Python Base 36 encode and decode

base36.py

#!/usr/bin/env python
'''
Python Base 36 encode and decode
'''
__author__ = 'Moises P Sena <moisespsena@gmail.com>'
__version__ = 0.1

CHARS = ''.join(map(str, range(10))) + 'abcdefghijklmnopqrstuvwxyz'
L_CHARS = len(CHARS)

def encode(num):
    return (encode((num / L_CHARS) - 1) 
        if num >= L_CHARS
        else '') + CHARS[num % L_CHARS]

if __name__ == '__main__':
  print(encode(long(sys.argv[1])))

Can you explain how this is decoding base36 as I only see it encoding it to base36