diff --git a/lib/spack/spack/util/unparse/unparser.py b/lib/spack/spack/util/unparse/unparser.py
index 9ca8eb90a94..bae05989768 100644
--- a/lib/spack/spack/util/unparse/unparser.py
+++ b/lib/spack/spack/util/unparse/unparser.py
@@ -413,6 +413,7 @@ def visit_Try(self, node):
                 self.dispatch(node.finalbody)
 
     def visit_TryExcept(self, node):
+        # this construct only exists in Python < 3.3
         self.fill("try")
         with self.block():
             self.dispatch(node.body)
@@ -425,7 +426,8 @@ def visit_TryExcept(self, node):
                 self.dispatch(node.orelse)
 
     def visit_TryFinally(self, node):
-        if len(node.body) == 1 and isinstance(node.body[0], ast.TryExcept):
+        # this construct only exists in Python < 3.3
+        if len(node.body) == 1 and isinstance(node.body[0], ast.TryExcept):  # type: ignore
             # try-except-finally
             self.dispatch(node.body)
         else: