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Handling Application Errors

Applications fail, server fail. Sooner or later you will see an exception in production. Even if your code is 100% correct, you will still see exceptions from time to time. Why? Because everything else involved will fail. Here some situations where perfectly fine code can lead to server errors:

  • the client terminated the request early and the application was still reading from the incoming data.
  • the database server was overloaded and could not handle the query.
  • a filesystem is full
  • a harddrive crashed
  • a backend server overloaded
  • a programming error in a library you are using
  • network connection of the server to another system failed.

And that's just a small sample of issues you could be facing. So how to deal with that sort of problem? By default if your application runs in production mode, Flask will display a very simple page for you and log the exception to the :attr:`~flask.Flask.logger`.

But there is more you can do, and we will cover some better setups to deal with errors.

Error Mails

If the application runs in production mode (which it will do on your server) you won't see any log messages by default. Why that? Flask tries to be a zero-configuration framework and where should it drop the logs for you if there is no configuration. Guessing is not a good idea because changes are, the place it guessed is not the place where the user has the permission to create a logfile. Also, for most small applications nobody will look at the logs anyways.

In fact, I promise you right now that if you configure a logfile for the application errors you will never look at it except for debugging an issue when a user reported it for you. What you want instead is a mail the second the exception happened. Then you get an alert and you can do something about it.

Flask is using the Python builtin logging system and that one can actually send you mails for errors which is probably what you want. Here is how you can configure the Flask logger to send you mails for exceptions:

ADMINS = ['']
if not app.debug:
    import logging
    from logging.handlers import SMTPHandler
    mail_handler = SMTPHandler('',
                               ADMINS, 'YourApplication Failed')

So what just happened? We created a new :class:`~logging.handlers.SMTPHandler` that will send mails with the mail server listening on to all the ADMINS from the address with the subject "YourApplication Failed". If your mail server requires credentials these can also provided, for that check out the documentation for the :class:`~logging.handlers.SMTPHandler`.

We also tell the handler to only send errors and more critical messages. Because we certainly don't want to get a mail for warnings or other useless logs that might happen during request handling.

Before you run that in production, please also look at :ref:`log-format` to put more information into that error mail. That will save you from a lot of frustration.

Logging to a File

Even if you get mails, you probably also want to log warnings. It's a good idea to keep as much information around that might be required to debug a problem. Please note that Flask itself will not issue any warnings in the core system, so it's your responsibility to warn in the code if something seems odd.

There are a couple of handlers provided by the logging system out of the box but not all of them are useful for basic error logging. The most interesting are probably the following:

Once you picked your log handler, do like you did with the SMTP handler above, just make sure to use a lower setting (I would recommend WARNING):

if not app.debug:
    import logging
    from logging.handlers import TheHandlerYouWant
    file_handler = TheHandlerYouWant(...)

Controlling the Log Format

By default a handler will only write the message string into a file or send you that message as mail. But a log record stores more information and it makes a lot of sense to configure your logger to also contain that information so that you have a better idea of why that error happened, and more importantly, where it did.

A formatter can be instanciated with a format string. Note that tracebacks are appended to the log entry automatically. You don't have to do that in the log formatter format string.

Here some example setups:


from logging import Formatter
Message type:       %(levelname)s
Location:           %(pathname)s:%(lineno)d
Module:             %(module)s
Function:           %(funcName)s
Time:               %(asctime)s



File logging

from logging import Formatter
    '%(astime)s %(levelname)s: %(message)s '
    '[in %(pathname)s:%(lineno)d]'

Complex Log Formatting

Here is a list of useful formatting variables for the format string. Note that this list is not complete, consult the official documentation of the :mod:`logging` package for a full list.

Format Description
%(levelname)s Text logging level for the message ('DEBUG', 'INFO', 'WARNING', 'ERROR', 'CRITICAL').
%(pathname)s Full pathname of the source file where the logging call was issued (if available).
%(filename)s Filename portion of pathname.
%(module)s Module (name portion of filename).
%(funcName)s Name of function containing the logging call.
%(lineno)d Source line number where the logging call was issued (if available).
%(asctime)s Human-readable time when the LogRecord` was created. By default this is of the form "2003-07-08 16:49:45,896" (the numbers after the comma are millisecond portion of the time). This can be changed by subclassing the formatter and overriding the :meth:`~logging.Formatter.formatTime` method.
%(message)s The logged message, computed as msg % args

If you want to further customize the formatting, you can subclass the formatter. The formatter has three interesting methods:

handles the actual formatting. It is passed a :class:`~logging.LogRecord` object and has to return the formatted string.
called for asctime formatting. If you want a different time format you can override this method.
called for exception formatting. It is passed a :attr:`~sys.exc_info` tuple and has to return a string. The default is usually fine, you don't have to override it.

For more information, head over to the official documentation.

Other Libraries

So far we only configured the logger your application created itself. Other libraries might log themselves as well. For example, SQLAlchemy use logging heavily in the core. While there is a method to configure all loggers at once in the :mod:`logging` package, I would not recommend using it. There might be a situation in which you want to have multiple separate applications running side by side in the same Python interpreter and then it becomes impossible to have different logging setups for those.

Instead, I would recommend figuring out which loggers you are interested in, getting the loggers with the :func:`~logging.getLogger` function and iterating over them to attach handlers:

from logging import getLogger
loggers = [app.logger, getLogger('sqlalchemy'),
for logger in loggers:
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