Guido van Rossum writes:
> And my mind boggles when considering a generator expression
> containing yield that is returned from a function. I tried this
> and cannot say I expected the outcome:
>
> def f():
> return ((yield i) for i in range(3))
> print(list(f()))
>
> In both Python 2 and Python 3 this prints
>
> [0, None, 1, None, 2, None]
>
> Even if there's a totally logical explanation for that, I still
> don't like it, and I think yield in a comprehension should be
> banned. From this it follows that we should also simply ban
> yield from comprehensions.
>
Serhiy Storchaka writes:
> This behavior doesn't look correct to me and Ivan.
The behavior is surprising, but it seems quite consistent with how
generator expressions are defined in the language. A generator
expression is defined by the language reference as "compact generator
notation in parentheses", which yields (sic!) a "new generator object".
I take that to mean that a generator expression is equivalent to
defining and calling a generator function. f() can be transformed to:
def f():
def _gen():
for i in range(3):
ret = yield i
yield ret
return _gen()
The transformed version shows that there are *two* yields per iteration
(one explicitly written and one inserted by the transformation), which
is the reason why 6 values are produced. The None values come from list
constructor calling __next__() on the generator, which (as per
documentation) sends None into the generator. This None value is yielded
after the "i" is yielded, which is why Nones follow the numbers.
Hrvoje
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