Python条件对象实例化

Question

我正在参加在线 MOOC 类(class)，但我很难弄清楚这一点，甚至无法准确地表达我正在试图弄清楚的内容。问题是要求仅当某个字符串作为参数传入时才创建对象。您可以在此处查看问题的描述:https://docs.google.com/forms/d/1gt4McfP2ZZkI99JFaHIFcP26lddyTREq4pvDnl4tl0w/viewform?c=0&w=1具体部分在第三段。使用“if”作为 init 的条件是否合法？谢谢。

Answer 1

用途:

def __new__( cls, *args):

而不是

def __init__( self, *args):

参见abort instance creation尤其是 new and init 已接受的答案

编辑:我自己添加了以下代码作为其工作原理的更简单示例 - 在现实生活场景中您需要的不仅仅是这个:

class MyClass:
    def __new__(cls,**wargs):
        if "create" in wargs: # This is just an example, obviously
            if wargs["create"] >0: # you can use any test here
                # The point here is to "forget" to return the following if your
                # conditions aren't met:
                return super(MyClass,cls).__new__(cls)
        return None
    def __init__(self,**wargs): # Needs to match __new__ in parameter expectations
        print ("New instance!")
a=MyClass()         # a = None and nothing is printed
b=MyClass(create=0) # b = None and nothing is printed
c=MyClass(create=1) # b = <__main__.MyClass object> and prints "New instance!"

__new__ 在实例创建之前被调用，与 __init__ 不同，它返回一个值 - 该值是实例。有关详细信息，请参阅上面的第二个链接 - 那里有代码示例，可以借用其中之一:

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