[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Ivan Levkivskyi]

Serhiy,

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 "_make_list" 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.

--
Ivan
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