[Python-Dev] Scoping vs augmented assignment vs sets (Re: 'fast locals' in Python 2.5)
tjreedy at udel.edu
Tue Jun 13 19:48:24 CEST 2006
"Boris Borcic" <bborcic at gmail.com> wrote in message
news:e6mcfg$iao$1 at sea.gmane.org...
>being transformed to profit from simplifications I expected sets to allow.
>There, itemwise augmented assigments in loops very naturally transform to
>wholesale augmented assignments without loops. Except for this wart.
Your transformation amounted to switching from collection mutation to
object rebinding. In Python, that is a crucial difference. That the
mutation and rebinding were both done with augmented assignments is not
terribly important except as this masks the difference. When *you* read
your code, you know that you will only call the inner function with a
mutable collection object, so you know that the name will be rebound to the
same object after mutation, so you can think of the augmented assignment as
being the same as collection mutation.
But the compiler does not know any such thing about the target of the
augmented assignment and must therefore treat the statement as an
assigment. It was a bug for a2 to do otherwise, even though the bug was
locally optimal for this particular usage.
Terry Jan Reedy
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