Error concat dataset and dataarray (#5425)

* add type checking for datasets concatenation

* add type checking for dataarrays concatenation

* add concatenation type checking test

* edit type checking for datasets concatenation

* fixing W293 concat.py

* edit error message

* edit error message

* fix typos

* fix typos

* fix linting

* add trailing commas

* Update whats-new.rst

* fix linting

* fix linting

Author: thomashirtz

doc/whats-new.rst

@@ -29,6 +29,8 @@ New Features
   values if inputs are dask arrays (:issue:`4804`, :pull:`5284`).
   By `Andrew Williams <https://github.com/AndrewWilliams3142>`_.
 
+- Attempting to ``concat`` list of elements that are not all ``Dataset`` or all ``DataArray`` now raises an error (:issue:`5051`, :pull:`5425`).
+  By `Thomas Hirtz <https://github.com/thomashirtz>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

xarray/core/concat.py

@@ -422,6 +422,13 @@ def _dataset_concat(
     """
     from .dataset import Dataset
 
+    datasets = list(datasets)
+
+    if not all(isinstance(dataset, Dataset) for dataset in datasets):
+        raise TypeError(
+            "The elements in the input list need to be either all 'Dataset's or all 'DataArray's"
+        )
+
     dim, coord = _calc_concat_dim_coord(dim)
     # Make sure we're working on a copy (we'll be loading variables)
     datasets = [ds.copy() for ds in datasets]
@@ -541,8 +548,15 @@ def _dataarray_concat(
     join: str = "outer",
     combine_attrs: str = "override",
 ) -> "DataArray":
+    from .dataarray import DataArray
+
     arrays = list(arrays)
 
+    if not all(isinstance(array, DataArray) for array in arrays):
+        raise TypeError(
+            "The elements in the input list need to be either all 'Dataset's or all 'DataArray's"
+        )
+
     if data_vars != "all":
         raise ValueError(
             "data_vars is not a valid argument when concatenating DataArray objects"

xarray/tests/test_concat.py

@@ -730,3 +730,20 @@ def test_concat_preserve_coordinate_order():
     for act, exp in zip(actual.coords, expected.coords):
         assert act == exp
         assert_identical(actual.coords[act], expected.coords[exp])
+
+
+def test_concat_typing_check():
+    ds = Dataset({"foo": 1}, {"bar": 2})
+    da = Dataset({"foo": 3}, {"bar": 4}).to_array(dim="foo")
+
+    # concatenate a list of non-homogeneous types must raise TypeError
+    with pytest.raises(
+        TypeError,
+        match="The elements in the input list need to be either all 'Dataset's or all 'DataArray's",
+    ):
+        concat([ds, da], dim="foo")
+    with pytest.raises(
+        TypeError,
+        match="The elements in the input list need to be either all 'Dataset's or all 'DataArray's",
+    ):
+        concat([da, ds], dim="foo")