Raise TypeError for non-datetime x_new

spencerkclark · spencerkclark · commit 5377b164fff4 · 2018-09-27T09:16:04.000-04:00
diff --git a/xarray/core/dataset.py b/xarray/core/dataset.py
@@ -1984,11 +1984,26 @@ def maybe_variable(obj, k):
             except KeyError:
                 return as_variable((k, range(obj.dims[k])))
 
+        def _validate_interp_indexer(x, new_x):
+            # In the case of datetimes, the restrictions placed on indexers
+            # used with interp are stronger than those which are placed on
+            # isel, so we need an additional check after _validate_indexers.
+            if (_contains_datetime_like_objects(x) and
+               not _contains_datetime_like_objects(new_x)):
+               raise TypeError('When interpolating over a datetime-like '
+                               'coordinate, the coordinates to '
+                               'interpolate to must be either datetime '
+                               'strings or datetimes. '
+                               'Instead got\n{}'.format(new_x))
+            else:
+                return (x, new_x)
+            
         variables = OrderedDict()
         for name, var in iteritems(obj._variables):
             if name not in indexers:
                 if var.dtype.kind in 'uifc':
-                    var_indexers = {k: (maybe_variable(obj, k), v) for k, v
+                    var_indexers = {k: _validate_interp_indexer(
+                        maybe_variable(obj, k), v) for k, v
                                     in indexers.items() if k in var.dims}
                     variables[name] = missing.interp(
                         var, var_indexers, method, **kwargs)
diff --git a/xarray/tests/test_interp.py b/xarray/tests/test_interp.py
@@ -555,3 +555,21 @@ def test_cftime_single_string():
     expected = xr.DataArray(0.5, coords={'time': times_new_array})
 
     assert_allclose(actual, expected)
+
+
+@requires_scipy
+def test_datetime_to_non_datetime_error():
+    da = xr.DataArray(np.arange(24), dims='time',
+                      coords={'time': pd.date_range('2000-01-01', periods=24)})
+    with pytest.raises(TypeError):
+        da.interp(time=0.5)
+
+
+@requires_cftime
+@requires_scipy
+def test_cftime_to_non_cftime_error():
+    times = xr.cftime_range('2000', periods=24, freq='D')
+    da = xr.DataArray(np.arange(24), coords=[times], dims='time')
+
+    with pytest.raises(TypeError):
+        da.interp(time=0.5)
