[Tutor] Recognising Errors

[Saba Usmani]

Hi,
I have designed some code, but I want it to be able to recognize invalid inputs - such as a binary number with more than 8 digits or non-binary values. What do I have to add and where do I add it?
print "Welcome to the binary -> decimal / decimal -> binary converter!"loop = Truewhile loop:    choice = raw_input("Enter b to convert from binary to decimal, d to convert from decimal to binary or e to exit")    if choice == "b":        decimal_num = 0        binary_num = 0        factor = 1;        binary_num = raw_input ("Enter Binary Number:")        binary_num=binary_num.lstrip("0")        binary_num = int(binary_num)        while(binary_num > 0):            if((int(binary_num) % 10) == 1):                decimal_num += factor            binary_num /= 10            factor = factor * 2        print "The Decimal Equivalent is: ", decimal_num           elif choice == "d":        z=0        n=int(input('Enter Decimal Number: '))                z=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 Equivalent is %d is %s'%(z, string))     elif choice == "e" :        print "Thanks For Using This Converter!"        loop = False

If for some reason you can't read this code properly as outlook has formatted it to look messy/cluttered; you do not have to respond. 
Thanks 		 	   		  
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