Don't change concat_dims

Author: jsignell

xarray/backends/api.py

@@ -1607,6 +1607,9 @@ def open_mfdataset(
 
     paths1d: list[str | ReadBuffer]
     if combine == "nested":
+        if isinstance(concat_dim, str | DataArray) or concat_dim is None:
+            concat_dim = [concat_dim]  # type: ignore[assignment]
+
         # This creates a flat list which is easier to iterate over, whilst
         # encoding the originally-supplied structure as "ids".
         # The "ids" are not used at all if combine='by_coords`.
@@ -1655,7 +1658,7 @@ def open_mfdataset(
             # along each dimension, using structure given by "ids"
             combined = _nested_combine(
                 datasets,
-                concat_dim=concat_dim,
+                concat_dims=concat_dim,
                 compat=compat,
                 data_vars=data_vars,
                 coords=coords,

xarray/structure/combine.py

@@ -352,7 +352,7 @@ def _new_tile_id(single_id_ds_pair):
 
 def _nested_combine(
     datasets,
-    concat_dim,
+    concat_dims,
     compat,
     data_vars,
     coords,
@@ -364,9 +364,6 @@ def _nested_combine(
     if len(datasets) == 0:
         return Dataset()
 
-    if isinstance(concat_dim, str | DataArray) or concat_dim is None:
-        concat_dim = [concat_dim]
-
     # Arrange datasets for concatenation
     # Use information from the shape of the user input
     if not ids:
@@ -383,7 +380,7 @@ def _nested_combine(
     # Apply series of concatenate or merge operations along each dimension
     combined = _combine_nd(
         combined_ids,
-        concat_dims=concat_dim,
+        concat_dims=concat_dims,
         compat=compat,
         data_vars=data_vars,
         coords=coords,
@@ -598,10 +595,13 @@ def combine_nested(
     if mixed_datasets_and_arrays:
         raise ValueError("Can't combine datasets with unnamed arrays.")
 
+    if isinstance(concat_dim, str | DataArray) or concat_dim is None:
+        concat_dim = [concat_dim]
+
     # The IDs argument tells _nested_combine that datasets aren't yet sorted
     return _nested_combine(
         datasets,
-        concat_dim=concat_dim,
+        concat_dims=concat_dim,
         compat=compat,
         data_vars=data_vars,
         coords=coords,