Iterators and the Python for statement

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

Iterators and the Python `for` statement

The Python compiler automatically generates code to use an iterator when the for statement is used. Thus, given an object c that is an instance of a concrete class SomeContainer derived from the abstract Container base class, we can use the for statement to enumerate the objects in the container as follows:

c = new SomeContainer();
# ...
for obj in c:
    print obj

One of the advantages of using an iterator object that is separate from the container is that it is possible to have more than one iterator associated with a given container. For example, consider the following code fragment:

c = SomeContainer()
# ...
for obj1 in c:
    for obj2 in c:
	if obj1 == obj2:
	    print obj1, obj2

This code implicitly uses two distinct iterators, one for each for loop. This code compares all ordered-pairs of objects in the container c and prints out those which are equal.