Python-MySQL中的错误处理

Question

我正在运行一个基于python
flask的小型Web服务，我想在其中执行一个小型MySQL查询。当我获得SQL查询的有效输入时，一切都按预期工作，并且我获得了正确的值。但是，如果该值未存储在数据库中，则会收到一个TypeError

    Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1478, in full_dispatch_request
    response = self.make_response(rv)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1566, in make_response
    raise ValueError('View function did not return a response')
ValueError: View function did not return a response

我试图利用错误处理自己，并在我的项目中使用此代码，但看来这无法正常工作。

#!/usr/bin/python

from flask import Flask, request
import MySQLdb

import json

app = Flask(__name__)


@app.route("/get_user", methods=["POST"])
def get_user():
    data = json.loads(request.data)
    email = data["email"]

    sql = "SELECT userid FROM oc_preferences WHERE configkey='email' AND configvalue LIKE '" + email + "%';";

    conn = MySQLdb.connect( host="localhost",
                            user="root",
                            passwd="ubuntu",
                            db="owncloud",
                            port=3306)
    curs = conn.cursor()

    try:
        curs.execute(sql)
        user = curs.fetchone()[0]
        return user
    except MySQLdb.Error, e:
        try:
            print "MySQL Error [%d]: %s" % (e.args[0], e.args[1])
            return None
        except IndexError:
            print "MySQL Error: %s" % str(e)
            return None
    except TypeError, e:
        print(e)
        return None
    except ValueError, e:
        print(e)
        return None
    finally:
        curs.close()
        conn.close()

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=5000, debug=True)

基本上，我只想返回一个值，当一切都正常工作时，如果最好不要在服务器上显示错误消息，则不返回任何值。如何正确使用错误处理？

编辑 更新了当前代码+错误消息。

Answer 1

第一点：try / except块中的代码太多。当您有两个可能引发不同错误的语句（或两组语句）时，最好使用不同的try / except块：

try:
    try:
        curs.execute(sql)
        # NB : you won't get an IntegrityError when reading
    except (MySQLdb.Error, MySQLdb.Warning) as e:
        print(e)
        return None

    try: 
        user = curs.fetchone()[0]
        return user
    except TypeError as e:
        print(e)
        return None

finally:
    conn.close()

现在您真的必须在这里捕获TypeError吗？如果你在阅读回溯，你会发现，你的错误来自调用__getitem__()上None（注意：__getitem__()是实施下标运算符[]），这意味着，如果你有没有匹配行cursor.fetchone()的回报None，这样你就可以测试的回报currsor.fetchone()：

try:
    try:
        curs.execute(sql)
        # NB : you won't get an IntegrityError when reading
    except (MySQLdb.Error, MySQLdb.Warning) as e:
        print(e)
        return None

    row = curs.fetchone()
    if row:
        return row[0]
    return None

finally:
    conn.close()

Now do you really need to catch MySQL errors here ? Your query is supposed to
be well tested and it’s only a read operation so it should not crash - so if
you have something going wrong here then you obviously have a bigger problem,
and you don’t want to hide it under the carpet. IOW: either log the exceptions
(using the standard logging package and logger.exception()) and re-raise
them or more simply let them propagate (and eventually have an higher level
component take care of logging unhandled exceptions):

try:
    curs.execute(sql)
    row = curs.fetchone()
    if row:
        return row[0]
    return None

finally:
    conn.close()

And finally: the way you build your sql query is utterly
unsafe. Use sql placeholders instead:

q = "%s%%" % data["email"].strip() 
sql = "select userid from oc_preferences where configkey='email' and configvalue like %s"
cursor.execute(sql, [q,])

Oh and yes: wrt/ the “View function did not return a response” ValueError,
it’s because, well, your view returns None in many places. A flask view is
supposed to return something that can be used as a HTTP response, and None
is not a valid option here.