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# SPDX-FileCopyrightText: © 2022 Mars Landis
# SPDX-FileCopyrightText: © 2024 Christian BUHTZ <c.buhtz@posteo.jp>
#
# SPDX-License-Identifier: CC0-1.0
#
# This file is released under Creative Commons Zero 1.0 (CC0-1.0) and part of
# the program "Back In Time". The program as a whole is released under GNU
# General Public License v2 or any later version (GPL-2.0-or-later).
# See file/folder LICENSE or
# go to <https://spdx.org/licenses/CC0-1.0.html>
# and <https://spdx.org/licenses/GPL-2.0-or-later.html>.
#
# Credits to Mr. Mars Landis describing that solution and comparing it to
# alternatives in his article 'Better Python Singleton with a Metaclass' at
# <https://python.plainenglish.io/better-python-singleton-with-a-metaclass-41fb8bfe2127>
# himself referring to this Stack Overflow
# <https://stackoverflow.com/q/6760685/4865723> question as his inspiration.
#
# Original code adapted by Christian Buhtz.
"""Flexible and pythonic singleton implementation.
Support inheritance and multiple classes. Multilevel inheritance is
theoretically possible if the '__allow_reinitialization' approach would be
implemented as described in the original article.
Example ::
>>> from singleton import Singleton
>>>
>>> class Foo(metaclass=Singleton):
... def __init__(self):
... self.value = 'Alyssa Ogawa'
>>>
>>> class Bar(metaclass=Singleton):
... def __init__(self):
... self.value = 'Naomi Wildmann'
>>>
>>> f = Foo()
>>> ff = Foo()
>>> f'{f.value=} :: {ff.value=}'
"f.value='Alyssa Ogawa' :: ff.value='Alyssa Ogawa'"
>>> ff.value = 'Who?'
>>> f'{f.value=} :: {ff.value=}'
"f.value='Who?' :: ff.value='Who?'"
>>>
>>> b = Bar()
>>> bb = Bar()
>>> f'{b.value=} :: {bb.value=}'
"b.value='Naomi Wildmann' :: bb.value='Naomi Wildmann'"
>>> b.value = 'thinking ...'
>>> f'{b.value=} :: {bb.value=}'
"b.value='thinking ...' :: bb.value='thinking ...'"
>>>
>>> id(f) == id(ff)
True
>>> id(b) == id(bb)
True
>>> id(f) == id(b)
False
"""
class Singleton(type):
"""Singleton implementation supporting inheritance and multiple classes."""
_instances = {}
"""Hold single instances of multiple classes."""
def __call__(cls, *args, **kwargs):
try:
# Reuse existing instance
return cls._instances[cls]
except KeyError:
# Create new instance
cls._instances[cls] = super().__call__(*args, **kwargs)
return cls._instances[cls]
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