Clarify index inference condition

Author: Zac-HD

xarray/core/combine.py

@@ -333,8 +333,11 @@ def _dataarray_concat(arrays, dim, data_vars, coords, compat,
         )
     datasets = [arr.rename(name)._to_temp_dataset() for arr in arrays]
 
-    if isinstance(dim, str) and len(set(names) - {None}) == len(names) \
-            and not any(dim in a.dims or dim in a.coords for a in arrays):
+    if (
+        isinstance(dim, str) 
+        and len(set(names) - {None}) == len(names)
+        and not any(dim in a.dims or dim in a.coords for a in arrays
+    ):
         # We're concatenating arrays with unique non-None names along
         # a new dimension, so we use the existing names as coordinates.
         dim = pd.Index(names, name=dim)