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Allow Generator type annotation to have one argument. #975
Description
To annotate a generator yielding an int, it’s
--> Generator[int, None, None]
That's really quite ugly.
It should be Generator[int]
Passing None does not fit the rest of Python where arguments are optional. Also in the rest of Python, the common cases are the default. This makes an unlikely case most prominent over the far more common case. It’s Python’s job to provide syntactic sugar to deal with the low-level details, not the user. This looks more like C.
https://docs.python.org/3/library/typing.html#typing.Generator
“Alternatively, annotate your generator as having a return type of either Iterable[YieldType] or Iterator[YieldType]”
Yes a generator is an iterator and an iterator is an iterable but then why not set everything as Any? Are we annotating the types or not? That advice makes a mockery of the whole idea. Makes Python harder to teach for no good reason. "Put the type the function returns, unless it's a generator then do this..."
Several people have pointed this out before, their bugs got closed without explanation. One suggested making a bug here, so this is it. Enjoy.
python/mypy#4221
https://bugs.python.org/issue31700
I'm hoping this bug gets closed because someone's already fixed it in a development version 😎.
Cheers.
Activity
erictraut commentedon Dec 1, 2021
There's a good reason that
Generatorhas three type parameters. If you omit type arguments, they will be assumed to beAny(by PEP 484 rules), and I don't think that's what you want.It sounds like what you're looking for is a simplified version of
Generatorthat assumes the last two type arguments areNone. One option is to create a type alias such asSimpleGenerator = Generator[T, None, None]. Another option, as others have suggested, is to useIterable[YieldType]orIterator[YieldType].There has been some discussion about adding support for default type arguments, but nothing has been finalized. If default type arguments are added to the type, it would be possible to omit type arguments without having them default to
Any.JelleZijlstra commentedon Dec 1, 2021
#307 proposes supporting defaults for TypeVars. Generator is definitely one of the most compelling use cases. However, I don't think we should special-case just Generator, especially with the workarounds @erictraut lists, so I'll close this issue as a duplicate.
If you're interested in pursuing this, we should discuss it further in #307 and write a PEP.
zeth commentedon Dec 1, 2021
Why would Any be a problem here if they are never used?
Alias is better advise, that probably should be in the documentation in my opinion.