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POPython += versus .extend() inside a function on a global variable
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copied!<p>I've read a few other SO (<a href="https://stackoverflow.com/questions/146359/python-scope">PythonScope</a> and <a href="https://stackoverflow.com/questions/14323817/global-dictionaries-dont-need-keyword-global-to-modify-them">globals don't need global</a>) but nothing seems to explain as explicitly as I would like and I'm having trouble mentally sifting through whether or not <a href="http://docs.python.org/3/reference/lexical_analysis.html#operators" rel="nofollow noreferrer">PyDocs</a> tells me the answer to my question:</p> <pre><code>myList = [1] def foo(): myList = myList + [2, 3] def bar(): myList.extend([2, 3]) def baz(): myList += [2, 3] </code></pre> <p>Now, understandably, </p> <pre><code>>>> foo() UnboundLocalError </code></pre> <p>and</p> <pre><code>bar() # works myList # shows [1, 2, 3] </code></pre> <p>but then</p> <pre><code>>>> baz() UnboundLocalError </code></pre> <p>I thought, however, that things like <code>+=</code> implicitly called the method operators, in this case <code>extend()</code>, but the error implies that for some reason it does not actually treat <code>+=</code> as <code>extends()</code>. Is this consistent with how Python parsing ought to work?</p> <p>I would have thought that calling functions that are equivalent to method-operators, they would be equivalent in all cases. Instead it seems that it treats <code>+=</code> as an actual assignment operator. Except, this isn't completely true, because if I do something (admittedly contrived):</p> <pre><code>myList = range(50000000) # wait a second or two on my laptop before returning myList += [0] # returns instantly myList = myList + [1] # wait a second or two before returning </code></pre> <p>all of which is expected, if <code>+=</code> actually just calls <code>extend()</code>. </p> <p>Is there some finer distinction (or very obvious point...) that I'm missing that makes it clear that <code>myList</code> in <code>baz()</code> needs to be treated as a local variable, and that therefore the <code>+=</code> cannot be implicitly converted to an <code>extend()</code> such that it recognizes the global variable?</p>

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