The issue is in lib2to3. tuple_params does not work with lambdas. I confirm the bug.

$ ./python -m lib2to3 -f tuple_params lambda_tuple_params.py 
RefactoringTool: Refactored lambda_tuple_params.py
--- lambda_tuple_params.py      (original)
+++ lambda_tuple_params.py      (refactored)
@@ -1,3 +1,3 @@
 lambda (a, b), c: a + b + c
-def f((a, b), c): return a + b + c
+def f(xxx_todo_changeme, c): (a, b) = xxx_todo_changeme; return a + b + c
 
RefactoringTool: Files that need to be modified:
RefactoringTool: lambda_tuple_params.py