akdom akdom - 2 months ago 7x
Python Question

Build a Basic Python Iterator

How would one create an iterative function (or iterator object) in python?

ars ars

Iterator objects in python conform to the iterator protocol, which basically means they provide two methods: __iter__() and next(). The __iter__ returns the iterator object and is implicitly called at the start of loops. The next() method returns the next value and is implicitly called at each loop increment. next() raises a StopIteration exception when there are no more value to return, which is implicitly captured by looping constructs to stop iterating.

Here's a simple example of a counter:

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

    def __iter__(self):
        return self

    def next(self): # Python 3: def __next__(self)
        if self.current > self.high:
            raise StopIteration
            self.current += 1
            return self.current - 1

for c in Counter(3, 8):
    print c

This will print:


This is easier to write using a generator, as covered in a previous answer:

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

for c in counter(3, 8):
    print c

The printed output will be the same. Under the hood, the generator object supports the iterator protocol and does something roughly similar to the class Counter.

David Mertz's article, Iterators and Simple Generators, is a pretty good introduction.