A Python Model for Ping Pong Matches

See Python: Tips and Tricks for similar articles.

A while back, I started playing table tennis more seriously. I’m now in sort of a no-man’s land, in which I can beat most people who consider it a basement game, but can’t beat very many people who consider it a sport.

Because I’d rather write Python code than practice against my robot, I’ve created a model in Python to try to get an idea of what I should practice most to win more games. I’m unlikely to follow through on the practice, but I am likely to refine the model if people have ideas of how it could be made better. So, if there are any ping pong programmers out there, please feel free to make suggestions.

Before we get into the Python code, let me explain a little about table tennis.

Relevant Table Tennis Rules:

  1. Matches are best of 5 games.
  2. Games are played to 11 points, but you must win by 2.
  3. The server changes every two points up until deuce deuce (11-11), at which point it changes every point.
  4. The first to serve in the first game is chosen randomly and then alternated each subsequent game.
  5. In table tennis, you only get one fault.
If your serve doesn’t go in, you lose the point.

How Points Play Out

The server tries to make it difficult for his opponent to return the serve well. Ideally, the opponent will not be able to return the serve at all, but if he does, the server hopes the return is weak enough so that he can easily put the third ball away. You can think of a point like this:

  1. Server serves
  2. Returner returns serve
  3. Server attempts putaway
  4. Returner volleys
  5. Server volleys
  6. Returner volleys

Once the volley has lasted for five hits, the odds of a player successfully returning a shot probably don’t change much with each successive hit.

So, when two players face each other, you can create a model to show how the matches are likely to come out based on the players’ likelihood to fail on any given shot. For example, imagine these two players failure rates when they play each other:

LeiraNat
Serve.05.1
Return of Serve.1.5
3rd Shot.3.25
4th Shot.2.4
5th Shot.2.25
6th Shot.2.25
  • When Leira serves first, he almost always gets it in.
  • Nat has a lot of trouble with Leira’s serve. He hits half of them into the net or off the table.
  • When Nat does get it back, he sets Leira up for a putaway. And Leira generally goes for it, but he hits it out 30% of the time.
  • As per above, Leira’s 3rd shot is often, but not always, an attempted putaway. Nat fails to return that shot about 40% of the time.
  • If it gets to a fifth shot, the point becomes more of a volley and the chances of a putaway or an error on any given shot are lower. Leira only fails to return the fifth and subsequent shots 20% of the time.
  • At this point in the volley, Nat fails to return shots 25% of the time.
  • When Nat serves first, he gets 90% of his serves in.
  • Leira has little trouble returning Nat’s serve. He gets 90% of them back.
  • Nat doesn’t often set himself up for the 3rd shot kill, so he moves right to volley mode, in which he fails to return shots 25% of the time.
  • Leira is in volley mode at this point too and fails to return shots 20% of the time.

So, let’s consider our players in Python:

# Leira
player.name = 'Leira'
player.fail_rates = [.05, .1, .3, .2]

# Nat
player.name = 'Nat'
player.fail_rates = [.1, .5, .25, .4, .25]

My program assumes that the fail rate for all hits after the last one indicated in the player’s fail_rates list is the same as the fail rate for the last one listed. So, because Leira has the same fail rate for shots 4, 5, and 6 and all subsequent shots, we only have to indicate the fail rate for his first four shots.

So, those are my assumptions. Here’s the code:

import random


class Player:
    def __init__(self, name, fail_rates):
        """Creates a new player

        Keyword arguments:
        name (str) -- Player's name
        fail_rates (list of floats) -- Fail rates of each shot in point
        """
        self.name = name
        self.fail_rates = fail_rates

    def get_fail_rate(self, hit_num):
        """Returns the fail rate based on the hit number in the point.
        If fail_rates doesn't go up that high, it returns the last
        element in fail_rates.

        Keyword arguments:
        hit_num (int): Hit number in point
        """
        try:
            return self.fail_rates[hit_num]
        except IndexError as e:
            return self.fail_rates[-1]

    def add_fail_rate(self, fr):
        """Appends fr to fail_rates.

