[Python-ideas] Improving the expressivity of function annotations

[Carl M. Johnson]

>
> Anyway this is where "structural types" come in: defining a type not by its
> name but by its shape (a set of methods and properties, and their
> signatures).
>


Again, this can be handled by defining (enough) abcs, but it soon becomes
> tiresome as ABCs are still pretty verbose to write and I don't doubt there
> would soon be about 5 million versions of ``Readable`` (requirement of a
> single no-args method ``read`` producing bytes). As a result, the ability to
> define a type as a set of methods (and their signatures) without having to
> give it an explicit name would — I think — be great.


OK, reading this again, maybe you mean something more like a Java interface?

@interface
class Writer:
     def write(output: bytes) -> None:
     "Takes in bytes, returns None. Raises Foo on error."
     pass

class DuckWriter(Writer):
    def write(output):
       #do something
        return 1

d = DuckWriter()
d.write("hello") #Raises TypeError, output must be bytes
d.write(b"hello") #Raises TypeError, return type must be NoneType
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