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copied!<p>Iterator objects in python conform to the iterator protocol, which basically means they provide two methods: <code>__iter__()</code> and <code>next()</code>. The <code>__iter__</code> returns the iterator object and is implicitly called at the start of loops. The <code>next()</code> method returns the next value and is implicitly called at each loop increment. <code>next()</code> raises a StopIteration exception when there are no more value to return, which is implicitly captured by looping constructs to stop iterating.</p> <p>Here's a simple example of a counter:</p> <pre><code>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 else: self.current += 1 return self.current - 1 for c in Counter(3, 8): print c </code></pre> <p>This will print:</p> <pre><code>3 4 5 6 7 8 </code></pre> <p>This is easier to write using a generator, as covered in a previous answer:</p> <pre><code>def counter(low, high): current = low while current <= high: yield current current += 1 for c in counter(3, 8): print c </code></pre> <p>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.</p> <p>David Mertz's article, <a href="https://www.ibm.com/developerworks/library/l-pycon/" rel="noreferrer">Iterators and Simple Generators</a>, is a pretty good introduction. </p>

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