[Tutor] Creating an Invalid Message for user

[Saba Usmani]

Hi,
I am meant to design code for a program that converts from binary number to decimal and vice versa. 
This is what i have so far:
print "Welcome to the binary and decimal converter"loop = Truewhile loop:    bord = raw_input("Enter b for binary or d decimal or exit to exit")    if bord == "b":        d = 0        b = 0        factor = 1;        b = raw_input ("Enter Binary Number:")        b=b.lstrip("0")        b = int(b)        while(b > 0):            if((int(b) % 10) == 1):                d += factor            b /= 10            factor = factor * 2        print "The Decimal Number is: ", d           elif bord == "d":        x=0        n=int(input('Enter Decimal Number: '))                x=n        k=[] # array        while (n>0):            a=int(float(n%2))            k.append(a)            n=(n-a)/2        k.append(0)        string=""        for j in k[::-1]:            string=string+str(j)        print('The binary Number for %d is %s'%(x, string))     elif bord == "exit" :        print "Goodbye"        loop = False
- This code does not recognize invalid inputs e.g in the binary to decimal conversion, so if I enter e.g 10021, not a binary number, it will not inform me,the user, that the input is invalid. The same problem occurs with the decimal to binary conversion - if i enter e.g 123&&gf I am not told to try again with a valid input  - how do I implement this in the code above
ThanksSaba 		 	   		  
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