[abc] Add abstract attributes via `abstract` type-hint

[randolf-scholz]

@NeilGirdhar

Personally, I think the checks in ABCMeta are an anachronism. Inheriting from ABC brings in an unwanted metaclass (that’s usually unnecessary—the checks it does could have been provided by an ordinary base class, and the registration it supports is not needed here).

Currently, abstractmethod is checked by type checkers whether you inherit from ABC or not. And I think that’s the main benefit of this feature.

For example, if an attribute is added dynamically during __init__. But that’s why the type hint stuff is great, because it means you can have the cake and eat it too:

• Type hints /decorators only ⇝ static checking
• Subclassing ABC ⇝ uses type hints / decorators to automatically generate runtime checks

@NeilGirdhar

Also, you can’t generally verify the type.

Generally not, in many simple cases you can. But more crucially I think the runtime-check is about verifying the attribute/method exists at all. For example, if you work a lot in interactive notebooks the runtime checks are pretty useful since the static checking support is limited.