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POHow to make a repeating generator in Python
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  1. POHow to make a repeating generator in Python
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    copied!<p>How do you make a repeating generator, like xrange, in Python? For instance, if I do:</p> <pre><code>&gt;&gt;&gt; m = xrange(5) &gt;&gt;&gt; print list(m) &gt;&gt;&gt; print list(m) </code></pre> <p>I get the same result both times — the numbers 0..4. However, if I try the same with yield:</p> <pre><code>&gt;&gt;&gt; def myxrange(n): ... i = 0 ... while i &lt; n: ... yield i ... i += 1 &gt;&gt;&gt; m = myxrange(5) &gt;&gt;&gt; print list(m) &gt;&gt;&gt; print list(m) </code></pre> <p>The second time I try to iterate over m, I get nothing back — an empty list.</p> <p>Is there a simple way to create a repeating generator like xrange with yield, or generator comprehensions? I found <a href="http://bugs.python.org/msg89898" rel="nofollow noreferrer">a workaround on a Python tracker issue</a>, which uses a decorator to transform a generator into an iterator. This restarts every time you start using it, even if you didn't use all the values last time through, just like xrange. I also came up with my own decorator, based on the same idea, which actually returns a generator, but one which can restart after throwing a StopIteration exception:</p> <pre><code>@decorator.decorator def eternal(genfunc, *args, **kwargs): class _iterable: iter = None def __iter__(self): return self def next(self, *nargs, **nkwargs): self.iter = self.iter or genfunc(*args, **kwargs): try: return self.iter.next(*nargs, **nkwargs) except StopIteration: self.iter = None raise return _iterable() </code></pre> <p>Is there a better way to solve the problem, using only yield and/or generator comprehensions? Or something built into Python? So I don't need to roll my own classes and decorators?</p> <h1>Update</h1> <p>The <a href="https://stackoverflow.com/questions/1376438/how-to-make-a-repeating-generator-in-python#comment1221812_1376531">comment by u0b34a0f6ae</a> nailed the source of my misunderstanding:</p> <blockquote> <p>xrange(5) does not return an iterator, it creates an xrange object. xrange objects can be iterated, just like dictionaries, more than once.</p> </blockquote> <p>My "eternal" function was barking up the wrong tree entirely, by acting like an iterator/generator (<code>__iter__</code> returns self) rather than like a collection/xrange (<code>__iter__</code> returns a new iterator).</p>
 

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