diff --git a/xarray/core/concat.py b/xarray/core/concat.py
index f492269b916..67023c800f1 100644
--- a/xarray/core/concat.py
+++ b/xarray/core/concat.py
@@ -613,7 +613,11 @@ def get_indexes(name):
                 # do not concat if:
                 # 1. variable is only present in one dataset of multiple datasets and
                 # 2. dim is not a dimension of variable
-                if dim not in variables[0].dims and len(variables) == 1 and len(datasets) > 1:
+                if (
+                    dim not in variables[0].dims
+                    and len(variables) == 1
+                    and len(datasets) > 1
+                ):
                     combined_var = variables[0]
                 # only concat if variable is in multiple datasets
                 # or if single dataset (GH1988)