Paul Moore writes: > 1. List comprehensions expand into nested for/if statements in the > "obvious" way - with an empty list created to start and append used to > add items to it. > 1a. Variables introduced in the comprehension don't "leak" (see below). > 2. Generator expressions expand into generators with the same "nested > loop" behaviour, and a yield of the generated value. > 3. List comprehensions are the same as list(the equivalent generator > expression).
I'm a little late to this discussion, but I don't see how 3 can be true if 1 and 2 are. Because I believe my model of generator expressions is pretty simple, I'll present it here: - the generator expression in 2 and 3 implicitly creates a generator function containing the usual loop - the point being that it will "capture" all of the yields (implicit *and* explicit) in the generator expression - then implicitly invokes it, - passing the (iterable) generator object returned to the containing expression. "Look Ma, no yields (or generator functions) left!" However, my model of comprehensions is exactly a for loop that appends to an empty list repeatedly, but doesn't leak iteration variables. So a yield in a comprehension turns the function *containing* the comprehension (which may or may not exist) into a generator function. In other words, I don't agree with Ethan that a yield inside a list comprehension should not affect the generator-ness of the containing function. What would this mean under that condition: [f(x) for x in (yield iterable)] then? IOW, x = ((yield i) for i in iterable) IS valid syntax at top level, while x = [(yield i) for i in iterable] IS NOT valid syntax at top level given those semantics. The latter "works" in Python 3.6; >>> for i in [(yield i) for i in (1, 2, 3)]: ... i ... 1 2 3 though I think it should be a syntax error, and "bare" yield does not "work": >>> i = 1 >>> yield i File "<stdin>", line 1 SyntaxError: 'yield' outside function >>> (yield i) File "<stdin>", line 1 SyntaxError: 'yield' outside function FWIW, that's the way I'd want it, and the way I've always understood comprehensions and generator expressions. I think this is consistent with Yuri's, Serhiy's, and Ivan's claims, but I admit I'm not sure. Of course the compiler need not create a generator function and invoke it, but the resulting bytecode should be the same as if it did. This means the semantics of [FOR-EXPRESSION] and [(FOR-EXPRESSION)] should differ in the same way. Steve _______________________________________________ Python-Dev mailing list [email protected] https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com