“pymysql.err.InternalError: (1046, u'No database selected')” 从命令行运行 Python 脚本时出现错误消息答案

【问题标题】："pymysql.err.InternalError: (1046, u'No database selected')" error message when running Python script from command line“pymysql.err.InternalError: (1046, u'No database selected')” 从命令行运行 Python 脚本时出现错误消息
【发布时间】：2020-04-07 06:34:42
【问题描述】：

我正在尝试运行一个通过 PyMySQL 连接到 MySQL 数据库的 Python 脚本。该脚本实际上是：

import pymysql
cnx = pymysql.connect(read_default_file = "/directory/my.cnf", cursorclass = pymysql.cursors.DictCursor)
cursor = cnx.cursor()
# Do stuff.

当我在解释器中运行脚本时，我没有收到任何错误，但是当我尝试从命令行运行它时，我收到以下错误：

Traceback (most recent call last):
  File "s02_prepare_data_RNN.py", line 264, in <module>
    (omniture, urls, years, global_regions) = get_omniture_data("omniture_results")
  File "s02_prepare_data_RNN.py", line 76, in get_omniture_data
    sso_to_accountid = get_sso_accountids()
  File "s02_prepare_data_RNN.py", line 31, in get_sso_accountids
    cursor.execute(query)
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/cursors.py", line 134, in execute
    result = self._query(query)
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/cursors.py", line 282, in _query
    conn.query(q)
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 768, in query
    self._affected_rows = self._read_query_result(unbuffered=unbuffered)
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 929, in _read_query_result
    result.read()
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 1125, in read
    first_packet = self.connection._read_packet()
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 893, in _read_packet
    packet.check_error()
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 369, in check_error
    err.raise_mysql_exception(self._data)
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/err.py", line 120, in raise_mysql_exception
    _check_mysql_exception(errinfo)
  File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/err.py", line 115, in _check_mysql_exception
    raise InternalError(errno, errorvalue)
pymysql.err.InternalError: (1046, u'No database selected')

【问题讨论】：

标签： pymysql

【解决方案1】：

修改我的代码：

import pymysql
cnx = pymysql.connect(read_default_file = "/directory/my.cnf", cursorclass = pymysql.cursors.DictCursor)
cursor = cnx.cursor()
# Do stuff.

到：

import pymysql
cnx = pymysql.connect(read_default_file = "/directory/my.cnf", cursorclass = pymysql.cursors.DictCursor)
cursor = cnx.cursor()
if __name__ == "__main__":
    # Do stuff.

修复了错误。我认为 Python 可能会在建立连接之前尝试执行查询，所以我尝试将程序的主要部分放在 if __name__ == "__main__": 下并修复它。不过，我仍然不是 100% 发生了什么。我曾假设代码会等待连接建立，然后再继续执行以下几行，但此修复表明情况并非如此。

还值得注意的是，我只是在具有 Python 2.6 的服务器上从命令行运行原始脚本时才收到错误消息。当我在装有 Python 2.7 的本地计算机上从命令行运行原始脚本时，我没有收到错误消息。

不管怎样，if __name__ == "__main__": 是很好的 Python 风格，所以我以后一定会使用它。

【讨论】：

【解决方案2】：

有时，当数据库 URI 构建不正确时，可能会发生这种情况。对我来说，当我的 pymysql 连接字符串（URL）如下所示时发生错误：

mysql+pymysql://:@localhost/

要修复它，它需要看起来像：

mysql+pymysql://<my-database-user-name>:@localhost/<my-database-name> # 或将localhost 替换为您的特定主机名

【讨论】：

【解决方案3】：

如果不选择连接的数据库，则必须添加此行。

connection = pymysql.connect(host=DB_host, user=DB_user, password=DB_password)  # NO DB_NAME SPECIFED
cursor = connection.cursor()
cursor.execute(f'CREATE DATABASE IF NOT EXISTS {DB_NAME}')  # ADD THIS LINE!
connection.select_db(DB_NAME)

【讨论】：