api: fix zeroth order only and add test

devito/finite_differences/derivative.py

@@ -151,7 +151,7 @@ def _process_kwargs(cls, expr, *dims, **kwargs):
                 orders = kwargs.get('deriv_order', dims[0][1])
                 if dims[0][1] != orders:
                     raise ValueError("Two different values of `deriv_order`")
-                new_dims = tuple([dims[0][0]]*dims[0][1])
+                new_dims = tuple([dims[0][0]]*max(1, dims[0][1]))
             else:
                 # Single Dimension
                 orders = kwargs.get('deriv_order', 1)

tests/test_derivatives.py

@@ -662,6 +662,20 @@ def test_zero_spec(self):
         assert expr0.subs(drv0, drv1) == expr1
         assert expr1.subs(drv1, drv0) == expr0
 
+    def test_zero_fd_interp(self):
+        """
+        Test that zero-order derivatives are generating interpolation coefficients
+        """
+        grid = Grid((11,))
+        x, = grid.dimensions
+        f = Function(name="f", grid=grid, space_order=4)
+
+        assert Derivative(f, x, deriv_order=0).evaluate == f
+        expected = (-f.subs(x, x - x.spacing) / 16 + f.subs(x, x) * 9 / 16 +
+                    f.subs(x, x + x.spacing) * 9 / 16 - f.subs(x, x + 2*x.spacing) / 16)
+        finterp = Derivative(f, (x, 0), x0={x: x+x.spacing/2}).evaluate
+        assert simplify(finterp - expected) == 0
+
 
 class TestTwoStageEvaluation(object):