在python类中将整数作为静态变量引用

Question

I'm trying to have a static int inside a class in python. But it doesn't work.

Here's an example of what I've implemented :

class MyDict(dict):

    STR_DEPTH = -1

    def __init__(self, **kwargs):
        super(MyDict, self).__init__(**kwargs)
        self.__dict__.update(name = kwargs.get("name", ""))

    def __str__(self):
        self.STR_DEPTH += 1
        res = self.name + '\n'
        for k in self.keys():
            res += '\t'*self.STR_DEPTH + k + " = "  + str(self[k])
            res += '\n'
        self.STR_DEPTH -= 1
        return res

def main():
    d1 = MyDict(one=MyDict())
    d1["two"] = 2
    d1["one"]["one"] = 1
    d1["one"]["two"] = MyDict(three=3)
    d1["four"] = 4
    print d1

if __name__ == "__main__":
    main()

and i'm expecting :

four = 4
two = 2
one = 
    two = 
        three = 3

    one = 1

but it doesn't work that way. If i'm not mistaking, int aren't references and it's not the same "STR_DEPTH" in every instances of my class.

I already know the list-of-length-1 trick and the empty-type trick, but do i really need to resort do clumsy unreadable trick ?

Isn't there a better way since i'm inside a class ?

Answer 1

Where you have:

    self.STR_DEPTH += 1

replace that with:

    MyDict.STR_DEPTH += 1

and the same where you decrement the value.

Assigning to self.STR_DEPTH will create a new instance variable which hides access to the class variable through self . You can use self.STR_DEPTH to access the class variable provided you don't have an instance variable of the same name, but if you want to rebind the class variable you have to refer to it directly.

Note that self.STR_DEPTH += 1 is really just shorthand for self.STR_DEPTH = self.STR_DEPTH + 1 so even if the right hand self.STR_DEPTH picks up the class variable the assignment still happens back to the instance variable.