Feature/merge errormsg

doc/whats-new.rst

@@ -48,6 +48,8 @@ Enhancements
   helpful for avoiding file-lock errors when trying to write to files opened
   using ``open_dataset()`` or ``open_dataarray()``. (:issue:`2887`)
   By `Dan Nowacki <https://github.com/dnowacki-usgs>`_.
+- Better warning message when supplying invalid objects to ``xr.merge``
+  (:issue:`2948`).  By `Mathias Hauser <https://github.com/mathause>`_.
 - Added ``strftime`` method to ``.dt`` accessor, making it simpler to hand a
   datetime ``DataArray`` to other code expecting formatted dates and times.
   (:issue:`2090`). By `Alan Brammer <https://github.com/abrammer>`_ and

xarray/core/merge.py

@@ -533,9 +533,14 @@ def merge(objects, compat='no_conflicts', join='outer', fill_value=dtypes.NA):
     from .dataarray import DataArray
     from .dataset import Dataset
 
-    dict_like_objects = [
-        obj.to_dataset() if isinstance(obj, DataArray) else obj
-        for obj in objects]
+    dict_like_objects = list()
+    for obj in objects:
+        if not (isinstance(obj, (DataArray, Dataset, dict))):
+            raise TypeError("objects must be an iterable containing only "
+                            "Dataset(s), DataArray(s), and dictionaries.")
+
+        obj = obj.to_dataset() if isinstance(obj, DataArray) else obj
+        dict_like_objects.append(obj)
 
     variables, coord_names, dims = merge_core(dict_like_objects, compat, join,
                                               fill_value=fill_value)

xarray/tests/test_merge.py

@@ -67,6 +67,15 @@ def test_merge_alignment_error(self):
         with raises_regex(ValueError, 'indexes .* not equal'):
             xr.merge([ds, other], join='exact')
 
+    def test_merge_wrong_input_error(self):
+        with raises_regex(TypeError, "objects must be an iterable"):
+            xr.merge([1])
+        ds = xr.Dataset(coords={'x': [1, 2]})
+        with raises_regex(TypeError, "objects must be an iterable"):
+            xr.merge({'a': ds})
+        with raises_regex(TypeError, "objects must be an iterable"):
+            xr.merge([ds, 1])
+
     def test_merge_no_conflicts_single_var(self):
         ds1 = xr.Dataset({'a': ('x', [1, 2]), 'x': [0, 1]})
         ds2 = xr.Dataset({'a': ('x', [2, 3]), 'x': [1, 2]})