hbutils.design.singleton

Overview:

Implementation of singleton design pattern.

This module provides metaclasses and utilities for implementing singleton patterns in Python. It includes:

SingletonMeta: A metaclass for creating traditional singleton classes
ValueBasedSingletonMeta: A metaclass for creating value-based singleton classes
SingletonMark: A utility class for creating unique singleton marker objects

SingletonMeta

class hbutils.design.singleton.SingletonMeta[source]

Meta class for singleton mode.

This metaclass ensures that only one instance of a class is created throughout the application’s lifetime. Subsequent calls to the class constructor will return the same instance.

Example::

>>> class MyService(metaclass=SingletonMeta):
>>>     def get_value(self):
>>>         return 233
>>>
>>> s = MyService()
>>> s.get_value()    # 233
>>> s1 = MyService()
>>> s1 is s          # True

Note

In native singleton pattern, the constructor is not needed because only one instance will be created in the whole lifetime. So when SingletonMeta is used as metaclass, please keep the constructor be non-argument, or just ignore the __init__ function.

__call__()[source]

Override the call method to control instance creation.

Returns:: The singleton instance of the class.
Return type:: object

ValueBasedSingletonMeta

class hbutils.design.singleton.ValueBasedSingletonMeta[source]

Meta class for value based singleton mode.

This metaclass creates singleton instances based on a value parameter. Multiple calls with the same value will return the same instance, while different values will create different instances.

Example::

>>> class MyData(metaclass=ValueBasedSingletonMeta):
>>>     def __init__(self, value):
>>>         self.__value = value
>>>
>>>     @property
>>>     def value(self):
>>>         return self.__value
>>>
>>> d1 = MyData(1)
>>> d1.value       # 1
>>> d2 = MyData(1)
>>> d3 = MyData(2)
>>> d2 is d1       # True
>>> d2 is d3       # False

Note

This is an external case of singleton pattern. It can only contain one argument (must be positional-supported), which differs from the native singleton case.

__call__(value)[source]

Override the call method to control instance creation based on value.

Parameters:: value (Any hashable type) – The value used as key for singleton instance lookup.
Returns:: The singleton instance associated with the given value.
Return type:: object

SingletonMark

class hbutils.design.singleton.SingletonMark(value)[source]

Singleton mark for some situation.

Can be used when some default value is needed, especially when None has meaning which is not default. This class creates unique marker objects that can be used as sentinel values.

Example::

>>> NO_VALUE = SingletonMark("no_value")
>>> NO_VALUE is SingletonMark("no_value")  # True

Note

SingletonMark is a value-based singleton class, can be used to create an unique value, especially in the cases which None is not suitable for the default value.

__eq__(other) → bool[source]

Compare equality between singleton marks.

SingletonMark objects can be directly compared with ==.

Parameters:: other (Any) – The object to compare with.
Returns:: True if marks are equal, False otherwise.
Return type:: bool

Examples::

>>> mark1 = SingletonMark('mark1')
>>> mark1x = SingletonMark('mark1')
>>> mark2 = SingletonMark('mark2')
>>> mark1 == mark1
True
>>> mark1 == mark1x
True
>>> mark1 == mark2
False

__hash__() → int[source]

Compute hash value for the singleton mark.

SingletonMark objects are hash supported and can be directly used in dict and set.

Returns:: Hash value of the mark.
Return type:: int

__init__(mark: str)[source]

Constructor of SingletonMark, can create a singleton mark object.

Parameters:: mark (str) – The string identifier for this mark.

__repr__() → str[source]

Return string representation of the singleton mark.

When you try to print a SingletonMark object, its mark content will be displayed.

Returns:: String representation of the mark.
Return type:: str

Examples::

>>> mark1 = SingletonMark('mark1')
>>> print(mark1)
<SingletonMark 'mark1'>

property mark: str

Get mark string of this mark object.

Returns:: Mark string identifier.
Return type:: str