Odd name shadowing in comprehension

[eryk sun]

On Sat, Oct 22, 2016 at 11:57 PM, Chris Angelico <rosuav at gmail.com> wrote:
>
> Normally, a comprehension is described as being equivalent to an
> unrolled loop, inside a nested function.
> ...
> But it seems that the first iterator (and only that one) is evaluated
> in the parent context:
>
> Why is this? It seems rather curious.

It matches the behavior of generator expressions, for which Guido
gives the following example, as quoted in PEP 289:

    Consider sum(x for x in foo()). Now suppose there's a bug in foo()
    that raises an exception, and a bug in sum() that raises an
    exception before it starts iterating over its argument. Which
    exception would you expect to see? I'd be surprised if the one in
    sum() was raised rather the one in foo(), since the call to foo()
    is part of the argument to sum(), and I expect arguments to be
    processed before the function is called.