[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Thu Nov 23 05:55:42 EST 2017
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On 23 November 2017 at 18:11, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
Ivan Levkivskyi wrote:
"People sometimes want to refactor for-loops containing
yieldinto a comprehension but that doesn't work (particularly because of the hidden function scope) - lets make it a SyntaxError"Personally I'd be fine with removing the implicit function scope from comprehensions and allowing yield in them, since the semantics of that are clear.
People keep saying this, but seriously, those semantics aren't clear at all once you actually start trying to implement it.
Yes, they're obvious in simple cases, but it isn't the simple cases that are the problem.
Instead, things start getting hard once you're dealing with:
- unpacking to multiple variable names
- nested loops in the comprehension
- lexical closures inside the comprehension (e.g. lambda expressions, comprehensions inside comprehensions)
Hence the approach we ended up going with for https://bugs.python.org/issue1660500, which was to use a real function scope that already handled all of those potential problems in a well defined way.
Technically we could define new answers to all of those situations, but then we're stuck explaining to everyone what those new behaviours actually are, and I think that will actually be harder than the status quo, where we only have to explain why these implicit scopes act much the same way that "lambda: await expr" and "lambda: yield expr" do.
Cheers, Nick.
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