pep8

        return a

    def evaluate_dp(self, dx, dydx):

        """Evalate arc length along the trace given trace polynomial coefficients

        # """Evalate arc length along the trace given trace polynomial coefficients

        Parameters

        ----------

        dx : array-like

            x pixel to evaluate

        # Parameters

        # ----------

        # dx : array-like

        #     x pixel to evaluate

        dydx : array-like

            Coefficients of the trace polynomial

        # dydx : array-like

        #     Coefficients of the trace polynomial

        Returns

        -------

        dp : array-like

            Arc length along the trace at position `dx`.

        # Returns

        # -------

        # dp : array-like

        #     Arc length along the trace at position `dx`.

        For `dydx` polynomial orders 0, 1 or 2, integrate analytically.

        Higher orders must be integrated numerically.

        # For `dydx` polynomial orders 0, 1 or 2, integrate analytically.

        # Higher orders must be integrated numerically.

        **Constant:**

            .. math:: dp = dx

        # **Constant:**

        #     .. math:: dp = dx

        **Linear:**

            .. math:: dp = \sqrt{1+\mathrm{DYDX}[1]}\cdot dx

        # **Linear:**

        #     .. math:: dp = \sqrt{1+\mathrm{DYDX}[1]}\cdot dx

        **Quadratic:**

            .. math:: u = \mathrm{DYDX}[1] + 2\ \mathrm{DYDX}[2]\cdot dx

        # **Quadratic:**

        #     .. math:: u = \mathrm{DYDX}[1] + 2\ \mathrm{DYDX}[2]\cdot dx

            .. math:: dp = (u \sqrt{1+u^2} + \mathrm{arcsinh}\ u) / (4\cdot \mathrm{DYDX}[2])

        #     .. math:: dp = (u \sqrt{1+u^2} + \mathrm{arcsinh}\ u) / (4\cdot \mathrm{DYDX}[2])

        """

        # """

        # dp is the arc length along the trace

        # $\lambda = dldp_0 + dldp_1 dp + dldp_2 dp^2$ ...