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copied!<p>As Mark Tolonen pointed out in a concise comment, the reason why your itertools attempts are cycling indefinitely is because, for the list assignment, python is checking the length of the right hand side.</p> <p>Now to really dig in...</p> <p>When you say:</p> <pre><code>x[::2] = itertools.repeat(False) </code></pre> <p>The left hand side (<code>x[::2]</code>) is a list, and you are assigning a value to a list where the value is the <code>itertools.repeat(False)</code> iterable, which will iterate forever since it wasn't given a length (as per <a href="http://docs.python.org/library/itertools.html#itertools.repeat" rel="nofollow">the docs</a>).</p> <p>If you dig into the list assignment code in the cPython implementation, you'll find the unfortunately/painfully named function <a href="http://hg.python.org/cpython/file/8527427914a2/Objects/listobject.c#l614" rel="nofollow"><code>list_ass_slice</code></a>, which is at the root of a lot of list assignment stuff. In that code you'll see <a href="http://hg.python.org/cpython/file/8527427914a2/Objects/listobject.c#l646" rel="nofollow">this segment</a>:</p> <pre><code>v_as_SF = PySequence_Fast(v, "can only assign an iterable"); if(v_as_SF == NULL) goto Error; n = PySequence_Fast_GET_SIZE(v_as_SF); </code></pre> <p>Here it is trying to get the length (<code>n</code>) of the iterable you are assigning to the list. However, before even getting there it is getting stuck on <a href="http://hg.python.org/cpython/file/8527427914a2/Objects/abstract.c#l2257" rel="nofollow"><code>PySequence_Fast</code></a>, where it ends up trying to convert your iterable to a list (with <a href="http://hg.python.org/cpython/file/8527427914a2/Objects/abstract.c#l2235" rel="nofollow"><code>PySequence_List</code></a>), within which it ultimately creates an empty list and tries to simply extend it with your iterable.</p> <p>To extend the list with the iterable, it uses <a href="http://hg.python.org/cpython/file/8527427914a2/Objects/listobject.c#l799" rel="nofollow"><code>listextend()</code></a>, and in there you'll see the root of the problem:</p> <pre><code>/* Run iterator to exhaustion. */ for (;;) { </code></pre> <p>and there you go.</p> <p>Or least I think so... :) It was an interesting question so I thought I'd have some fun and dig through the source to see what was up, and ended up there.</p> <p>As to the different behaviour with numpy arrays, it will simply be a difference in how the <code>numpy.array</code> assignments are handled.</p> <p>Note that using <code>itertools.repeat</code> doesn't work in numpy, but it doesn't hang up (I didn't check the implementation to figure out why):</p> <pre><code>>>> import numpy, itertools >>> x = numpy.ones(10,dtype='bool') >>> x[::2] = itertools.repeat(False) >>> x array([ True, True, True, True, True, True, True, True, True, True], dtype=bool) >>> #but the scalar assignment does work as advertised... >>> x = numpy.ones(10,dtype='bool') >>> x[::2] = False >>> x array([False, True, False, True, False, True, False, True, False, True], dtype=bool) </code></pre>

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