我只是试图简化我的一个类,并引入了一些与flyweight设计模式相同风格的功能。
然而,我有点困惑,为什么__init__总是在__new__之后被调用。我没想到会这样。有人能告诉我为什么会发生这种情况,我如何才能实现这个功能吗?(除了将实现放在__new__中,这感觉相当粗糙。)
这里有一个例子:
class A(object):
_dict = dict()
def __new__(cls):
if 'key' in A._dict:
print "EXISTS"
return A._dict['key']
else:
print "NEW"
return super(A, cls).__new__(cls)
def __init__(self):
print "INIT"
A._dict['key'] = self
print ""
a1 = A()
a2 = A()
a3 = A()
输出:
NEW
INIT
EXISTS
INIT
EXISTS
INIT
Why?
当__new__返回同一类的实例时,__init__随后在返回的对象上运行。也就是说,你不能使用__new__来阻止__init__被运行。即使你从__new__返回之前创建的对象,它也会被__init__一次又一次地初始化。
下面是单例模式的通用方法,它扩展了上面的vartec答案并修复了它:
def SingletonClass(cls):
class Single(cls):
__doc__ = cls.__doc__
_initialized = False
_instance = None
def __new__(cls, *args, **kwargs):
if not cls._instance:
cls._instance = super(Single, cls).__new__(cls, *args, **kwargs)
return cls._instance
def __init__(self, *args, **kwargs):
if self._initialized:
return
super(Single, self).__init__(*args, **kwargs)
self.__class__._initialized = True # Its crucial to set this variable on the class!
return Single
完整的故事在这里。
另一种方法,实际上涉及__new__,是使用类方法:
class Singleton(object):
__initialized = False
def __new__(cls, *args, **kwargs):
if not cls.__initialized:
cls.__init__(*args, **kwargs)
cls.__initialized = True
return cls
class MyClass(Singleton):
@classmethod
def __init__(cls, x, y):
print "init is here"
@classmethod
def do(cls):
print "doing stuff"
请注意,使用这种方法,你需要用@classmethod装饰你的所有方法,因为你永远不会使用MyClass的任何真实实例。
现在我又遇到了同样的问题,出于某些原因,我决定避免使用装饰器、工厂和元类。我是这样做的:
主文件
def _alt(func):
import functools
@functools.wraps(func)
def init(self, *p, **k):
if hasattr(self, "parent_initialized"):
return
else:
self.parent_initialized = True
func(self, *p, **k)
return init
class Parent:
# Empty dictionary, shouldn't ever be filled with anything else
parent_cache = {}
def __new__(cls, n, *args, **kwargs):
# Checks if object with this ID (n) has been created
if n in cls.parent_cache:
# It was, return it
return cls.parent_cache[n]
else:
# Check if it was modified by this function
if not hasattr(cls, "parent_modified"):
# Add the attribute
cls.parent_modified = True
cls.parent_cache = {}
# Apply it
cls.__init__ = _alt(cls.__init__)
# Get the instance
obj = super().__new__(cls)
# Push it to cache
cls.parent_cache[n] = obj
# Return it
return obj
示例类
class A(Parent):
def __init__(self, n):
print("A.__init__", n)
class B(Parent):
def __init__(self, n):
print("B.__init__", n)
在使用
>>> A(1)
A.__init__ 1 # First A(1) initialized
<__main__.A object at 0x000001A73A4A2E48>
>>> A(1) # Returned previous A(1)
<__main__.A object at 0x000001A73A4A2E48>
>>> A(2)
A.__init__ 2 # First A(2) initialized
<__main__.A object at 0x000001A7395D9C88>
>>> B(2)
B.__init__ 2 # B class doesn't collide with A, thanks to separate cache
<__main__.B object at 0x000001A73951B080>
警告:你不应该初始化父类,它会与其他类冲突——除非你在每个子类中定义了单独的缓存,这不是我们想要的。
警告:以Parent为祖父母的类似乎行为怪异。(未经证实的)
在网上试试!
当__new__返回同一类的实例时,__init__随后在返回的对象上运行。也就是说,你不能使用__new__来阻止__init__被运行。即使你从__new__返回之前创建的对象,它也会被__init__一次又一次地初始化。
下面是单例模式的通用方法,它扩展了上面的vartec答案并修复了它:
def SingletonClass(cls):
class Single(cls):
__doc__ = cls.__doc__
_initialized = False
_instance = None
def __new__(cls, *args, **kwargs):
if not cls._instance:
cls._instance = super(Single, cls).__new__(cls, *args, **kwargs)
return cls._instance
def __init__(self, *args, **kwargs):
if self._initialized:
return
super(Single, self).__init__(*args, **kwargs)
self.__class__._initialized = True # Its crucial to set this variable on the class!
return Single
完整的故事在这里。
另一种方法,实际上涉及__new__,是使用类方法:
class Singleton(object):
__initialized = False
def __new__(cls, *args, **kwargs):
if not cls.__initialized:
cls.__init__(*args, **kwargs)
cls.__initialized = True
return cls
class MyClass(Singleton):
@classmethod
def __init__(cls, x, y):
print "init is here"
@classmethod
def do(cls):
print "doing stuff"
请注意,使用这种方法,你需要用@classmethod装饰你的所有方法,因为你永远不会使用MyClass的任何真实实例。