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from functools import cached_property
from pathlib import Path
from typing import Optional, ClassVar
class SingletonMeta(type):
"""
This is a thread-safe implementation of Singleton.
"""
_instances: ClassVar = {}
_lock: Lock = Lock()
"""
We now have a lock object that will be used to synchronize threads during
first access to the Singleton.
"""
def __call__(cls, *args, **kwargs):
"""
Possible changes to the value of the `__init__` argument do not affect
the returned instance.
"""
# Now, imagine that the program has just been launched. Since there's no
# Singleton instance yet, multiple threads can simultaneously pass the
# previous conditional and reach this point almost at the same time. The
# first of them will acquire lock and will proceed further, while the
# rest will wait here.
with cls._lock:
# The first thread to acquire the lock, reaches this conditional,
# goes inside and creates the Singleton instance. Once it leaves the
# lock block, a thread that might have been waiting for the lock
# release may then enter this section. But since the Singleton field
# is already initialized, the thread won't create a new object.
if cls not in cls._instances:
instance = super().__call__(*args, **kwargs)
cls._instances[cls] = instance
return cls._instances[cls]
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