decode_it.py

# Programming in Python 3.x

encoded = "IHPSLY PZ H NVVK IVF HUK OL'Z NVA H AVF"           # Your encoded string
encoded_as_list = list(encoded)                               # Convert string to list of characters.

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
alphabet_as_list = list(alphabet)

decoded_as_list=[]                                            # Create an empty list for our decoded characters.
decoded=""                                                    # Create an empty string for our decoded message.

for character in encoded_as_list:                             # Do something for each character.
    if character.isalpha():                                   # Do something only if character is from the slphabet.
        character_index = alphabet_as_list.index(character)   # Get the numerical location of the character in the alphabet list.
        character_index = character_index - 7                 # Subtract 7 from that index.
        if character_index < 0:                               # Do something if the index is less than zero.
            character_index = character_index + 26            # Add 26 so that way we esentially wrap to the end of the alphabet.
                                                              #     26+(-7) is equal to 26-7.
        new_character=alphabet_as_list[character_index]       # Get the character at the new index
        decoded_as_list.append(new_character)                 # Add that new character to our decoded list

    else:                                                     # We get here is the character was never in the alphabet, like a space or punctuation.
        decoded_as_list.append(character)                     # Add that non-alpha character to our decoded list.

# Finally convert our list of decoded characters into a string.
decoded=''.join(decoded_as_list)

# Now we will display that message in the commapt prompt
print(decoded)