运行GAE开发服务器时，本地MySQLdb连接与AttributeError的参数样式失败

Question

我正在针对Cloud构建一个GAE水瓶应用程序，并在构建过程中运行dev_appserver来测试应用程序。

但是，如果我将SQLALCHEMY_DATABASE_URI设置为mysql+gaerdbms:///appname?instance=instanceid URL，则在尝试调用db.create_all()时会得到以下跟踪

Traceback (most recent call last):
  # earlier lines omitted for brevity
  File "/Project/app/foo.bar/foo/bar/admin/__init__.py", line 26, in init_db
    db.create_all()
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 856, in create_all
    self._execute_for_all_tables(app, bind, 'create_all')
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 848, in _execute_for_all_tables
    op(bind=self.get_engine(app, bind), tables=tables)
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 797, in get_engine
    return connector.get_engine()
  File "/Project/app/distlib/flask_sqlalchemy/__init__.py", line 473, in get_engine
    self._engine = rv = sqlalchemy.create_engine(info, **options)
  File "/Project/app/distlib/sqlalchemy/engine/__init__.py", line 332, in create_engine
    return strategy.create(*args, **kwargs)
  File "/Project/app/distlib/sqlalchemy/engine/strategies.py", line 69, in create
    dialect = dialect_cls(**dialect_args)
  File "/Project/app/distlib/sqlalchemy/dialects/mysql/base.py", line 1986, in __init__
    default.DefaultDialect.__init__(self, **kwargs)
  File "/Project/app/distlib/sqlalchemy/engine/default.py", line 124, in __init__
    self.paramstyle = self.dbapi.paramstyle
AttributeError: 'module' object has no attribute 'paramstyle'

怎么回事？为什么缺少(DB-API2.0必需的) paramstyle属性？

Answer 1

这意味着MySQLdb模块丢失，导入失败。GAE本身并不随MySQLdb客户端库一起提供；安装MySQLdb (as instructed in the SDK documentation)：

venv/bin/pip install mysql-python

或者使用操作系统包管理器在系统python中安装MySQLdb。

此错误是由充当存根的Google google.appengine.api.rdbms_mysqldb模块引起的，在paramstyle失败时没有paramstyle属性。它提供了一个存根connect()函数，它将引发一个更有用的异常，但是由于与SQLAlchemy的不幸交互，错误的信息量要少得多。