[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Serhiy Storchaka storchaka at gmail.com
Wed Nov 22 08:53:41 EST 2017

22.11.17 15:25, Ivan Levkivskyi пише:

I think this is indeed a problem.. For me the biggest surprise was that yield inside a comprehension does not turn a surrounding function into comprehension, see also https://stackoverflow.com/questions/29334054/why-am-i-getting-different-results-when-using-a-list-comprehension-with-coroutin

In fact there is a b.p.o. issue for this https://bugs.python.org/issue10544, it is assigned to me since July, but I was focused on other things recently. My plan was to restore the Python 2 semantics while still avoiding the leak of comprehension variable to the enclosing scope (the initial reason of introducing auxiliary "makelist" function IIUC). So that: 1) g = [(yield i) for i in range(3)] outside a function will be a SyntaxError (yield outside a function) 2) g = [(yield i) for i in range(3)] inside a function will turn that enclosing function into generator. 3) accessing i after g = [(yield i) for i in range(3)] will give a NameError: name 'i' is not defined If you have time to work on this, then I will be glad if you take care of this issue, you can re-assign it.

I have the same plan. I know how implement this for comprehensions, but the tricky question is what to do with generator expressions? Ideally

 result = [expr for i in iterable]

and

 result = list(expr for i in iterable)

should have the same semantic. I.e. if expr is "(yield i)", this should turn the enclosing function into a generator function, and fill the list with values passed to the generator's .send(). I have no idea how implement this.

More information about the Python-Dev mailing list