[Python-ideas] free variables in generator expressions

Georg Brandl g.brandl at gmx.net
Thu Dec 13 00:41:22 CET 2007

Arnaud Delobelle schrieb:

> Let's test this (python 2.5):
>  >>> A = '12'
>  >>> B = 'ab'
>  >>> gen = (x + y for x in A for y in B)
>  >>> A = '34'
>  >>> B = 'cd'
>  >>> list(gen)
> ['1c', '1d', '2c', '2d']
> So in the generator expression, A is remains bound to the string '12'
> but B gets rebound to 'cd'.  This may make the implementation of
> generator expressions more straighforward, but from the point of view
> of a user of the language it seems rather arbitrary. What makes A so
> special as opposed to B?  Ok it belongs to the outermost loop, but
> conceptually in the example above there is no outermost loop.

Well, B might depend on A so it can't be evaluated in the outer context
at the time the genexp "function" is called. It has to be evaluated
inside the "function".


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