Understanding Python's augmented assignment (a += b)

Nice article, though I already understood the matter. Thanks for the link to PEP 463[1], though; I can see it cleaning up some of our code. Do you know if there have been any developments into implementing it?<p>[1] <a href="http://legacy.python.org/dev/peps/pep-0463/" rel="nofollow">http:&#x2F;&#x2F;legacy.python.org&#x2F;dev&#x2F;peps&#x2F;pep-0463&#x2F;</a>

What this article says about C++ is not true.

Why couldn&#x27;t Python do something like:<p><pre><code> &gt;&gt;&gt; _tmp = getitem(t, 0) &gt;&gt;&gt; setitem(t, 0, _tmp) &gt;&gt;&gt; try: &gt;&gt;&gt; _tmp2 = _tmp.__iadd__([2]) &gt;&gt;&gt; except AttributeError: &gt;&gt;&gt; _tmp2 = _tmp.__add__([2]) &gt;&gt;&gt; setitem(t, 0, _tmp2) </code></pre> Then if the first setitem raises, the rest of the code will be skipped. Obviously this isn&#x27;t perfect:<p>1. If setitem has strange side-effects, this will cause them to happen twice. 2. If setitem raises based on the value (rather than the key), this won&#x27;t notice that.<p>But otherwise, it seems like a pretty straightforward improvement, no?

The example look ill-conceived to begin with. If you want to extend the list that is the first element of the tuple just use t[0].extend([1]). Using an augmented assignment operator makes no sense if the left hand side can not be assigned to. I don&#x27;t know Python well enough to tell why it does not check if the left hand side can be assigned to before evaluating the right hand side, i.e. why it dos not check that __setitem__ is present. This would make the behavior more similar to a compiled statically typed language where this would be a compile time error and the right hand side would never get evaluated.

I ran into this when computing PSNR comparisons using a Numpy function that returned a numpy integer type:<p><pre><code> &gt;&gt;&gt; import numpy &gt;&gt;&gt; x=0 &gt;&gt;&gt; y=numpy.int32(2*10**9) &gt;&gt;&gt; x+=y &gt;&gt;&gt; x+=y &gt;&gt;&gt; x -294967296 </code></pre> In later versions of Python this gives a RuntimeWarning so this is less likely to cause problems now.

Official FAQ is easier to understand. I recommend reading that first before the article.

This is a wonderful explanation of a deeply confusing situation. Thank you for posting.