我试图了解如何在 Python 中创建 Singleton 类。以下是我的尝试
class Singleton(object):
_instance = None
def __new__(class_, *args, **kwargs):
if not isinstance(class_._instance, class_):
class_._instance = object.__new__(class_)
return class_._instance
class MyClass(Singleton):
num_of_instances = 0
def __init__(self, real = 5, imaginary = 6):
self.real = real
self.imaginary = imaginary
MyClass.num_of_instances += 1
a = MyClass(10, 20)
print(a.real)
print(a.imaginary)
b = MyClass()
print(MyClass.num_of_instances) # 2
理想情况下,__new__()
使用对象实例调用 __init__()
,但在上述情况下,当我尝试创建第二个对象时 b
>, __new__
不会被调用,因为 MyClass
的实例已经存在那么为什么 print 语句打印 num_of_instances
打印 2
?
最佳答案
每次 MyClass(...)
调用都会调用
__new__
。如果没有被调用,它将无法返回单例实例。
当__new__
方法返回一个对象并且该对象是传递给__new__
(或子类)的cls
参数的实例时,那么 __init__
方法也会被调用。
因此,对于每个 MyClass(...)
调用,都会调用 __new__
。 __new__
方法始终返回当前类的实例,因此每次都会调用 __init__
。这里每次都是同一个实例并不重要。
来自__new__
method documentation :
If
__new__()
returns an instance ofcls
, then the new instance’s__init__()
method will be invoked like__init__(self[, ...])
, whereself
is the new instance and the remaining arguments are the same as were passed to__new__()
.
如果您在方法中添加一些 print()
调用,您就会看到这种情况发生:
>>> class Singleton(object):
... _instance = None
... def __new__(class_, *args, **kwargs):
... print(f'Calling {class_!r}(*{args!r}, **{kwargs!r})')
... if not isinstance(class_._instance, class_):
... print(f'Creating the singleton instance for {class_!r}')
... class_._instance = object.__new__(class_)
... return class_._instance
...
>>> class MyClass(Singleton):
... num_of_instances = 0
... def __init__(self, real=5, imaginary=6):
... print(f'Calling {type(self)!r}.__init__(self, real={real!r}, imaginary={imaginary!r})')
... self.real = real
... self.imaginary = imaginary
... MyClass.num_of_instances += 1
...
>>> a = MyClass(10, 20)
Calling <class '__main__.MyClass'>(*(10, 20), **{})
Creating the singleton instance for <class '__main__.MyClass'>
Calling <class '__main__.MyClass'>.__init__(self, real=10, imaginary=20)
>>> b = MyClass()
Calling <class '__main__.MyClass'>(*(), **{})
Calling <class '__main__.MyClass'>.__init__(self, real=5, imaginary=6)
您无法阻止自动 __init__
调用,至少在不覆盖其他内容的情况下是这样。如果您想避免每次调用 __init__
,您有一些选择:
您不必在子类上使用 __init__
方法。您可以发明自己的机制,__new__
可以查找 __singleton_init__
方法并调用它:
class Singleton(object):
_instance = None
def __new__(class_, *args, **kwargs):
if not isinstance(class_._instance, class_):
class_._instance = object.__new__(class_)
if hasattr(class_._instance, '__singleton_init__'):
class_._instance.__singleton_init__(*args, **kwargs)`
return class_._instance
或者你的__init__
方法可以检查vars(self)
(或self.__dict__
)中是否已经设置了属性,但不是再次设置属性:
class MyClass(Singleton):
def __init__(self, real=5, imaginary=6):
if vars(self):
# we already set attributes on this instance before
return
self.real = real
self.imaginary = imaginary
__new__
和__init__
逻辑在type.__call__
中实现;你可以创建一个metaclass这超越了这个逻辑。虽然您可以简单地仅调用 __new__
(并保留所有内容),但让元类负责处理单例模式是有意义的:
class SingletonMeta(type):
def __new__(mcls, *args, **kwargs):
cls = super().__new__(mcls, *args, **kwargs)
cls._instance = None
return cls
def __call__(cls, *args, **kwargs):
if cls._instance is None:
cls._instance = super().__call__(*args, **kwargs)
return cls._instance
然后不要将其用作基类,而是与 metaclass=...
一起使用。如果更容易的话,您可以创建一个空基类:
class Singleton(metaclass=SingletonMeta):
pass
class MyClass(Singleton):
# ...
上面的代码将在类上调用 __new__
,可选地在结果实例上调用 __init__
,仅一次。然后,SingletonMeta.__call__
实现将永远返回单例实例,而无需进一步调用:
>>> class SingletonMeta(type):
... def __new__(mcls, *args, **kwargs):
... cls = super().__new__(mcls, *args, **kwargs)
... cls._instance = None
... return cls
... def __call__(cls, *args, **kwargs):
... print(f'Calling {cls!r}(*{args!r}, **{kwargs!r})')
... if cls._instance is None:
... cls._instance = super().__call__(*args, **kwargs)
... return cls._instance
...
>>> class Singleton(metaclass=SingletonMeta):
... pass
...
>>> class MyClass(Singleton):
... def __init__(self, real=5, imaginary=6):
... print(f'Calling {type(self)!r}.__init__(self, real={real!r}, imaginary={imaginary!r})')
... self.real = real
... self.imaginary = imaginary
...
>>> a = MyClass(10, 20)
Calling <class '__main__.MyClass'>(*(10, 20), **{})
Calling <class '__main__.MyClass'>.__init__(self, real=10, imaginary=20)
>>> MyClass()
Calling <class '__main__.MyClass'>(*(), **{})
<__main__.MyClass object at 0x10bf33a58>
>>> MyClass() is a
Calling <class '__main__.MyClass'>(*(), **{})
True
>>> MyClass().real
Calling <class '__main__.MyClass'>(*(), **{})
10
关于python - 在Python中创建Singleton类并计算实例数量,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52968491/