Iterators

Data Structures and Algorithms with Object-Oriented Design Patterns in Python

Iterators

In this section we introduce an abstraction called an iterator . An iterator provides the means to access one-by-one all the objects in a container.

A Python iterator is any class that provides two methods called __iter__ and next and that implements the iterator protocol which we describe below. In order to use an iterator to access the objects in a container, that container must provide iteration support. Specifically, that class must implement a method called __iter__ that returns the corresponding iterator for that class.

The idea is that for every concrete container class derived from the Container base class we will also implement a related class derived from the abstract Iterator class shown in Program

program4594
Program: Iterator class.

The Iterator class comprises three methods, __init__, __iter__, and next. The __init__ and __iter__ methods are straightforward. The abstract next method provided in a derived class is required to implement the iterator protocol.

In order to understand the iterator protocol, it is best to consider first an example which illustrates the use of an iterator. Consider a concrete container class, say SomeContainer, that implements the Container interface. The following code fragment illustrates the use of an iterator to access one-by-one the objects contained in the container:

c = SomeContainer()
# ...
i = iter(c) # Equivalent to c.__iter__()
while True:
    try:
	obj = i.next()
	print obj
    except StopIteration:
	break

In order to have the desired effect, the next method must behave as follows:

Each time the next method is called, it returns the next element in the container.
When all the elements in the container have been returned, the next method raises the StopIteration exception.

Given these semantics for the next method, the program fragment shown above systematically visits all of the objects in the container and prints each one on its own line of the console.

A certain amount of care is required when defining and using iterators. In order to simplify the implementation of iterators, we shall assume that while an iterator is in use, the associated container will not be modified.

Iterators and the Python for statement