Dataclass-ABC

Dataclass-ABC is a Python library that bridges the gap between abstract base classes (ABCs) and dataclasses. It allows you to define and automatically implement abstract properties in dataclasses when these properties are overridden by fields.

Features

• Abstract properties: Implement abstract properties by inheritence using dataclasses.
• Trait Behavior: Allows to reproduce Trait-like behavior (as in Scala or Rust) using Python ABC classes.

Installation

Install Dataclass-ABC using pip:

pip install dataclassabc

Usage

The dataclassabc decorator enables the use of abstract properties within dataclasses. It resolves abstract properties defined in an abstract base class (ABC) and enforces their implementation through fields in the derived dataclass.

Example

Here's how you can define an abstract property in an abstract class and implement it in a dataclass:

from abc import ABC, abstractmethod
from dataclassabc import dataclassabc

# Define an abstract base class with an abstract property
class A(ABC):
    @property
    @abstractmethod
    def name(self) -> str: ...

# Use the dataclassabc decorator to implement the abstract property in a dataclass
@dataclassabc(frozen=True)
class B(A):
    # Implementing the abstract property 'name'
    name: str

# Works as expected
b1 = B(name='A')

# TypeError: B.__init__() missing 1 required positional argument: 'name'
b2 = B()

Define mutable variables

The dataclassabc library also supports defining mutable abstract properties. Use the @property decorator alongside a setter to define mutable properties in the abstract class.

from abc import ABC, abstractmethod
from dataclassabc import dataclassabc

class A(ABC):
    @property
    @abstractmethod
    def name(self) -> str: ...

    @name.setter
    @abstractmethod
    def name(self, val: str): ...

@dataclassabc
class B(A):
    name: str

Comparison with the standard dataclass

Here are known issues when using the standard dataclass decorator in combination with abc library:

• TypeError: "Can't instantiate abstract class" when trying to override an abstract property with a dataclass field.

  from abc import ABC, abstractmethod
  from dataclasses import dataclass
  
  class A(ABC):
      @property
      @abstractmethod
      def name(self) -> str:
          ...
  
  @dataclass(frozen=True)
  class B(A):
      name: str
  
  # TypeError: Can't instantiate abstract class B without an implementation for abstract method 'name'
  b = B(name='A')
  

• Unexpected Default Value <property object at ...> when using dataclass option slots=True.

  from abc import ABC, abstractmethod
  from dataclasses import dataclass
  
  class A(ABC):
      @property
      @abstractmethod
      def name(self) -> str:
          ...
  
  @dataclass(slots=True)
  class B(A):
      name: str
  
  # No exception is raised when name is not provided
  b = B()
  
  # The output will be <property object at ...>
  print(b.name)
  

• TypeError: "Non-default argument follows default argument" when using dataclass option slots=True.

  from abc import ABC, abstractmethod
  from dataclasses import dataclass
  
  class A(ABC):
      @property
      @abstractmethod
      def name(self) -> str:
          ...
  
  @dataclass(slots=True)
  class B(A):
      name: str
      age: int
  
  # TypeError: non-default argument 'age' follows default argument 'name'
  b = B(age=12, name='A')