[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)
Hrvoje Niksic hrvoje.niksic at avl.com
Fri Nov 24 04:58:19 EST 2017
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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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