Self-generating code (sub optimal)

import os, random, traceback

# Select two random files from the folder this file is in and read them. Make sure there are at least two files. This will work best if all files are executable python programs.
a, b = random.sample(os.listdir(os.path.dirname(os.path.realpath(__file__))), 2)
a = open(a).read()
b = open(b).read()

# Loop until an executable program is found
while 1:

    # Combinate the two files randomly
    ai = 1
    bi = 1
    s = ''
    while ai < len(a) or bi < len(a):

        # Append one byte at time
        if bi == len(b) or ai < len(a) and random.getrandbits(1):
            s += a[ai-1]
            ai += 1
        else:
            s += b[bi-1]
            bi += 1

    # After combining two files successfully, try executing new code
    try:
        exec(s)
        # If the execution happens to be successful, print the code and exit
        print s
        break
    except:
        # Most likely the execution will fail. Try again.
        traceback.print_exc()