[Python-ideas] Why does += trigger UnboundLocalError?

andrew cooke andrew at acooke.org
Wed Jun 1 14:29:10 CEST 2011

On Wed, Jun 01, 2011 at 07:48:19AM +0200, Georg Brandl wrote:
> Because x += y is equivalent to
> x = x.__iadd__(y)
> and therefore an assignment is going on here.  Therefore, it's only logical to
> treat it as such when determining scopes.
> Georg

And it's like this so that immutable classes work correctly, as far as I can

So one way to answer the original idea is: because of immutable classes, +=
does not have the same semantics as .append()


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