Write a Python program to replace whitespaces with an underscore and vice versa.

Sure, here is a simple Python program that replaces whitespaces with underscores and vice versa:

def replace_characters(input_string):
    # Replace whitespaces with underscores
    string_with_underscores = input_string.replace(' ', '_')

    # Replace underscores with whitespaces
    string_with_spaces = string_with_underscores.replace('_', ' ')

    return string_with_spaces

# Test the function
print(replace_characters("Hello World"))

This program defines a function replace_characters that takes an input string, replaces all whitespaces with underscores using the replace method, then replaces all underscores with whitespaces. The result is then returned.

Please note that this program will first replace all whitespaces with underscores, and then immediately replace those underscores with whitespaces, effectively returning the original string. If you want to keep the underscores, you should return string_with_underscores instead of string_with_spaces.