In Python, without using built‑in functions, write a function to reverse a string.

To reverse a string in Python without built‑in functions, you can iterate over the string from the end to the beginning and build a new string character by character. One common approach is to use a while loop with an index that starts at the last character and decrements until it reaches the first. At each step, you concatenate the current character to a result string. This method avoids slicing or the reversed() built‑in. The time complexity is O(n) because each character is processed once, and the space complexity is also O(n) for the new string. Here’s a concise implementation:

def reverse_string(s):
    result = ''
    i = len(s) - 1
    while i >= 0:
        result += s[i]
        i -= 1
    return result

This function works for any string, including empty strings and Unicode characters. It demonstrates fundamental string manipulation and control flow, which are essential skills for a junior Python developer.