dataclass-abc 0.0.4

Library that lets you define abstract properties for dataclasses.

Dataclass ABC

Library that lets you define abstract properties for dataclasses.

Installation

pip install dataclass-abc

Usage

The dataclass_abc class decorator:

• erases the default value of the fields (see comment below)
• resolves the abstract properties overwritten by a field

from abc import ABC, abstractmethod

from dataclass_abc import dataclass_abc

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

@dataclass_abc(frozen=True)
class B(A):
    val: str        # overwrites the abstract property 'val' in 'A'

Erase default value

dataclass_abc erases the default values of fields. This has the advantage that additional fields that do not refer to an abstract property can be added without running into the "non-default argument follows default argument" exception.

Therefore, do not define default values, work with initialize functions instead (see Design Pattern below).

from abc import ABC, abstractmethod

from dataclass_abc import dataclass_abc

class A(ABC):
    @property
    @abstractmethod
    def val1(self) -> str:
        ...

@dataclass_abc(frozen=True)
class B(A):
    val1: str
    val2: str

# work with initialize functions when dealing with default values
def init_b(
    val1: str,
    val2: str = 'default',
):
    return B(val1=val1, val2=val2)

Example

The example implements the code snippets taken from RealPython with abstract properties.

Design pattern

This library suggests the design pattern as implemented in the example:

• mixins - a mixin is an abstract class that implements data as abstract properties and methods based on the abstract properties.
• classes - an abstract class inherits from one or more mixins (see City or CapitalCity in the example). This class is used for pattern matching, e.g. using isinstance method.
• impl - an implementation class implements the abstract properties. (see CityImpl or CapitalCityImpl in the example). This class is decorated with dataclass_abc and resolve_abc_prop and should always be called through an initialize function.
• init - an initialize function (or constructor function) initializes an implementation class.