tests/utils/test_singleton.py

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from __future__ import annotations

from airflow.utils.singleton import Singleton


class A(metaclass=Singleton):
    pass


class Counter(metaclass=Singleton):
    """Singleton class that counts how much __init__ and count was called."""

    counter = 0

    def __init__(self):
        self.counter += 1

    def count(self):
        self.counter += 1


def test_singleton_refers_to_same_instance():
    a, b = A(), A()
    assert a is b


def test_singleton_after_out_of_context_does_refer_to_same_instance():
    # check if setting something on singleton is preserved after instance goes out of context
    def x():
        a = A()
        a.a = "a"

    x()
    b = A()
    assert b.a == "a"


def test_singleton_does_not_call_init_second_time():
    # first creation of Counter, check if __init__ is called
    c = Counter()
    assert c.counter == 1

    # check if "new instance" calls __init__ - it shouldn't
    d = Counter()
    assert c.counter == 1

    # check if incrementing "new instance" increments counter on previous one
    d.count()
    assert c.counter == 2