Python支持SQL数据库返回psycopg2.ProgrammingError：在尝试删除表中的数据时，关系不存在错误？

Question

简介：我一直致力于使用Vagrant为Udacity中的关系数据库课程运行Ubuntu VM，构建一个瑞士风格锦标赛的Python支持的PostgreSQL数据库。

问题：当我尝试删除现有表中的信息时，为什么说该表不存在？

我做了一般搜索研究

psycopg2.ProgrammingError

，但信息主要针对其他情况，我无法建立逻辑连接，可以解决这个问题。我还发现另一个Stack Overflow线程接近同一个问题other thread here，但是这个问题也从未得到解决。所以我创造了这个问题，试图找到一个能够理解我的难题的人，并提供某种暗示或线索让我朝着正确的方向努力。

我已将锦标赛数据库SQL文件导入Vagrant VM。这删除了两个表（玩家和匹配）和数据库（锦标赛），然后重新创建了这个数据库和这些表。所以我知道表（匹配）存在。但是，这就是我得到的......

流浪者:(当前的流浪错误）

vagrant@vagrant:/vagrant/tournament$ python tournament_test.py
Traceback (most recent call last):
  File "tournament_test.py", line 151, in <module>
    testCount()
  File "tournament_test.py", line 17, in testCount
    deleteMatches()
  File "/vagrant/tournament/tournament.py", line 18, in deleteMatches
    cursor.execute("DELETE FROM matches;")
psycopg2.ProgrammingError: relation "matches" does not exist
LINE 1: DELETE FROM matches;
                    ^

SQL :(全部来自tournament.sql）

-- Table definitions for the tournament project.
--
-- Put your SQL 'create table' statements in this file; also 'create view'
-- statements if you choose to use it.
--
-- You can write comments in this file by starting them with two dashes, 
like
-- these lines here.
DROP TABLE matches;
DROP TABLE players;
DROP DATABASE tournament;
CREATE DATABASE tournament;


CREATE TABLE players (player_id SERIAL UNIQUE PRIMARY KEY, name 
VARCHAR(40));
CREATE TABLE matches (match_id SERIAL UNIQUE PRIMARY KEY,
                      winner INTEGER REFERENCES players(player_id),
                      loser INTEGER REFERENCES players (player_id));

Python :(来自tournament.py的最小值）

#!/usr/bin/env python
#
# tournament.py -- implementation of a Swiss-system tournament
#

import psycopg2


def connect():
    """Connect to the PostgreSQL database.  Returns a database 
connection."""
    return psycopg2.connect("dbname=tournament")


def deleteMatches():
    """Remove all the match records from the database."""
    db = connect()
    cursor = db.cursor()
    cursor.execute("TRUNCATE matches CASCADE;")
    db.commit()
    db.close()

Python :(最少来自tournament_test.py）这是验证tournament.py函数正在使用tournament.sql数据库的文件。

#!/usr/bin/env python
#
# Test cases for tournament.py

from tournament import *

def testCount():
    """
    Test for initial player count,
             player count after 1 and 2 players registered,
             player count after players deleted.
    """
    deleteMatches()
    deletePlayers()
    c = countPlayers()
    if c == '0':
        raise TypeError(
            "countPlayers should return numeric zero, not string '0'.")
    if c != 0:
        raise ValueError("After deletion, countPlayers should return zero.")
    print "1. countPlayers() returns 0 after initial deletePlayers() 
execution."
    registerPlayer("Chandra Nalaar")
    c = countPlayers()
    if c != 1:
        raise ValueError(
            "After one player registers, countPlayers() should be 1. Got 
{c}".format(c=c))
    print "2. countPlayers() returns 1 after one player is registered."
    registerPlayer("Jace Beleren")
    c = countPlayers()
    if c != 2:
        raise ValueError(
            "After two players register, countPlayers() should be 2. Got 
{c}".format(c=c))
    print "3. countPlayers() returns 2 after two players are registered."
    deletePlayers()
    c = countPlayers()
    if c != 0:
        raise ValueError(
            "After deletion, countPlayers should return zero.")
    print "4. countPlayers() returns zero after registered players are 
deleted.
5. Player records successfully deleted."

testCount()

Vagrant（确认表'匹配'存在于数据库中）：

vagrant=> dt
         List of relations
 Schema |  Name   | Type  |  Owner
--------+---------+-------+---------
 public | matches | table | vagrant
 public | players | table | vagrant
(2 rows)

Update2：澄清评论

我在python文件的开头连接到db。

def connect()
    """Connect to the PostgreSQL database.  Returns a database 
    connection."""
    return psycopg2.connect("dbname=tournament")