Merge pull request #306 from MSeifert04/arit_with_quantities_fix

fixes add/subtract arithmetics with quantities of different unit.

CHANGES.rst

@@ -30,6 +30,10 @@ Other Changes and Additions
 Bug Fixes
 ^^^^^^^^^
 
+- Adding/Subtracting a CCDData instance with a Quantity with a different unit
+  produced wrong results. [#291]
+
+
 0.3.2 (unreleased)
 ------------------
 

ccdproc/ccddata.py

@@ -321,7 +321,14 @@ def _ccddata_arithmetic(self, other, operation, scale_uncertainty=False):
         # and a Quantity is currently broken, but it works with two Quantity
         # arguments.
         if isinstance(other, u.Quantity):
-            other_value = other.value
+            if (operation.__name__ in ['add', 'subtract'] and
+                    self.unit != other.unit):
+                # For addition and subtraction we need to convert the unit
+                # to the same unit otherwise operating on the values alone will
+                # give wrong results (#291)
+                other_value = other.to(self.unit).value
+            else:
+                other_value = other.value
         elif isinstance(other, numbers.Number):
             other_value = other
         else:

ccdproc/tests/test_ccddata.py

@@ -410,6 +410,29 @@ def test_arithmetic_overload_ccddata_operand(ccd_data):
                                expected_uncertainty)
 
 
+def test_arithmetic_overload_differing_units():
+    a = np.array([1, 2, 3]) * u.m
+    b = np.array([1, 2, 3]) * u.cm
+    ccddata = CCDData(a)
+
+    # TODO: Could also be parametrized.
+    res = ccddata.add(b)
+    np.testing.assert_array_almost_equal(res.data, np.add(a, b).value)
+    assert res.unit == np.add(a, b).unit
+
+    res = ccddata.subtract(b)
+    np.testing.assert_array_almost_equal(res.data, np.subtract(a, b).value)
+    assert res.unit == np.subtract(a, b).unit
+
+    res = ccddata.multiply(b)
+    np.testing.assert_array_almost_equal(res.data, np.multiply(a, b).value)
+    assert res.unit == np.multiply(a, b).unit
+
+    res = ccddata.divide(b)
+    np.testing.assert_array_almost_equal(res.data, np.divide(a, b).value)
+    assert res.unit == np.divide(a, b).unit
+
+
 def test_ccddata_header_does_not_corrupt_fits(ccd_data, tmpdir):
     # This test is for the problem described in astropy/ccdproc#165
     # The issue comes up when a long FITS keyword value is in a header