Signer uses python2 "__metaclass__"

[jku]

signer.py does __metaclass__ = abc.ABCMeta. This is python2 syntax and does nothing. So calling Signer() works fine but the resulting object does not implement the Signer API (the sign method is unimplemented)

The actually supported methods are

from abc import ABC
class MyABC(ABC)

and

from abc import ABCMeta
class MyABC(metaclass=ABCMeta)

the latter may be safer (WRT multiple inheritance issues)

Found in #456

[jku]

also StorageBackendInterface

[lukpueh]

This might be a bit of a digression, but I think we should be using typing.Protocol in most cases, where we now use abc. This is at least true for Signer and StorageBackendInterface), where the base classes don't provide any re-usable code and we don't care if the implementations are actually subclasses of these interfaces. We just want to make sure that the implementations quack like Signers or StorageBackendInterface.

typing.Protocol is not in Python 3.7, but a backport exists. What do others think?

[jku]

In general I like the protocol idea for things like Signer (where the intent is to document to 3rd parties what they need to do). Caveats though:

• signer dispatch would still need to be somewhere
• anything with serialize/deserialize (like Key) seems trickier

The one issue I have with Protocol is that the implementations never declare themselves (compare with rust Traits which you always declare with a impl ThisTrait for ThatType). This is very pythonic of course but it's also annoying in a very pythonic way: Does MySpecialClass implement the Signer protocol? Who knows! Go ahead and read the full source code to find out...

[joshuagl]

I agree that typing.Protocol seems like a good fit for our ABC's which exist only to define an interface and do not provide any base or shared functionality.