42. Working with abstract base classes and protocols

`abc.ABC` and `@abstractmethod` define interfaces that subclasses must implement, raising `TypeError` if instantiated without fulfilling the contract.

python
from abc import ABC, abstractmethod

class ChainClient(ABC):
    @abstractmethod
    def get_balance(self, address: str) -> float:
        ...

class MockClient(ChainClient):
    def get_balance(self, address: str) -> float:
        return 1.5

`typing.Protocol` offers structural typing ("duck typing" checked statically): a class satisfies a Protocol just by having the right methods, without explicit inheritance — useful for decoupling code from concrete implementations while keeping static type checking.

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