Re: [Python-ideas] A comprehension scope issue in PEP 572

On Mon, May 7, 2018 at 9:42 PM, Steven D'Aprano wrote:

On Mon, May 07, 2018 at 12:48:53PM +1000, Chris Angelico wrote:

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On Mon, May 7, 2018 at 12:34 PM, Tim Peters wrote:

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There's a difference, though: if `y` "leaks", BFD. Who cares? ;-) If `y` remains inaccessible, there's no way around that.

That's Steve D'Aprano's view - why not just let them ALL leak? I don't like it though.

I know popular opinion is against me, and backward compatibility and all that, but I wish that generator expressions and comprehensions ran in their surrounding scope, like regular for statements.

(Yes, I know that makes generator expressions tricky to implement. As the guy who doesn't have to implement it, I don't have to care :-)

Yeah, it's really easy when you don't have to worry about how on earth you can implement the concept of "unexecuted block of code that can be executed later even after the surrounding context has returned, but which isn't a function". :)

(3) A compromise: binding assignments are scoped local to the comprehension, but they are initialised from their surrounding scope.

This would be similar to the way Lua works, as well as function parameter defaults.

I have some vague ideas about implementation, but there's no point discussing that unless people actually are interested in this option.

This will *half* satisfy the running-total example:

total = 0 running_totals = [(total := total + x) for x in [98, 99]] assert total == 0

Guaranteed to generate at least two Stackoverflow posts a month complaining about it, but better than nothing :-)

Does it HAVE to be initialised from the surrounding scope? What if the surrounding scope doesn't have that variable? stuff = [spam for x in items if (spam := f(x)) < 0] Is this going to NameError if you haven't defined spam? Or is the compiler to somehow figure out whether or not to pull in a value? Never mind about implementation - what are the semantics?

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Having the iteration variable NOT leak means it's a self-contained unit that simply says "that thing we're iterating over".

Assuming there are no side-effects to any of the operations inside the comprehension.

Which is often the case.

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Part of it is just that people seem to be fighting for the sake of fighting.

Them's fightin' words! *wink*

Honestly Chris, I know this must be frustrating, but I'm not fighting for the sake of it, and I doubt Tim is either. I'm arguing because there are real use-cases which remain unmet if binding-variables inside comprehensions are confined to the comprehension.

I don't think you are and I don't think Tim is. But do you honestly want to say that about EVERY person in these threads? ChrisA