decorators

12.1. Iterators
12.2. Generators
12.3. Generator expressions
12.4. Clousers

In this chapter we will learn about iterators, generators and decorators.

12.1. Iterators

Python iterator objects required to support two methods while following the iterator protocol.

__iter__ returns the iterator object itself. This is used in for and in statements.

next method returns the next value from the iterator. If there is no more items to return then it should raise StopIteration exception.

class Counter(object):
    def __init__(self, low, high):
        self.current = low
        self.high = high

    def __iter__(self):
        'Returns itself as an iterator object'
        return self

    def next(self):
        'Returns the next value till current is lower than high'
        if self.current > self.high:
            raise StopIteration
        else:
            self.current += 1
            return self.current - 1

Now we can use this iterator in our code.

>>> c = Counter(5,10)
>>> for i in c:
...   print i,
... 
5 6 7 8 9 10

Remember that an iterator object can be used only once. It means after it raises StopIteration once, it will keep raising the same exception.

>>> c = Counter(5,6)
>>> next(c)
5
>>> next(c)
6
>>> next(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 11, in next
StopIteration
>>> next(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 11, in next
StopIteration

Using the iterator in for loop example we saw, the following example tries to show the code behind the scenes.

>>> iterator = iter(c)
>>> while True:
...     try:
...         x = iterator.next()
...         print x,
...     except StopIteration as e:
...         break
... 
5 6 7 8 9 10