实现Singleton模式会导致TypeError:未绑定方法foobar()必须以Singleton实例作为第一个参数

Question

我正在尝试在Python(2.7)中实现Singleton模式。

我读了几个关于实现的帖子(1,2,3,4)，我想编写我自己的版本。一个我能理解的版本。I'm new to Python.)

所以我用一个方法创建了一个单例，这个方法将创建我的单个对象，它将在每次Singleton.Instance()调用时返回。

但是错误信息总是一样的:

Traceback (most recent call last):
  File "./test4.py", line 24, in <module>
    print id(s.Instance())
  File "./test4.py", line 15, in Instance
    Singleton._instance = Singleton._creator();
TypeError: unbound method foobar() must be called with Singleton instance as first argument (got nothing instead)

Here I roll:

class Singleton(object):
    _creator = None
    _instance = None
    def __init__(self, creator):
        if Singleton._creator is None and creator is not None:
            Singleton._creator = creator
    def Instance(self):
        if Singleton._instance is not None:
            return Singleton._instance
        Singleton._instance = Singleton._creator();
        return Singleton._instance;
def foobar():
    return "foobar"
s = Singleton( foobar )
print id(s.Instance())

为什么?更具体地说:我如何在Python中调用存储在类变量中的def方法?

Answer 1

问题是当你把它插入到类中时，Python会自动把它变成一个方法。您需要使它成为一个静态方法来避免这种情况。

class Singleton(object):
    _creator = None
    _instance = None
    def __init__(self, creator):
        if Singleton._creator is None and creator is not None:
            Singleton._creator = staticmethod(creator)
    def Instance(self):
        if Singleton._instance is not None:
            return Singleton._instance
        Singleton._instance = Singleton._creator();
        return Singleton._instance;
def foobar():
    return "foobar"
s = Singleton( foobar )
print id(s.Instance())