Fixed a problem with diff'ing masked_arrays with units.

The issue occurs only, when "x" in first_derivative is a masked array
with units. As all arrays stemming from NetCDF access are basically
masked arrays, we do not have non-masked arrays in our code, even though
the mask is almost always 'False'.

I added a test case demonstrating the problem, it fails without the
contained fix.

See issue Unidata#983

src/metpy/units.py

@@ -175,7 +175,7 @@ def diff(x, **kwargs):
         # Can't just use units because of how things like temperature work
         it = x.flat
         true_units = (next(it) - next(it)).units
-        return ret * true_units
+        return true_units * ret
     else:
         return np.diff(x, **kwargs)
 

tests/calc/test_calc_tools.py

@@ -560,6 +560,20 @@ def test_first_derivative_masked():
     assert_array_equal(df_dx.mask, truth.mask)
 
 
+def test_first_derivative_masked_units():
+    """Test that first_derivative properly propagates masks with units."""
+    data = units('K') * np.ma.arange(7)
+    data[3] = np.ma.masked
+    x = units('m') * np.ma.arange(7)
+    df_dx = first_derivative(data, x=x)
+
+    truth = units('K / m') * np.ma.array(
+        [1., 1., 1., 1., 1., 1., 1.],
+        mask=[False, False, True, True, True, False, False])
+    assert_array_almost_equal(df_dx, truth)
+    assert_array_equal(df_dx.mask, truth.mask)
+
+
 def test_second_derivative(deriv_1d_data):
     """Test second_derivative with a simple 1D array."""
     d2v_dx2 = second_derivative(deriv_1d_data.values, x=deriv_1d_data.x)