They say, (deep voice) "Never roll your own cryptography!". And that's right!
Except for one thing, how does one learn if (s)he's not allowed to make mistakes?

WARNING: weak cryptography ahead! Use weakCryptogra.py as a sandbox for introductory courses and experiments.

The code has been entirely rewritten but it's heavily inspired by Al Sweigart's wonderful series of books. Particularly Cracking Codes with Python.
The repository stores a copy of CrackingCodesFiles.zip from the same website.

Disclaimer: the files contained herewith are clearly an implementation of weak cryptographic functions.
Use them to try and learn as you please. Never use them for any serious, real-life applications!

>>> import weakCryptogra

>>> weakCryptogra.reverse('Hello, world!')
'!dlrow ,olleH'

>>> weakCryptogra.caesar('Hello, world!', 17)
'YvCCF{ NFICu='

>>> weakCryptogra.columnar('Hello, world!', 3)
'Hl r!eowll,od'

>>> import random
>>> alphabet = 'abcdefghijklmnopqrstuvwxyz'
>>> key = list(alphabet)
>>> random.shuffle(key)
>>> key
['d', 'q', 'u', 't', 'n', 'w', 'g', 'k', 'm', 'o', 'c', 'f', 'y', 's', 'x', 'p', 'j', 'e', 'b', 'z', 'r', 'a', 'v', 'l', 'h', 'i']
>>> weakCryptogra.substitution('Hello!', key)
'Hrxxj!'
>>> weakCryptogra.substitution('Hrxxj!', key, 1)
'Hello!'