Thinking about the prisoners dilemma with iPython. This is an iPython notebook. I made it python to share it with people.

iterated-prisoners-dilemma.py

# -*- coding: utf-8 -*-
# <nbformat>3.0</nbformat>

# <headingcell level=1>

# The Game World

# <codecell>

def pd(previous_games, p1_fun, p2_fun):
    """Run one iteration of the prisoners dilemna"""
    p1_result = p1_fun(previous_games, player_identity='p1')
    p2_result = p2_fun(previous_games, player_identity='p2')
    
    previous_games.append({'p1': p1_result, 'p2': p2_result})
    return previous_games

   

# <codecell>

def ipd(p1_fun, p2_fun, iterations = 100):
    games = []
    for i in range(iterations):
        result = pd(games, p1_fun, p2_fun)
        games = result
        
    scores = score(games)
    # Make the players names readable
    scores['game'] = p1_fun.func_name + " vs " + p2_fun.func_name
    return scores

# <codecell>

def score_a_game(game):
    p1 = game['p1']
    p2 = game['p2']
    
    if p1 is True and p2 is True:
        return {'p1': 3, 'p2': 3}
    
    if p1 is True and p2 is False:
        return {'p1': 0, 'p2': 5}
    
    if p1 is False and p2 is False:
        return {'p1': 1, 'p2': 1}
    
    if p1 is False and p2 is True:
        return {'p1': 5, 'p2': 0}
    
    

# <codecell>

def score(games):
    # Convert game outcomes into scores
    scores = [ score_a_game(x) for x in games ]
    
    p1_score = 0
    p2_score = 0
    
    for score in scores:
        p1_score += score['p1']
        p2_score += score['p2']
    
    return {'p1': p1_score,
            'p2': p2_score}

# <headingcell level=1>

# Helper functions, imports, etc

# <codecell>

from random import choice
import itertools
import operator

def most_common(L):
  # get an iterable of (item, iterable) pairs
  SL = sorted((x, i) for i, x in enumerate(L))
  # print 'SL:', SL
  groups = itertools.groupby(SL, key=operator.itemgetter(0))
  # auxiliary function to get "quality" for an item
  def _auxfun(g):
    item, iterable = g
    count = 0
    min_index = len(L)
    for _, where in iterable:
      count += 1
      min_index = min(min_index, where)
    # print 'item %r, count %r, minind %r' % (item, count, min_index)
    return count, -min_index
  # pick the highest-count/earliest item
  return max(groups, key=_auxfun)[0]

# <headingcell level=1>

# The Players

# <codecell>

def tit_for_tat(previous_games, player_identity):
    """Just do whatever the opponent did to you last time. Cooperate the first time."""
    
    opponent = 'p1' if player_identity == 'p2' else 'p2'
    
    if len(previous_games) == 0:
        return True
    else:
        return previous_games[-1][opponent]

# <codecell>

def tat_for_tit(previous_games, player_identity):
    """Do the opposite of whatever the opponent did last time. Cooperate the first time."""
    opponent = 'p1' if player_identity == 'p2' else 'p2'
    
    if len(previous_games) == 0:
        return True
    
    if previous_games[-1][opponent]:
        return False
    else:
        return True

# <codecell>

def be_an_ass(previous_games, player_identity):
    """Dumb and mean."""
    return False

# <codecell>

def be_naive(previous_games, player_identity):
    """Cooperative, and pretty stupid."""
    return True

# <codecell>

def be_crazy(previous_games, player_identity):
    """I AM SO RANDOM"""
    return choice([True, False])

# <codecell>

def have_a_long_memory(previous_games, player_identity):
    """Evaluate all of the games played, and do what the opponent usually does. Cooperate the first time."""
    
    opponent = 'p1' if player_identity == 'p2' else 'p2'
    
    if len(previous_games) == 0:
        return True
    else:
        opponents_moves = [ x[opponent] for x in previous_games ] 
        return most_common(opponents_moves)

# <headingcell level=1>

# Let the games begin!

# <codecell>

# Get the players in a list
players = [tit_for_tat, tat_for_tit, be_an_ass, be_naive, be_crazy, have_a_long_memory]
matches = itertools.combinations_with_replacement(players, 2)
results = [ ipd(match[0], match[1]) for match in matches ]

# <codecell>

for result in results:
    print "%s: %s to %s" % (result['game'], result['p1'], result['p2'])