Lære Abstract Method Variants

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Abstract methods are not limited to regular instance methods. You can require subclasses to implement properties, class methods, and static methods using the correct combination of decorators. The modern approach differs from older deprecated decorators that you may still encounter in legacy code.

Abstract Properties

To require a subclass to implement a property, combine @property and @abstractmethod. @abstractmethod must be innermost:


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from abc import ABC, abstractmethod

# Requiring subclasses to implement a property
class DatabaseConnection(ABC):
    @property
    @abstractmethod
    def connection_string(self):
        pass

    @property
    @abstractmethod
    def is_connected(self):
        pass

class PostgresConnection(DatabaseConnection):
    def __init__(self, host, port, database):
        self._host = host
        self._port = port
        self._database = database
        self._connected = False

    @property
    def connection_string(self):
        return f"postgresql://{self._host}:{self._port}/{self._database}"

    @property
    def is_connected(self):
        return self._connected

conn = PostgresConnection("localhost", 5432, "analytics")
print(conn.connection_string)  # "postgresql://localhost:5432/analytics"
print(conn.is_connected)       # False

Abstract Class Methods

Require subclasses to implement a class method using @classmethod and @abstractmethod:


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from abc import ABC, abstractmethod

# Requiring subclasses to provide a factory class method
class DataLoader(ABC):
    @classmethod
    @abstractmethod
    def from_config(cls, config):
        pass

    @abstractmethod
    def load(self):
        pass

class CsvLoader(DataLoader):
    def __init__(self, filepath, delimiter):
        self._filepath = filepath
        self._delimiter = delimiter

    @classmethod
    def from_config(cls, config):
        return cls(config["filepath"], config.get("delimiter", ","))

    def load(self):
        return f"Loading CSV from {self._filepath}"

config = {"filepath": "/data/transactions.csv"}
loader = CsvLoader.from_config(config)
print(loader.load())

Abstract Static Methods

Require subclasses to implement a static method using @staticmethod and @abstractmethod:


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from abc import ABC, abstractmethod

# Requiring subclasses to provide a schema validator
class ApiClient(ABC):
    @staticmethod
    @abstractmethod
    def validate_response(response):
        pass

    @abstractmethod
    def fetch(self, endpoint):
        pass

class RestApiClient(ApiClient):
    @staticmethod
    def validate_response(response):
        return isinstance(response, dict) and "data" in response

    def fetch(self, endpoint):
        return {"data": f"results from {endpoint}"}

client = RestApiClient()
response = client.fetch("/transactions")
print(RestApiClient.validate_response(response))  # True

Deprecated Decorators

The abc module originally provided @abstractproperty, @abstractclassmethod, and @abstractstaticmethod as dedicated decorators. These were deprecated in Python 3.3 and should not be used in new code:

You may still encounter the deprecated forms in older codebases – they still work but emit deprecation warnings.

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