即使在调用MySQLdb之前执行，仍会引发" execute () first“错误

Question

在使用MySQLdb时，我得到了以下错误。_mysql_exceptions.ProgrammingError: execute() first

print输出告诉我“获取后”从未被打印，这意味着脚本在row = g.db.cursor().fetchall()上崩溃了。为什么我要得到这个错误，以及如何修复它？

def connect_db():
    return mysql.connection

@app.before_request
def before_request():
    g.db = connect_db()

@app.teardown_request
def teardown_request(exception):
    g.db.cursor().close()

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        g.db.cursor().execute('SELECT * FROM users WHERE email = %s', [request.form['email']])
        print('Before fetch')
        row = g.db.cursor().fetchall()
        print('After fetch')

        if not row:
            flash('No such user exists!')
            return redirect(url_for('show_home'))
        elif request.form['password'] == row[0][1]:
            return redirect(url_for('show_dashboard'))
        else:
            flash('Wrong password / login error.')
            return redirect(url_for('show_home'))

File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1836, in __call__
return self.wsgi_app(environ, start_response)
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1820, in wsgi_app
response = self.make_response(self.handle_exception(e))
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1403, in handle_exception
reraise(exc_type, exc_value, tb)
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1817, in wsgi_app
response = self.full_dispatch_request()
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1477, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1381, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/_compat.py", line 33, in reraise
raise value
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1475, in full_dispatch_request
rv = self.dispatch_request()
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/flask/app.py", line 1461, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/media/common/code/python/projects/up2date/up2date.py", line 47, in login
row = g.db.cursor().fetchall()
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/MySQLdb/cursors.py", line 406, in fetchall
self._check_executed()
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/MySQLdb/cursors.py", line 107, in _check_executed
self.errorhandler(self, ProgrammingError, "execute() first")
File "/media/common/code/python/projects/up2date/venv/lib/python3.4/site-packages/MySQLdb/connections.py", line 38, in defaulterrorhandler
Open an interactive python shell in this frameraise errorclass(errorvalue)

Answer 1

您要多次调用cursor，这将创建多个游标。在第一个游标上执行查询，然后从第二个游标中获取查询。因为您没有在第二次执行任何操作，所以会得到这个错误。只使用一个光标。

cursor = g.db.cursor()
cursor.execute(...)
rows = cursor.fetchall()