python單例模式的五種實現方式

1.__new__特殊方法實現

class singleton:

def __new__(cls, *args, **kwargs):
if not hasattr(cls, '_instance'):
cls._instance = super(singleton, cls).__new__(cls)
return cls._instance
def __init__(self, name):
self.name = name
s1 = singleton('first')
s2= singleton('last')
print(s1 == s2)
>> true
print(s1.name)
>> last

2.裝飾器實現

def singleton(cls):

_instance = {}
def inner(*args, **kwargs):
if cls not in _instance:
_instance[cls] = cls(*args, **kwargs)
return _instance[cls]
return inner
@singleton
class test:
def __init__(self, name):
self.name = name
t1 = test('first')
t2 = test('last')
print(t1==t2)
>> true
print(t2.name)
>> first

class singleton:

def __init__(self, cls):
self._cls = cls
self._instance = {}
def __call__(self, *args):
if self._cls not in self._instance:
self._instance[self._cls] = self._cls(*args)
return self._instance[self._cls]
@singleton
class cls2:
def __init__(self, name):
self.name = name
cls1 = cls2('first')
cls2 = cls2('last')
print(id(cls1) == id(cls2))
>> true
print(cls1.name)
>> first

class singleton1(type):

def __init__(self, *args, **kwargs):

self.__instance = none

super(singleton1, self).__init__(*args, **kwargs)

def __call__(self, *args, **kwargs):

if self.__instance is none:

self.__instance = super(singleton1, self).__call__(*args, **kwargs)

return self.__instance

class c(metaclass=singleton1):

def __init__(self, name):

self.name = name

c1 = c('first')

c2 = c('last')

print(c1 == c2)

>> true

print(c2.name)

>> first

5.模組實現

python 的模組就是天然的單例模式，因為模組在第一次匯入時，會生成 .pyc 檔案，當第二次匯入時，就會直接載入 .pyc 檔案，而不會再次執行模組**。

#foo1.py

class singleton(object):
def foo(self):
pass
singleton = singleton()
#foo.py
from foo1 import singleton

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