On Sun, 23 Oct 2016 11:51 am, eryk sun wrote: > On Sat, Oct 22, 2016 at 11:57 PM, Chris Angelico <ros...@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.
I don't think that this is relevant. Or rather, it might explain why the behaviour is the way it is, but it isn't justification for it. Here's a link to the PEP, showing the quote in context: https://www.python.org/dev/peps/pep-0289/#early-binding-versus-late-binding But this isn't a question about early or late binding, it is asking why the variable y is treated as both global and local in the same comprehension. This may be an unexpected side-effect of other choices, but I don't see any discussion or consideration of this specific issue. [Aside: Guido's quote in the PEP is unsourced; there's a reference given, but it goes to a different email from Guido, not one that includes the claimed explanation.] -- Steve “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough, things got worse. -- https://mail.python.org/mailman/listinfo/python-list
