[Python-Dev] PEP 526 ready for review: Syntax for Variable and Attribute Annotations (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Thu Sep 1 22:38:27 EDT 2016

On 2 September 2016 at 02:21, Steven D'Aprano <steve at pearwood.info> wrote:

Unless I've missed something, there's no way to pre-declare an instance attribute without specifying a type. (Even if that type is Any.) So how about we allow None as a type-hint on its own:

NAME: None

None already has a meaning as an annotation - it's a shorthand for "type(None)".

While for variables and parameters, that's usually only seen in combination with Union, and even though Union[T, None] has a preferred spelling as Optional[T], there's also the "-> None" case to specify that a function doesn't return a value. Having "-> None" mean "no return value" and "NAME: None" mean "infer type from later assignment" would be quite confusing.

However, a standalone Ellipsis doesn't currently have a meaning as a type annotation (it's only meaningful when subscripting Tuple and Callable), so a spelling like this might work:

NAME: ...

That spelling could then also be used in function definitions to say "infer the return type from the return statements rather than assuming Any":

def inferred_return_type(): -> ...
    return some_other_function()

Cheers, Nick.

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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