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copied!<p>There are four ways to build an iterative function:</p> <ul> <li>create a generator (uses the <a href="http://docs.python.org/py3k/reference/expressions.html#yield-expressions">yield keyword</a>)</li> <li>use a generator expression (<a href="http://docs.python.org/py3k/reference/expressions.html#generator-expressions">genexp</a>)</li> <li>create an iterator (defines <a href="http://docs.python.org/py3k/library/stdtypes.html?highlight=__iter__#iterator-types"><code>__iter__</code> and <code>__next__</code></a> (or <code>next</code> in Python 2.x))</li> <li>create a function that Python can iterate over on its own (<a href="http://docs.python.org/py3k/reference/datamodel.html?highlight=__getitem__#object.__getitem__">defines <code>__getitem__</code></a>)</li> </ul> <p>Examples:</p> <pre><code># generator def uc_gen(text): for char in text: yield char.upper() # generator expression def uc_genexp(text): return (char.upper() for char in text) # iterator protocol class uc_iter(): def __init__(self, text): self.text = text self.index = 0 def __iter__(self): return self def __next__(self): try: result = self.text[self.index].upper() except IndexError: raise StopIteration self.index += 1 return result # getitem method class uc_getitem(): def __init__(self, text): self.text = text def __getitem__(self, index): result = self.text[index].upper() return result </code></pre> <p>To see all four methods in action:</p> <pre><code>for iterator in uc_gen, uc_genexp, uc_iter, uc_getitem: for ch in iterator('abcde'): print ch, print </code></pre> <p>Which results in:</p> <pre><code>A B C D E A B C D E A B C D E A B C D E </code></pre> <p><strong>Note</strong>:</p> <p>The two generator types (<code>uc_gen</code> and <code>uc_genexp</code>) cannot be <code>reversed()</code>; the plain iterator (<code>uc_iter</code>) would need the <code>__reversed__</code> magic method (which must return a new iterator that goes backwards); and the getitem iteratable (<code>uc_getitem</code>) must have the <code>__len__</code> magic method:</p> <pre><code> # for uc_iter def __reversed__(self): return reversed(self.text) # for uc_getitem def __len__(self) return len(self.text) </code></pre> <hr> <p>To answer Colonel Panic's secondary question about an infinite lazily evaluated iterator, here are those examples, using each of the four methods above:</p> <pre><code># generator def even_gen(): result = 0 while True: yield result result += 2 # generator expression def even_genexp(): return (num for num in even_gen()) # or even_iter or even_getitem # not much value under these circumstances # iterator protocol class even_iter(): def __init__(self): self.value = 0 def __iter__(self): return self def __next__(self): next_value = self.value self.value += 2 return next_value # getitem method class even_getitem(): def __getitem__(self, index): return index * 2 import random for iterator in even_gen, even_genexp, even_iter, even_getitem: limit = random.randint(15, 30) count = 0 for even in iterator(): print even, count += 1 if count >= limit: break print </code></pre> <p>Which results in (at least for my sample run):</p> <pre><code>0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 </code></pre>

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