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copied!<p><code>yield element</code> can be executed if <code>nested</code> is a list but <code>sublist</code> is not (i.e., if <code>nested</code> is a normal "flat" list). In this case, <code>for sublist in nested</code> will work fine. When the next line recursively calls <code>flatten sublist</code> a typerror will be raised when the recursive call tries to iterate over the "sublist" (which is not iterable). This TypeError is caught and the recursive call yields the <em>entire input list</em> back, so it is then iterated over by the <code>for element in flatten(sublist)</code> call. In other words, <code>for element in flatten(sublist)</code> winds up doing <code>for element in sublist</code> if sublist is already flat.</p> <p>The key thing to recognize is that even a non-nested list will result in a recursive call. A call like <code>flatten([1])</code> will result in <em>two</em> yields: the recursive call will yield <code>[1]</code> to the outer call, and the outer call immediately re-yields <code>1</code>.</p> <p>This version of the function may help to understand what's going on:</p> <pre><code> def flatten(nested, indent=""): try: print indent, "Going to iterate over", nested for sublist in nested: print indent, "Going to iterate over flattening of", sublist for element in flatten(sublist, indent+" "): print indent, "Yielding", element yield element except TypeError: print indent, "Type Error! Yielding", nested yield nested >>> list(flatten([[1,2],3])) Going to iterate over [[1, 2], 3] Going to iterate over flattening of [1, 2] Going to iterate over [1, 2] Going to iterate over flattening of 1 Going to iterate over 1 Type Error! Yielding 1 Yielding 1 Yielding 1 Going to iterate over flattening of 2 Going to iterate over 2 Type Error! Yielding 2 Yielding 2 Yielding 2 Going to iterate over flattening of 3 Going to iterate over 3 Type Error! Yielding 3 Yielding 3 [1, 2, 3] </code></pre>

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