Ronald Oussoren added the comment:

issue-17973-experimental.txt is a very crude first attempt at catching augment 
assignment to an immutable LHS. On first glance the code works, but it causes 
problems in the test suite and hence isn't correct yet. The generated code also 
isn't optimal, the LHS is evaluated too many times.

With the patch ``([],)[0] += ['a']`` raises an exception, and doesn't update 
the list. 

I also haven't done benchmarking of the code, I'd expect a slowdown in micro 
benchmarks for augmented assignment when the LHS isn't a simple name because 
there is more byte code (``a[0] += b`` disassembly expands from 8 to 14 lines).

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Added file: http://bugs.python.org/file30318/issue-17973-experimental.txt

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