If, else return else value even when the condition is true, inside a for loop

Here is the function i defined:

def count_longest(field, data):
    l = len(field)
    count = 0
    final  = 0
    n = len(data)
    for i in range(n):
        count = 0
        if data[i:i + l] is field:
            while data[i - l: i] == data[i:i + l]:
                count = count + 1
                i = i + 1
        else:
            print("OK")
        if final == 0 or count >= final:
            final = count
    return final

a = input("Enter the field - ")
b = input("Enter the data - ")
print(count_longest(a, b))

It works in some cases and given incorrect output in most cases. I checked by printing the strings being compared, and even after matching the requirement, the loop results in "OK" which is to be printed when the condition is not true! I don't get it! Taking the simplest example, if i enter 'as', when prompted for field, and 'asdf', when prompted for data, i should get count = 1, as the longest iteration of the substring 'as' is once in the string 'asdf'. But i still get final as 0 at the end of the program. I added the else statement just to check the if the condition was being satisfied, but the program printed 'OK', therefore informing that the if condition has not been satisfied. While in the beginning itself, data[0 : 0 + 2] is equal to 'as', 2 being length of the "field".