On Sat, Jan 23, 2016 at 3:50 PM, Andrew Barnert via Python-ideas <python-ideas@python.org> wrote:
Finally, Terry suggested a completely different solution to the problem: don't change closures; change for loops. Make them create a new variable each time through the loop, instead of reusing the same variable. When the variable isn't captured, this would make no difference, but when it is, closures from different iterations would capture different variables (and therefore different cells). For backward-compatibility reasons, this might have to be optional, which means new syntax; he proposed "for new i in range(10):".
Not just for backward compatibility. Python's scoping and assignment rules are currently very straight-forward: assignment creates a local name unless told otherwise by a global/nonlocal declaration, and *all* name binding follows the same rules as assignment. Off the top of my head, I can think of two special cases, neither of which is truly a change to the binding semantics: "except X as Y:" triggers an unbinding at the end of the block, and comprehensions have a hidden function boundary that means their iteration variables are more local than you might think. Making for loops behave differently by default would be a stark break from that tidiness. It seems odd to change this on the loop, though. Is there any reason to use "for new i in range(10):" if you're not making a series of nested functions? Seems most logical to make this a special way of creating functions, not of looping. ChrisA