Write a function that takes a string as input and returns the reversed string using a stack.

def reverse_string(input_string):
    stack = []
    reversed_string = ""

    # Push each character of the input string onto the stack
    for char in input_string:
        stack.append(char)

    # Pop characters from the stack to construct the reversed string
    while stack:
        reversed_string += stack.pop()

    return reversed_string

# Example usage:
input_str = "python"
reversed_str = reverse_string(input_str)
print(reversed_str)  # Output: "nohtyp"