def extractDigits(num):
    d4 = num%10
    d3 = (num//10)%10
    d2 = (num//100)%10
    d1 = (num//1000)%10
    return (d1,d2,d3,d4)

def noRepeatingDigits(num):
    (d1,d2,d3,d4) = extractDigits(num)
    return not (d1 == d2 or d1 == d3 or d1 == d4 or d2 == d3 or d2 == d4 or d3 == d4)

# bulls and cows program. read user input 4-digit number with no repeats
# invalid input: (1) enter non-digits (2) number of digits is not 4 (3) no repeats
def readUserInput(gnum):
    s = input("Enter guess number "+str(gnum)+": ")
    while not s.isnumeric():
        print("Please enter only digits")
        s = input("Enter guess number "+str(gnum)+": ")
    num = int(s)
    while not noRepeatingDigits(num) or num < 1000 or num > 9999:
        print("Please provide 4-digit number with no repeats")
        s = input("Enter guess number "+str(gnum)+": ")
        while not s.isnumeric():
            print("Please enter only digits")
            s = input("Enter guess number "+str(gnum)+": ")
        num = int(s)
    return num  

readUserInput(5)

Please enter only digits

Please enter only digits

Please provide 4-digit number with no repeats

Please enter only digits

Please enter only digits

Please provide 4-digit number with no repeats

Please provide 4-digit number with no repeats

1234

s = "1234"
t = "1w34"
print(int(s))

1234

print(int(t))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-2-970374d8aa40> in <module>
----> 1 print(int(t))

ValueError: invalid literal for int() with base 10: '1w34'

s.isnumeric()

True

t.isnumeric()

False

u = "ab1d"

u.isalpha()

False

u.isalnum()

True

int("231")

231