diff --git a/Calc.py b/Calc.py
index 1baec3e..24c2188 100644
--- a/Calc.py
+++ b/Calc.py
@@ -14,9 +14,23 @@ class Expression:
         return self.valeur
     
     def __str__(self):
-        
-        pass
-
+        if self.gauche is None and self.droit is None:
+            return str(self.valeur)
+        expr = ""
+        if self.gauche is not None:
+            if self.gauche.valeur in ['+', '*']:
+                expr += "(" + str(self.gauche) + ")"
+            else:
+                expr += str(self.gauche)
+        expr += " " + str(self.valeur) + " "
+        if self.droit is not None:
+            if self.droit.valeur in ['+', '*']:
+                expr += "(" + str(self.droit) + ")"
+            else:
+                expr += str(self.droit)
+        return expr
+    
+    
 def npi2tree(ch):
     ch = ch.split()
     Expr = Pile_chaine()
@@ -29,21 +43,10 @@ def npi2tree(ch):
     return Expr.sommet()
     
 
-
-
-
-def dfs_in(a):
-    if a == None:
-        return
-    dfs_in(a.gauche)
-    print(a.valeur)
-    dfs_in(a.droit)
-        
     
 ch1 = '7 4 3 * +'
 exp = npi2tree(ch1)
 
-exp.evalue()