This python code converts integers to strings and strings to ints without the inbuilt functions "int()" and "str()"
Keep it simple
Your functions are way too complicated. First, you can convert a single digit from integer to character by using ord() and the fact that the ordinals of the characters '0' to '9' are consecutive. Furthermore, you don't need to calculate the length of an integer if you reverse the way you build up the string. Here are example implementations that are simpler:
def str_to_int(num_str):
    num = 0

    for char in num_str:
        num *= 10
        num += ord(char) - ord('0')

    return num

def int_to_str(num_int):
    word = &quot;&quot;

    while num_int:
        word = chr(ord('0') + num_int % 10) + word
        num_int //= 10

    return word or &quot;0&quot;

Integers can be negative
Your code (and my example simplification above) does not handle negative integers.