diff --git a/fatoptimizer/builtins.py b/fatoptimizer/builtins.py
index 4165251..06e455d 100644
--- a/fatoptimizer/builtins.py
+++ b/fatoptimizer/builtins.py
@@ -8,13 +8,13 @@
 
 
 def _chr_check_args(args):
-    code_point = args[0]
-    return 0 <= code_point <= 0x10ffff
+    #   args[0] => code_point
+    return 0 <= args[0] <= 0x10ffff
 
 
 def _complex_check_args(args):
-    code_point = args[0]
-    return 0 <= code_point <= 0x10ffff
+        #   args[0] => code_point
+    return 0 <= args[0] <= 0x10ffff
 
 
 def _divmod_check_args(args):
@@ -23,32 +23,31 @@ def _divmod_check_args(args):
 
 
 def _ord_check_args(args):
-    string = args[0]
-    return (len(string) == 1)
+    # param args[0] is a string
+    return (len(args[0]) == 1)
 
 
 def _bytes_check_args(args):
-    arg = args[0]
-    if not isinstance(arg, tuple):
+    if not isinstance(args[0], tuple):
         return True
-    return all(0 <= item <= 255 for item in arg)
+    return all(0 <= item <= 255 for item in args[0])
 
 
 def _pow_check_args(config, args):
-    num = args[0]
-    exp = args[1]
+    # num => args[0]
+    # exp => args[1]
     if len(args) >= 3:
-        mod = args[2]
+        # mod => args[2]
+        return check_pow(config, args[0], args[1], args[2])
     else:
-        mod = None
-    return check_pow(config, num, exp, mod)
+        # mod => None
+        return check_pow(config, args[0], args[1], None)
 
 
 def add_pure_builtins(config):
     def add(name, *args, **kw):
         func = getattr(builtins, name)
-        pure_func = PureFunction(func, name, *args, **kw)
-        config._pure_builtins[name] = pure_func
+        config._pure_builtins[name] = PureFunction(func, name, *args, **kw)
 
     ANY_TYPE = None
 
@@ -76,6 +75,7 @@ def add(name, *args, **kw):
     add('int', 1, FLOAT_TYPES + STR_TYPES,
         # catch ValueError for int('xyz')
         exceptions=ValueError)
+
     add('len', 1, ITERABLE_TYPES)
     add('list', 1, ITERABLE_TYPES)
     add('oct', 1, int)
diff --git a/fatoptimizer/config.py b/fatoptimizer/config.py
index 7ec2212..bdb577a 100644
--- a/fatoptimizer/config.py
+++ b/fatoptimizer/config.py
@@ -1,7 +1,8 @@
 import builtins
 
 from .tools import get_constant_size, ITERABLE_TYPES
-
+# ISSUE : SystemError: Parent module '' not loaded, cannot perform relative import
+# StackOverFlow search : http://stackoverflow.com/questions/16981921/relative-imports-in-python-3
 
 class Config:
     # FIXME: use dir()?
diff --git a/fatoptimizer/methods.py b/fatoptimizer/methods.py
index 5f37f44..8725f21 100644
--- a/fatoptimizer/methods.py
+++ b/fatoptimizer/methods.py
@@ -53,5 +53,6 @@ def add(obj_type, name, *args, **kw):
         check_args=check_encoding,
         exceptions=UnicodeEncodeError)
     # FIXME: add more str methods
-
+    add(str, 'upper', 0)
+    add(str,'lower', 0)
     # FIXME: tuple: count, index
diff --git a/test_fatoptimizer.py b/test_fatoptimizer.py
index a8badf6..72d200c 100644
--- a/test_fatoptimizer.py
+++ b/test_fatoptimizer.py
@@ -3206,6 +3206,13 @@ def test_str_encode(self):
         self.check_dont_optimize(r'"ab\xff".encode("big5")')
         self.check_dont_optimize(r'"ab\xff".encode("ascii", "backslashreplace")')
 
+    def test_str_upper(self):
+
+        self.check_optimize(r'"abc".upper()','"ABC"')
+
+    def test_str_lower(self):
+
+        self.check_optimize(r'"ABC".lower()','"abc"')
 
 if __name__ == "__main__":
     unittest.main()