Re: [PATCH v5 19/36] qapi/events.py: add type hint annotations

[Markus Armbruster]

John Snow <jsnow@redhat.com> writes:

> On 10/7/20 7:32 AM, Markus Armbruster wrote:
>> Ignorant question: what's the difference between -> None (like here) and
>> nothing (like __init__() above?
>
> This came up in Cleber's review, too.
>
> Mypy supports a gradual typing paradigm; it is designed to facilitate
> what we are doing here: the gradual adding of types until we are able
> to enforce static typing everywhere.
>
> To that end, mypy uses a simple heuristic to determine if a function
> is "typed" or "untyped": did you use any type annotations for it?
>
> Meanwhile, __init__ never returns anything. You do not need to
> annotate its return type. mypy knows what the return type is and must
> be. In the case of __init__ with no parameters, it is both untyped and
> strictly typed!
>
> Annotating the return type for no-parameter init methods convinces
> mypy that this is a strictly typed method. It doesn't do this
> automatically, because doing so might enable more checks than you were
> ready for simply because mypy was able to accurately surmise the
> typing of just __init__.
>
> So it's just a little flip switch to enable strict typing, really.
>
> --js
>
> https://mypy.readthedocs.io/en/stable/class_basics.html#annotating-init-methods

I now understand we can omit -> None, except when it's the only type
hint, because omitting it then would make it untyped, which is not what
we want.

Is this just for __init__(), or is it for any function that doesn't
return anything?