python - 为什么会抛出 IndexError？

Question

我正在尝试找出它抛出该错误的原因。元组的长度应该是 4，确实是。有什么提示、想法吗？

此代码适用于我正在为 Udacity 编程入门 nano 学位开展的瑞士风格项目。

相关代码Python代码来自tournament.py:

def playerStandings():
    """Returns a list of the players and their win records, sorted by wins.

    The first entry in the list should be the player in first place, or a player
    tied for first place if there is currently a tie.

    Returns:
     A list of tuples, each of which contains (id, name, wins, matches):
      id: the player's unique id (assigned by the database)
        name: the player's full name (as registered)
        wins: the number of matches the player has won
        matches: the number of matches the player has played 
    """
    conn = connect()
    c = conn.cursor()
    c.execute("SELECT COUNT(id) FROM Players;")
    total = c.fetchone()[0]

    num_of_players = countPlayers()
    standings = [None]*num_of_players

    c.execute("SELECT * FROM wincounter;")
    winners = c.fetchall()

    i = 0
    for player in winners:
        standings[i] = (player[0], player[1], player[2], player[3])
        i += 1

    conn.close()
    return standings

tournament_test.py中的相关代码:

def testStandingsBeforeMatches():
    """
    Test to ensure players are properly represented in standings prior
    to any matches being reported.
    """
    deleteMatches()
    deletePlayers()
    registerPlayer("Melpomene Murray")
    registerPlayer("Randy Schwartz")
    standings = playerStandings()
    if len(standings) < 2:
        raise ValueError("Players should appear in playerStandings even before "
                         "they have played any matches.")
    elif len(standings) > 2:
        raise ValueError("Only registered players should appear in standings.")
    if len(standings[0]) != 4:
        raise ValueError("Each playerStandings row should have four columns.")
    [(id1, name1, wins1, matches1), (id2, name2, wins2, matches2)] = standings
    if matches1 != 0 or matches2 != 0 or wins1 != 0 or wins2 != 0:
        raise ValueError(
            "Newly registered players should have no matches or wins.")
    if set([name1, name2]) != set(["Melpomene Murray", "Randy Schwartz"]):
        raise ValueError("Registered players' names should appear in standings, "
                         "even if they have no matches played.")
    print ("6. Newly registered players appear in the standings with no matches.")

PostgreSQL 模式:

DROP DATABASE IF EXISTS tournament;
CREATE DATABASE tournament;
\c tournament;


CREATE TABLE players (
    id serial PRIMARY KEY NOT NULL,
    Name text
);

CREATE TABLE matches (
    match_id serial PRIMARY KEY NOT NULL,
    winner int REFERENCES players(id),
    loser int REFERENCES players(id)
);

CREATE VIEW wincounter
AS
  SELECT players.id,
         players.name,
         COUNT(matches.winner) AS wins
  FROM   players
         LEFT JOIN matches
                ON players.id = matches.winner
  GROUP  BY players.id;

错误信息:

vagrant@vagrant-ubuntu-trusty-32:/vagrant/tournament$ python tournament_test.py
    1. countPlayers() returns 0 after initial deletePlayers() execution.
    2. countPlayers() returns 1 after one player is registered.
    3. countPlayers() returns 2 after two players are registered.
    4. countPlayers() returns zero after registered players are deleted.
    5. Player records successfully deleted.
    Traceback (most recent call last):
      File "tournament_test.py", line 152, in <module>
        testStandingsBeforeMatches()
      File "tournament_test.py", line 54, in testStandingsBeforeMatches
        standings = playerStandings()
      File "/vagrant/tournament/tournament.py", line 85, in playerStandings
        standings[i] = (player[0], player[1], player[2], player[3])
    IndexError: tuple index out of range
    vagrant@vagrant-ubuntu-trusty-32:/vagrant/tournament$

Answer 1

错误发生在线路上:

standings[i] = (player[0], player[1], player[2], player[3])

您的 player 变量是一个元组，代表数据库中 wincounter View 中的一行。该 View 只有三列(players.id、players.name 和 wins)，因此当您尝试访问时出现索引错误第四个值 (player[3])。

目前还不完全清楚为什么要那样设置那条线。如果你想把 player 元组中的所有项目都添加到 standings 中，你可以做 standings[i] = player，甚至可以得到去掉显式循环，只写standings = list(winners)。