Simple, thread-safe singleton mixin class for Python

singleton.py

class SingletonMixin:
    """ Threading safe singleton mixin class.  Will run __init__ on subclasses for every
    invocation, so check self._initialized from __init__ to get around this if not desired.

    Use:
    class MyClass(SingletonMixin):
        def __init__(self):
          if not self._initialized:
             # initialize here
             self.foo = 'bar'
             # when done, set initialized to True
             self._initialized = True
    """
    _instances: dict[type, Any] = {}
    _lock = threading.Lock()
    _initialized: bool

    def __new__(cls):
        with SingletonMixin._lock:
            if cls not in SingletonMixin._instances:
                # Create instance
                instance = super().__new__(cls)
                SingletonMixin._instances[cls] = instance
                instance._initialized = False
        # Return existing instance
        return SingletonMixin._instances[cls]

z_LICENSE

# Copyright 2024 Paul Durivage <[email protected]>

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#    http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.