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

Ivan Levkivskyi levkivskyi at gmail.com
Wed Nov 22 09:09:40 EST 2017

On 22 November 2017 at 14:53, Serhiy Storchaka <storchaka at gmail.com> wrote:

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-gettin g-different-results-when-using-a-list-comprehension-with-coroutin

In fact there is a b.p.o. issue for this https://bugs.python.org/issue1 0544, 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. Yes, generator expressions are also currently problematic with await:

async def g(i): ... print(i) ... async def f(): ... result = list(await g(i) for i in range(3)) ... print(result) ... f().send(None) Traceback (most recent call last): File "", line 1, in File "", line 2, in f TypeError: 'async_generator' object is not iterable

Maybe Yury can say something about this?

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