def on_assign(self, states, assign):
        """
        Called when the pass sees an ``ir.Assign``.

        Subclasses should override this for custom behavior

        Parameters
        -----------
        states : dict
        assign : numba.ir.Assign

        Returns
        -------
        stmt : numba.ir.Assign or None
            For rewrite passes, the return value is used as the replacement
            for the given statement.
        """

        Called when the pass sees an ``ir.Assign``.

        Subclasses should override this for custom behavior

        Parameters
        -----------
        states : dict
        assign : numba.ir.Assign

        Returns
        -------
        stmt : numba.ir.Assign or None
            For rewrite passes, the return value is used as the replacement
            for the given statement.
        
numba/core/ssa.py