Calculate the entropy of a password that would be generated.

def generated_password_entropy(self) -> float:
        """Calculate the entropy of a password that would be generated."""
        characters = self._get_password_characters()
        if (
                self.passwordlen is None
                or not characters
        ):
            raise ValueError("Can't calculate the password entropy: character"
                             " set is empty or passwordlen isn't set")

        if self.passwordlen == 0:
            return 0.0

        return calc_password_entropy(self.passwordlen, characters)