        Keyword arguments:
        fr (float): Fail rate
        """
        self.fail_rates.append(fr)

    def print_info(self):
        """Prints report on player"""
        print("{}:\n - Fail rates: ".format(self.name.upper()))
        for i, r in enumerate(self.fail_rates, 1):
            print("\t\t", i, ". ", "{:.0%}".format(r), sep="")


class Match:
    def __init__(self, players):
        """Creates a new match

        Keyword arguments:
        players: List of Player objects

        Attributes
        players: List of Player objects
        first_to_serve (Player): Player to serve first in game 1
        games: List of Game objects
        winner (Player): Match winner
        loser (Player): Match loser
        """
        random.shuffle(players)  # shuffles players in place
        self.players = players
        self.first_to_serve = self.players[0]
        self.games = []
        self.winner = None
        self.loser = None

    def play(self):
        """Play match"""
        while not self._is_match_over():
            game = Game(self.players)
            self.games.append(game)
            game.play()
            self.players.reverse()  # changes who serves first

    def _is_match_over(self):
        """Return True if match is over"""
        if len(self.games) < 3:
            return False

        if self._get_wins(self.players[0]) == 3:
            self.winner = self.players[0]
            self.loser = self.players[1]
            return True
        elif self._get_wins(self.players[1]) == 3:
            self.winner = self.players[1]
            self.loser = self.players[0]
            return True
        else:
            return False

    def _get_wins(self, player):
        """Return the number of wins a player had in the match

        Keyword arguments:
        player: Player object
        """
        wins = len([game for game in self.games if game.winner is player])
        return wins

    def print_results(self):
        """Prints report of match"""
        print("First to Serve: ", self.first_to_serve.name)
        print("Winner: ", self.winner.name)
        print("Loser: ", self.loser.name)
        print(
            "Score: {}-{}".format(
                self._get_wins(self.winner), self._get_wins(self.loser)
            )
        )
        more_details = input("Want more details? y/n: ").lower()
        if more_details != "n":
            for i, game in enumerate(self.games, 1):
                print("GAME", i)
                game.print_results()


class Game:
    def __init__(self, players):
        """Creates a new game

        Keyword arguments:
        players: List of Player objects

        Attributes
        players: List of Player objects (first in list serves first)
        first_to_serve (Player): Player to serve first in game
        winner (Player): Game winner
        loser (Player): Game loser
        """
        self.players = players
        self.first_to_serve = self.players[0]
        self.points = []
        self.winner = None
        self.loser = None

    def get_score(self):
        """Returns score as dictionary"""
        p1_points = len([p for p in self.points if p.winner is self.players[0]])
        p2_points = len(self.points) - p1_points
        return {self.players[0]: p1_points, self.players[1]: p2_points}

    def play(self):
        """Play game"""
        server, returner = self.players

        while not self._is_game_over():
            score = self.get_score()
            hit_num = 0
            if len(self.points) >= 22 or (self.points and len(self.points) % 2 == 0):
                server, returner = returner, server  # change server
            while True:  # rally continues
                if random.random() < server.get_fail_rate(hit_num):  # server missed
                    point = Point(server, returner, server, hit_num)
                    self.points.append(point)
                    break
                hit_num += 1
                if random.random() < returner.get_fail_rate(hit_num):  # returner missed
                    point = Point(server, server, returner, hit_num)
                    self.points.append(point)
                    break
                hit_num += 1

    def _is_game_over(self):
        """Return True if game is over"""
        score = self.get_score()
        p1 = score[self.players[0]]
        p2 = score[self.players[1]]
        if abs(p1 - p2) > 1 and (p1 >= 11 or p2 >= 11):
            if p1 > p2:
                self.winner = self.players[0]
                self.loser = self.players[1]
            else:
                self.winner = self.players[1]
                self.loser = self.players[0]
            return True
        else:
            return False

