airflow_scheduler/.venv/Lib/site-packages/airflow/utils/singleton.py

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#   http://www.apache.org/licenses/LICENSE-2.0
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from __future__ import annotations

from typing import Generic, TypeVar

T = TypeVar("T")


class Singleton(type, Generic[T]):
    """Metaclass that allows to implement singleton pattern."""

    _instances: dict[Singleton[T], T] = {}

    def __call__(cls: Singleton[T], *args, **kwargs) -> T:
        if cls not in cls._instances:
            cls._instances[cls] = super().__call__(*args, **kwargs)
        return cls._instances[cls]