raffle.py: Randomly choose N unique entries from a list

Raffle script

README.md

raffle.py

This script will run a raffle with the following rules:

1. A predefined number of winners will be selected
2. People are allowed to have multiple entries in the raffle
3. However, each person can only win once
4. Drawings are made at random

Mathematical Properties

It's possible to work out your statistical odds of winning based on the number of winners, how many entries you have, and the total number of entries. I'll leave that as an exercise for the reader.

raffle.py

#!/usr/bin/env python
import random

# ------------------
#  Raffle algorithm
# ------------------

def raffle(number_of_winners, entries, random_source):   
    

    winners = set() # hold unique values only
    hat = list(entries)

    while len(winners) < number_of_winners:
        hat_size = len(hat)
        if hat_size == 0:
            raise ValueError("Not enough unique entries for {} winners".format(number_of_winners))

        random_index = random_source.randrange(hat_size)
        drawn_entry = hat.pop(random_index) # get and remove random_index
        winners.add(drawn_entry)

    return winners


# ------------------------
#  Command-line interface
# ------------------------

USAGE="""USAGE: raffle.py number_of_winners [entries_file...]

    number_of_winners (required):
        The number of unique winners for the raffle
    entries_file (optional, repeated):
        Text file(s) with one raffle entry per line
        If none given, entries will be read from standard input (`-`)
                 
"""

if __name__ == '__main__':
    import sys
    import os
    import fileinput

    number_of_winners = 0;
    montecarlo = False;
    monte_carlo_runs  = 0;
    i = 1;
    loops = 1;

    try:
        while i < len(sys.argv):
            if sys.argv[i] == "-montecarlo":
                montecarlo = True;
                loops = int(sys.argv[i+1])
                i += 1
            elif number_of_winners == 0:
                number_of_winners = int(sys.argv[i])
            else:
                entry_files = sys.argv[i]
            i += 1;
    except (IndexError, ValueError) as e:
        sys.stderr.write(USAGE)
        sys.exit(1)

    entries = [line.strip() for line in fileinput.input(files=entry_files)]
    names = {x: 0 for x in list(entries)}

    seed = os.environ.get('RANDOM_SEED')
    random_source = random.Random(seed) if seed else random.SystemRandom()

    #sys.stderr.write("Selecting {} winners from {}...\n".format(number_of_winners, entries))
    for i in range(loops):
        winners = raffle(number_of_winners, entries, random_source)
        for i in range(number_of_winners):
            next_winner = winners.pop()
            new_value = names.get(next_winner) + 1
            names.update({next_winner: new_value})


    for k, v in names.items():
        print(k, '{0:.2f}%'.format((float(v)*100)/float(loops)))