Returns an identifier string for the PauliTerm (ignoring the coefficient).

        Don't use this to compare terms. This function will not work with qubits that
        aren't sortable.

        :param sort_ops: Whether to sort operations by qubit. This is True by default for
            backwards compatibility but will change in a future version. Callers should never rely
            on comparing id's for testing equality. See ``operations_as_set`` instead.
        :return: A string representation of this term's operations.
        :rtype: string

def id(self, sort_ops=True):
        """
        Returns an identifier string for the PauliTerm (ignoring the coefficient).

        Don't use this to compare terms. This function will not work with qubits that
        aren't sortable.

        :param sort_ops: Whether to sort operations by qubit. This is True by default for
            backwards compatibility but will change in a future version. Callers should never rely
            on comparing id's for testing equality. See ``operations_as_set`` instead.
        :return: A string representation of this term's operations.
        :rtype: string
        """
        if len(self._ops) == 0 and not sort_ops:
            # This is nefariously backwards-compatibility breaking. There's potentially
            # lots of code floating around that says is_identity = term.id() == ''
            # Please use `is_identity(term)`!
            # Therefore, we only return 'I' when sort_ops is set to False, which is the newer
            # way of calling this function and implies the user knows what they're doing.
            return 'I'

        if sort_ops and len(self._ops) > 1:
            warnings.warn("`PauliTerm.id()` will not work on PauliTerms where the qubits are not "
                          "sortable and should be avoided in favor of `operations_as_set`.",
                          FutureWarning)
            return ''.join("{}{}".format(self._ops[q], q) for q in sorted(self._ops.keys()))
        else:
            return ''.join("{}{}".format(p, q) for q, p in self._ops.items())