Returns mnemonics for a phone number

mnemonic.py

pad = {0: ['0'], 1: ['1'], 2: ['A', 'B', 'C'], 3: ['D', 'E', 'F'], 4: ['G', 'H', 'I'],\
        5: ['J', 'K', 'L'], 6:['M', 'N', 'O'], 7:['P', 'Q','R', 'S'], 8: ['T', 'U', 'V'],\
        9: ['W', 'X', 'Y', 'Z']}

def list_mnemonics(number):
    a = set()
    if number == '':
        a.add('')
        return a
    chars = pad[int(number[0])]
    for substring in list_mnemonics(number[1:]):
          for char in chars:
            a.add(char + substring)
    return list(a)