def derivative(self, x, der=<NUM_LIT:1>):
x, x_shape = self._prepare_x(x)<EOL>y = self._evaluate_derivatives(x, der+<NUM_LIT:1>)<EOL>return self._finish_y(y[der], x_shape)<EOL>
Evaluate one derivative of the polynomial at the point x

Parameters
----------
x : array-like
    Point or points at which to evaluate the derivatives

der : integer, optional
    Which derivative to extract. This number includes the
    function value as 0th derivative.

Returns
-------
d : ndarray
    Derivative interpolated at the x-points.  Shape of d is
    determined by replacing the interpolation axis in the
    original array with the shape of x.

Notes
-----
This is computed by evaluating all derivatives up to the desired
one (using self.derivatives()) and then discarding the rest.