Returns the `Constant` at `index`, raising a KeyError if it
        does not exist.

def get(self, index):
        """
        Returns the `Constant` at `index`, raising a KeyError if it
        does not exist.
        """
        constant = self._pool[index]
        if not isinstance(constant, Constant):
            constant = _constant_types[constant[0]](self, index, *constant[1:])
            self._pool[index] = constant
        return constant