How is the usage of chr(122 - (ord(char) - 97))

I'm not understanding how this line chr(122 - (ord(char) - 97)) will work.

def encrypt_message(message):
    encrypted_message = ""
    for char in message:
        if char.isalpha():
            encrypted_char = chr(122 - (ord(char) - 97))         
            encrypted_message += encrypted_char

This will replace a letter with it's "symmetric" letter from the other end of the alphabet i.e. a = z, b = y, c = x, etc...

So how does this work?

1. We need to measure the "distance" from our original letter to a. We do this by looking at the ord ord('a') happens to be 97.
2. We then move that distance away from z. ord('z') happens to 122. This will get the ord of our replacement letter
3. Once we have the ord, we take the chr to get the actual letter.

Basically, for readability, the code could be expressed as chr(ord(z) - (ord(char) - ord('a'))), but replacing the ord('z') and ord('a') with their actual values save some calculation. The most optimal would be to simplify the math as chr(219- ord(char)).

Basically this line of code gives the mirror letter of the alphabet...

You can inspect this line of code with the following:

characters = 'abcdefghijklmopqrstuvwxyz'

enc_string = ''
for char in characters:
    encrypted_char = chr(122 - (ord(char) - 97))
    enc_string += encrypted_char

print(enc_string)

Which returns this:

zyxwvutsrqponlkjihgfedcba

Which is exactly as John pointed out in the comment. The characters have been changed to the mirror letters in the alphabet.

• a becomes z
• b becomes y
  . . .

and so on...