Convert lower case characters to capitals and capitals to lower case

Question

I want to convert the lower case of a given string to capital letters and the capital letters to lower case letters.

For example: Initial string: Hello World! and the resulting output must be hELLO wORLD!.

My current code is:

# Ask the use to give a string
user_string = input("Please give a string: ")
new_string = user_string

# For each character in the given string
    # Check if it is a capital letter and convert it to lower case.
    # Check if it is a lower case letter and convert it to capital.

for char in user_string:
    if char.islower():
       new_string[char]=
    if char.isupper():