try except

Parser/ll1.py

@@ -1,3 +1,5 @@
+import os
+# parse table
 parse_table = {
     ("E","("):["T","E'"],
     ("E","id"):["T","E'"],
@@ -13,22 +15,28 @@
     ("F","("):["(","#",")"],
     ("F","id"):["id"],
 }
-
+# inputting string
 string = input("Enter input string from chars [ id , + , * ] separating each char by white space : ").split(" ") + ["$"]
-
+# creating stack
 stack = ["E","$"]
-
+# printing stack operations
 print("STACK".center(15,' '),"INPUT".center(15,' '),"MATCHED".center(15,' '),"ACTION".center(15,' '))
 print("".ljust(15,'-'),"".rjust(15,'-'),"".center(15,'-'),"".center(15,'-'))
 print(" "*5+"".join(stack[::-1]).ljust(10,' '),"".join(string).rjust(12,' ')+" "*3,"".center(15,' '),"".center(15,' '))
-
 while len(stack)>1:
+# found a match
     if stack[0] == string[0]:
         x = stack[0]
         stack, string = stack[1:], string[1:]
         print(" "*5+"".join(stack[::-1]).ljust(10,' '),"".join(string).rjust(12,' ')+" "*3,x.center(15,' '),"Matched {0}".format(x).center(15,' '))
     else:
-        x, y = stack[0], parse_table[(stack[0],string[0])]
-        stack = parse_table[(stack[0],string[0])] + stack[1:]
-        if stack[0] == '#':stack = stack[1:]
-        print(" "*5+"".join(stack[::-1]).ljust(10,' '),"".join(string).rjust(12,' ')+" "*3,"".center(15,' '),"{0} -> {1}".format(x,"".join(y)).center(15,' '))
+# checking for substitution
+        try:
+            x, y = stack[0], parse_table[(stack[0],string[0])]
+            stack = parse_table[(stack[0],string[0])] + stack[1:]
+            if stack[0] == '#':stack = stack[1:]
+            print(" "*5+"".join(stack[::-1]).ljust(10,' '),"".join(string).rjust(12,' ')+" "*3,"".center(15,' '),"{0} -> {1}".format(x,"".join(y)).center(15,' '))
+        except:
+            os.system('clear')
+            print("Given string cannot be parsed by this parser. String does not belong to this grammar ")
+            break