    def print_results(self):
        """Prints report of game"""
        print(" - First to Serve: ", self.first_to_serve.name)
        print(" - Winner: ", self.winner.name)
        print(" - Loser: ", self.loser.name)
        scores = list(self.get_score().values())
        print(" - Score: {}-{}".format(max(scores), min(scores)))
        for i, p in enumerate(self.points, 1):
            print(
                "\t{:>3}".format(i),
                ". Server: ",
                p.server.name,
                ", Winner: ",
                p.winner.name,
                ", Volley length: ",
                p.volley_length,
                sep="",
            )


class Point:
    def __init__(self, server, winner, loser, num_hits):
        """Creates a new point

        Keyword arguments:
        server (Player): server of point
        winner (Player): winner of point
        loser (Player): loser of point
        num_hits (int): number of hits in point, including serve
        """
        self.server = server
        self.winner = winner
        self.loser = loser
        self.volley_length = num_hits


def print_summary(matches, players):
    """Prints summary report.
    Used for when many matches are played.
    """
    num = len(matches)
    players[0].print_info()
    players[1].print_info()

    p1_wins = len([match for match in matches if match.winner is players[0]])
    p2_wins = len([match for match in matches if match.winner is players[1]])

    print(
        "{} Wins: {}. {} Wins: {}".format(
            players[0].name, p1_wins, players[1].name, p2_wins
        )
    )

    server_wins = len(
        [match for match in matches if match.winner is match.first_to_serve]
    )

    print("First to serve wins {:.0%} of the time.".format(server_wins / num))

    num_points = 0
    num_hits = 0
    for match in matches:
        for game in match.games:
            num_points += len(game.points)
            num_hits += sum(p.volley_length for p in game.points)

    volley_length = num_hits / num_points
    print("The average volley length is {:.2f} hits.".format(volley_length))


def play_one(players):
    """Plays one match and prints results"""
    print()
    match = Match(players)
    match.play()
    match.print_results()
    if input("Enter to play again. Q to quit. ").lower() == "q":
        print("Goodbye!")
    else:
        play_one(players)


def play(players, num):
    """Plays num matches

    Keyword arguments:
    num (int): Number of matches to play
    """
    if num == 1:
        play_one(players)
        return
    print()
    matches = []
    for i in range(num):
        match = Match(players)
        match.play()
        matches.append(match)

    print_summary(matches, players)

    if input("Enter to play again. Q to quit. ").lower() == "q":
        print("Goodbye!")
    else:
        play(players, num)


def create_player(name):
    """Creates and returns player based on user input

    Keyword arguments:
    name (str): Player name
    """
    try:
        player_sfr = float(input(name + "'s Serve Fail Rate: "))
        player_srfr = float(input(name + "'s Serve Return Fail Rate: "))
        player = Player(name, [player_sfr, player_srfr])
        i = 2
        while True:
            i += 1
            try:
                vfr = float(input(name + "'s hit " + str(i) + " fail rate: "))
            except:
                break
            else:
                player.add_fail_rate(vfr)
    except:
        print("Let's try this again. Enter floats.")
        return add_player(name)
    else:
        return player


testing = True


def main():
    """Main Program
    If num_matches is 1, then only one match is played and
    the results of all the games (including each point) are
    reported.

    If num_matches is more than 1, then a summary of the results
    is displayed.
    """
    if testing:
        # CHANGE PLAYER INFO BELOW
        players = [
            Player("Leira", [0.05, 0.1, 0.3, 0.3, 0.2]),
            Player("Nat", [0.05, 0.5, 0.25, 0.4, 0.25]),
        ]
        num_matches = 1000
    else:
        players = []
        player1_name = input("Player 1 Name: ")
        player2_name = input("Player 2 Name: ")
        players.append(create_player(player1_name))
        players.append(create_player(player2_name))
        num_matches = int(input("Num matches: "))

    play(players, num_matches)


main()

Feel free to play around with it and let me know if you have any suggestions for improving the model or the code.

Written by Nat Dunn. Follow Nat on Twitter.


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