diff --git a/pymath/operator.py b/pymath/operator.py
index 701486e..57466cc 100644
--- a/pymath/operator.py
+++ b/pymath/operator.py
@@ -49,9 +49,15 @@ class Operator(str):
         :returns: the string with operator and operands
 
         """
-        replacement = {"op"+str(i+1): ' '.join(self.add_parenthesis(op)) for (i,op) in enumerate(args)}
-        
-        ans = link.format(**replacement)
+        if self.arity == 1:
+            op1 = self.l_parenthesis(args[0], True)
+            ans = link.format(op1 = op1)
+
+        elif self.arity == 2:
+            op1 = self.r_parenthesis(args[0], True)
+            op2 = self.l_parenthesis(args[1], True)
+            ans = link.format(op1 = op1, op2 = op2)
+
         ans = save_mainOp(ans, self)
         return ans
 
@@ -127,24 +133,19 @@ class Operator(str):
         # TODO: Attention à gestion des fractions qui se comportent chelou avec les parenthèses |dim. nov.  9 09:21:52 CET 2014
         if self.arity == 1:
             # TODO: Marche juste avec -, il faudra voir quand il y aura d'autres operateurs unitaires |dim. nov.  9 09:24:53 CET 2014
-            op1 = self.add_parenthesis(args[0])
+            op1 = self.l_parenthesis(args[0])
             ans = flatten_list([self, op1])
 
         elif self.arity == 2:
-            op1 = self.add_parenthesis(args[0])
-            op2 = self.add_parenthesis(args[1])
+            op1 = self.l_parenthesis(args[0])
+            op2 = self.r_parenthesis(args[1])
             ans = flatten_list([op1, self, op2])
         
         ans = save_mainOp(ans, self)
         return ans
 
-    def add_parenthesis(self, op):
-        """ Add parenthesis if necessary 
-        
-        >>> from pymath.polynom import Polynom
-        >>> P = Polynom([1,2,3])
-        
-        """
+    def l_parenthesis(self, op, str_join=False):
+        """ Add parenthesis for left operand if necessary """
         try:
             if op.mainOp.priority < self.priority:
                 op = flatten_list(["(", op, ")"])
@@ -155,7 +156,14 @@ class Operator(str):
                     op = ['(', op, ')']
             except ValueError:
                 pass
-        return flatten_list([op])
+        ans = flatten_list([op])
+        if str_join:
+            ans = ' '.join([str(i) for i in ans])
+        return ans
+
+    def r_parenthesis(self, op, str_join=False):
+        """ Add parenthesis for left operand if necessary """
+        return self.l_parenthesis(op, str_join)
 
 def save_mainOp(obj, mainOp):
     """Create a temporary class build over built-in type to stock the main operation of a calculus
@@ -188,7 +196,8 @@ def operatorize(fun):
         * "_render": action use in __txt__ and __tex__
         * "__txt__": txt rendering
         * "__tex__": tex rendering
-        * "add_parenthesis": mechanism to add parenthesis
+        * "l_parenthesis": mechanism to add parenthesis for left operande
+        * "r_parenthesis": mechanism to add parenthesis for rigth operande
     """
     def mod_fun(self, *args):
         ans = fun(self, *args)
@@ -324,7 +333,7 @@ class op(object):
         >>> sub1.__tex__('-1')
         '- (-1)'
         """
-        def add_parenthesis(self, op):
+        def l_parenthesis(self, op, str_join=False):
             """ Add parenthesis if necessary """
             try:
                 if op.mainOp.priority <= self.priority:
@@ -336,7 +345,11 @@ class op(object):
                         op = ['(', op, ')']
                 except ValueError:
                     pass
-            return flatten_list([op])
+
+            ans = flatten_list([op])
+            if str_join:
+                ans = ' '.join([str(i) for i in ans])
+            return ans
 
         caract = {
             "operator" : "-", \
@@ -346,7 +359,7 @@ class op(object):
             "actions" : "__neg__",\
             "txt" :  "- {op1}",\
             "tex" :  "- {op1}",\
-            "add_parenthesis": add_parenthesis,\
+            "l_parenthesis": l_parenthesis,\
         }
 
         return caract
@@ -371,15 +384,17 @@ class op(object):
         # * can not be display in some cases
         def _render(self, link, *args):
 
-            replacement = {"op"+str(i+1): ' '.join(self.add_parenthesis(op)) for (i,op) in enumerate(args)}
+            op1 = self.r_parenthesis(args[0], True)
+            op2 = self.l_parenthesis(args[1], True)
+            ans = link.format(op1 = op1, op2 = op2)
 
-            if not self.visibility or args[1][0] == "(" or \
-                    (type(args[1][0]) == str and args[1][0].isalpha()):
-                ans = "{op1} {op2}".format(**replacement)
+            if not self.visibility or op2[0] == "(" or \
+                    (type(op2[0]) == str and op2[0].isalpha()):
+                ans = "{op1} {op2}".format(op1 = op1, op2 = op2)
                 ans = save_mainOp(ans, self)
                 return ans
             else:
-                ans = link.format(**replacement)
+                ans = link.format(op1 = op1, op2 = op2)
                 ans = save_mainOp(ans, self)
                 return ans