Lære What typing.Protocol Is and How It Differs

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Python 3.8 introduced typing.Protocol as a way to express structural typing – defining an interface based on what methods an object has, without requiring inheritance. Where ABCs use nominal typing ("this class is a subclass of X"), Protocols use structural typing ("this class has the right methods").

Nominal vs Structural Typing

Nominal typing (ABCs): a class must explicitly inherit from or register with the ABC to be considered compatible.

Structural typing (Protocols): a class is compatible if it has the required methods, regardless of its inheritance chain.


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from abc import ABC, abstractmethod
from typing import Protocol

# Nominal: must inherit from Serializable
class Serializable(ABC):
    @abstractmethod
    def serialize(self):
        pass

# Structural: any class with serialize() qualifies
class SerializableProtocol(Protocol):
    def serialize(self) -> str:
        ...

# A class that has serialize() but does not inherit from anything
class TradeRecord:
    def serialize(self):
        return '{"id": "TX-001", "amount": 4500.0}'

# ABC – fails without inheritance or register()
print(isinstance(TradeRecord(), Serializable))        # False

# Protocol – passes because the method exists
from typing import runtime_checkable

@runtime_checkable
class SerializableProtocol(Protocol):
    def serialize(self) -> str:
        ...

print(isinstance(TradeRecord(), SerializableProtocol))  # True

Defining a Protocol

A Protocol class inherits from typing.Protocol. Methods are defined with ... (ellipsis) as the body – they describe the interface, not an implementation:


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from typing import Protocol

# Protocol defining the interface for a data exporter
class Exporter(Protocol):
    def export(self, data: list) -> str:
        ...

    def get_format(self) -> str:
        ...

# Any class with these methods satisfies the protocol
class CsvExporter:
    def export(self, data):
        return ",".join(str(item) for item in data)

    def get_format(self):
        return "CSV"

class JsonExporter:
    def export(self, data):
        import json
        return json.dumps(data)

    def get_format(self):
        return "JSON"

# Both satisfy the protocol without inheriting from it
def run_export(exporter: Exporter, data: list) -> None:
    print(f"Format: {exporter.get_format()}")
    print(exporter.export(data))

run_export(CsvExporter(), ["TX-001", "TX-002"])
run_export(JsonExporter(), ["TX-001", "TX-002"])

`@runtime_checkable`

By default, Protocol membership cannot be checked at runtime with isinstance(). Decorating with @runtime_checkable enables it – but the check only verifies method presence, not signatures:


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from typing import Protocol, runtime_checkable

@runtime_checkable
class Runnable(Protocol):
    def run(self) -> None:
        ...

class BackgroundJob:
    def run(self):
        print("Running background job")

class BrokenJob:
    pass

print(isinstance(BackgroundJob(), Runnable))  # True – has run()
print(isinstance(BrokenJob(), Runnable))      # False – missing run()

ABCs vs Protocols at a Glance

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