Python class of a continuous label-to-integer mapping that converts labels to integer indices and automatically handles new labels

int_label.py

class IntLabel():

    def __init__(self):
        self.ints = {}
        self.keys = []
        super().__init__()

    def __getitem__(self, key):
        try:
            return self.ints[key]
        except KeyError:
            i = len(self.ints)
            self.keys.append(key)
            self.ints[key] = i
            return i

    def inv(self,i):
        return self.keys[i]

    def __str__(self):
        return str(self.ints)