A really simple ponysort implementation

pony.py

import random

"""
My 'probably good enough' implementation of the ponysort.
Create enough buckets (rakes) for each rider. Iterate over the
riders and only add a rider to a rake if it is not already
there. Does not attempt to optimize spacing.

There is a case where the callup can not be created: if a rider
appears more than NUM_RIDERS/RAKE_SIZE times.
"""

ALPHABET = 'abcdefghijklmnopqrstuvwxyz'
NUM_RIDERS = 42
RAKE_SIZE = 5

# Generate NUM_RIDERS random riders in the form 'aaa', 'bbb' ...
riders = [random.choice(ALPHABET) * 3 for i in range(NUM_RIDERS)]

# Create enough rakes for all the riders
num_rakes = len(riders) // RAKE_SIZE
if len(riders) % RAKE_SIZE != 0:
    num_rakes += 1

callup = [[] for j in range(num_rakes)]

for rider in riders:
    for rake in callup:
        if rider not in rake and len(rake) < RAKE_SIZE:
            rake.append(rider)
            break

# Flatten the callup
callup = [i for l in callup for i in l]

# Test to make sure all riders are accounted for
assert(sorted(callup) == sorted(riders))

print(callup)