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

[John Snow]

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