Encrypting a message using secret key in python

you can use a simple dictionary

def my_encryption(some_string):
    character_set= "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 "
    secret_key=    "Dd18Abz2EqNPW hYTOjBvtVlpXaH6msFUICg4o0KZwJeryQx3f9kSinRu5L7cGM"
    table = {x: y for (x, y) in zip(character_set,secret_key )}
    return "".join( map(lambda x:table.get(x,x),some_string) )

the get method can receive 2 arguments, the first is the key to search and the second is a value to return in case that the key is not present, in this case assign that as x to leave that unchanged

here a test

>>> my_encryption("Lets meet at the usual place at 9 am")
'oABjMWAABMDBMB2AMvjvDPMYPD1AMDBMGMDW'
>>> 

this is usually the first thing that come to my mind when I want to do this simple substitution cipher.

and the inverse is as simple as invert key-value

def my_decription(some_string):
    character_set= "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 "
    secret_key=    "Dd18Abz2EqNPW hYTOjBvtVlpXaH6msFUICg4o0KZwJeryQx3f9kSinRu5L7cGM"
    table = {x: y for (x, y) in zip(character_set,secret_key )}
    return "".join( map(lambda x:table.get(x,x),some_string) )

>>> my_decription('oABjMWAABMDBMB2AMvjvDPMYPD1AMDBMGMDW')
'Lets meet at the usual place at 9 am'
>>> 

A simple solution I use when making things less plain text is base64 module. This is not encryption by any means. Just makes the text a little harder to read:

>>> import base64
>>> base64.b64encode(b'This is a secret.')
b'VGhpcyBpcyBhIHNlY3JldC4='
>>> base64.b64decode(b'VGhpcyBpcyBhIHNlY3JldC4=').decode('utf-8')
'This is a secret.'

def my_encryption(some_string):
     output_string=""
     index=0
     character_set = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 "
     secret_key    = "Dd18Abz2EqNPW hYTOjBvtVlpXaH6msFUICg4o0KZwJeryQx3f9kSinRu5L7cGM"
     for character in some_string:
        index=character_set.find(character)
        output_string=output_string+secret_key[index]
      return output_string