Private Constructor in Python

Question

How do I create a private constructor which should be called only by the static function of the class and not from else where?

Answer 1

Though strictly private attributes do not exist in Python, you can use a metaclass to prevent the use of the MyClass() syntax to create a MyClass object.

Here is an example adapted from the Trio project:

from typing import Type, Any, TypeVar


T = TypeVar("T")


class NoPublicConstructor(type):
    """Metaclass that ensures a private constructor

    If a class uses this metaclass like this:

        class SomeClass(metaclass=NoPublicConstructor):
            pass

    If you try to instantiate your class (`SomeClass()`),
    a `TypeError` will be thrown.
    """

    def __call__(cls, *args, **kwargs):
        raise TypeError(
            f"{cls.__module__}.{cls.__qualname__} has no public constructor"
        )

    def _create(cls: Type[T], *args: Any, **kwargs: Any) -> T:
        return super().__call__(*args, **kwargs)  # type: ignore

Here is an example of use:

from math import cos, sin


class Point(metaclass=NoPublicConstructor):
     def __init__(self, x, y):
         self.x = x
         self.y = y

     @classmethod
     def from_cartesian(cls, x, y):
         return cls._create(x, y)
     
     @classmethod
     def from_polar(cls, rho, phi):
         return cls._create(rho * cos(phi), rho * sin(phi))

Point(1, 2) # raises a type error
Point.from_cartesian(1, 2) # OK
Point.from_polar(1, 2) # OK