BUG: GH37635

Previously, the variable "cols" was created solely to concatenate
the lists "index" and "columns" when the "values" parameter was
left as None.  It did so in a way that it modified the variable
passed as "index" using list.extend(), affecting the caller's
namespace.

This change simply uses an anonymous list in place of the
variable "cols"

Author: Jake Stevens-Haas

pandas/core/reshape/pivot.py

@@ -447,13 +447,12 @@ def pivot(
 
     if values is None:
         if index is not None:
-            cols = com.convert_to_list_like(index)
+            index = com.convert_to_list_like(index)
         else:
-            cols = []
-        cols.extend(columns)
+            index = []
 
         append = index is None
-        indexed = data.set_index(cols, append=append)
+        indexed = data.set_index(index + columns, append=append)
     else:
         if index is None:
             index = [Series(data.index, name=data.index.name)]