How Do I Create and Use Iterators in Python?

Creating an Iterator in Python

Building an iterator in Python requires implementing the iterator protocol, which specifies two essential methods: __iter__() and __next__(). These methods define how objects can initialize and iterate through a sequence of values.

Understanding the Iterator Protocol

In __iter__(), the iterator object is returned, which is typically called implicitly at the start of loops. __next__() is the primary method that returns the next value in the sequence. For Python 2 users, this method is known as next(). When all values are exhausted, __next__() raises a StopIteration exception, which looping constructs capture to terminate iteration.

An Example: The Counter Iterator

Let's create a simple Counter iterator that generates values within a specified range:

class Counter:
    def __init__(self, low, high):
        self.current = low - 1
        self.high = high

    def __iter__(self):
        return self

    def __next__(self): # Python 2: def next(self)
        self.current += 1
        if self.current < self.high:
            return self.current
        raise StopIteration

# Usage:
for c in Counter(3, 9):
    print(c)

This will produce the following output:

3
4
5
6
7
8

Using Generators for Iterators

Generators offer an alternative mechanism for creating iterators. Generator functions yield values one at a time, effectively implementing the iterator protocol.

def counter(low, high):
    current = low
    while current < high:
        yield current
        current += 1

# Usage:
for c in counter(3, 9):
    print(c)

The above code produces the same output as the Counter class.

Additional Resources

For a comprehensive understanding of iterators, refer to resources like David Mertz's article "Iterators and Simple Generators."

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