diff --git a/experiment.py b/experiment.py
new file mode 100644
index 0000000..988974b
--- /dev/null
+++ b/experiment.py
@@ -0,0 +1,15 @@
+from tests.test_utils import *
+
+@Commentor(fmt=[
+    (type(None), lambda o: None),
+    
+])
+def function():
+    mystring = 'hello'
+    print(mystring)
+    mystring = "hello"
+    print(mystring)
+    return 1
+
+
+print(function())
diff --git a/tests/test_definitions.py b/tests/test_definitions.py
index ae89efe..7d11a7d 100644
--- a/tests/test_definitions.py
+++ b/tests/test_definitions.py
@@ -11,3 +11,26 @@ def test_function_def():
     def target():
         pass
 ''')
+
+
+def test_return():
+
+    with closing(StringIO()) as f:
+
+        @Commentor(f)
+        def target():
+            a = 1
+            return a + 1
+
+        assert target() == 2
+
+        asserteq_or_print(
+            f.getvalue(), '''
+        def target():
+            a = 1
+            return a + 1
+            """
+            1 : a
+            2 : a + 1
+            """
+''')
diff --git a/tests/test_utils.py b/tests/test_utils.py
index b6b9dad..37dfb9d 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,7 +1,11 @@
+from io import StringIO
+from contextlib import closing
 from trace_commentor import flags, Commentor
 
 def asserteq_or_print(value, ground_truth):
     if flags.DEBUG or flags.PRINT:
         print(value)
     else:
-        assert value == ground_truth.strip("\n"), "\n".join(["\n\n<<<<<<<< VALUE", value, "========================", ground_truth.strip("\n"), ">>>>>>>> GROUND\n"])
+        value = value.strip("\n").rstrip(" ")
+        ground_truth = ground_truth.strip("\n").rstrip(" ")
+        assert value == ground_truth, "\n".join(["\n\n<<<<<<<< VALUE", value, "========================", ground_truth, ">>>>>>>> GROUND\n"])
diff --git a/trace_commentor/commentor.py b/trace_commentor/commentor.py
index 14702b9..0d92adc 100644
--- a/trace_commentor/commentor.py
+++ b/trace_commentor/commentor.py
@@ -5,6 +5,7 @@ import rich
 
 from inspect import getfullargspec
 from functools import wraps
+from io import IOBase
 
 from . import handlers
 from . import formatters
@@ -51,9 +52,13 @@ class Commentor(object):
                 rich.print(code, file=sys.stderr)
             elif self.file == "<stdout>":
                 rich.print(code, file=sys.stdout)
-            else:
+            elif isinstance(self.file, IOBase):
+                rich.print(code, file=self.file)
+            elif type(self.file) == str:
                 with open(self.file, "wt") as f:
                     rich.print(code, file=f)
+            else:
+                raise NotImplementedError(f"Unknown file protocal {self.file}")
             return self._return
             # }
 
@@ -66,11 +71,16 @@ class Commentor(object):
             raise NotImplementedError(f"Unknown how to handle {node_type} node.")
         return handler(node, self)
 
-    def eval(self, node: ast.Expr):
+    def eval(self, node: ast.Expr, format=True):
         src = to_source(node)
         obj = eval(src, self._globals, self._locals)
+        if not format:
+            return obj
+            
         fmt = self.get_formatter(obj)
-        return f"{fmt(obj)} : {src}"
+        fmt_obj = fmt(obj)
+        if fmt_obj is not None:
+            return f"{fmt(obj)} : {src}"
     
     def exec(self, node: ast.stmt):
         src = to_source(node)
diff --git a/trace_commentor/handlers/__init__.py b/trace_commentor/handlers/__init__.py
index 5abfd4c..8b4a20e 100644
--- a/trace_commentor/handlers/__init__.py
+++ b/trace_commentor/handlers/__init__.py
@@ -1,4 +1,4 @@
-from .definitions import FunctionDef
+from .definitions import FunctionDef, Return
 from .statements import Pass, Assign
 from .expressions import Expr, BinOp, Call
 from .literals import Constant
diff --git a/trace_commentor/handlers/definitions.py b/trace_commentor/handlers/definitions.py
index fcf7dfd..2b4f24c 100644
--- a/trace_commentor/handlers/definitions.py
+++ b/trace_commentor/handlers/definitions.py
@@ -15,3 +15,8 @@ def FunctionDef(self, cmtor):
     cmtor.append_source()
         
     cmtor.indent -= flags.INDENT
+
+
+def Return(self, cmtor):
+    cmtor.process(self.value)
+    cmtor._return = cmtor.eval(self.value, format=False)