2018-01-16T20:22:10Z

The Flask Mega-Tutorial Part VII: Error Handling

This is the seventh installment of the Flask Mega-Tutorial series, in which I'm going to tell you how to do error handling in a Flask application.

For your reference, below is a list of the articles in this series.

In this chapter I'm taking a break from coding new features into my microblog application, and instead will discuss a few strategies to deal with bugs, which invariably make an appearance in every software project. To help illustrate this topic, I intentionally let a bug slip in the code that I've added in Chapter 6. Before you continue reading, see if you can find it!

The GitHub links for this chapter are: Browse, Zip, Diff.

Error Handling in Flask

What happens when an error occurs in a Flask application? The best way to find out is to experience it first hand. Go ahead and start the application, and make sure you have at least two users registered. Log in as one of the users, open the profile page and click the "Edit" link. In the profile editor, try to change the username to the username of another user that is already registered, and boom! This is going to bring a scary looking "Internal Server Error" page:

Internal Server Error

If you look in the terminal session where the application is running, you will see a stack trace of the error. Stack traces are extremely useful in debugging errors, because they show the sequence of calls in that stack, all the way to the line that produced the error:

(venv) $ flask run
 * Serving Flask app "microblog"
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
[2021-06-14 22:40:02,027] ERROR in app: Exception on /edit_profile [POST]
Traceback (most recent call last):
  File "venv/lib/python3.6/site-packages/sqlalchemy/engine/base.py", in _execute_context
    context)
  File "venv/lib/python3.6/site-packages/sqlalchemy/engine/default.py", in do_execute
    cursor.execute(statement, parameters)
sqlite3.IntegrityError: UNIQUE constraint failed: user.username

The stack trace indicates what is the bug. The application allows a user to change the username, and does not validate that the new username chosen does not collide with another user already in the system. The error comes from SQLAlchemy, which tries to write the new username to the database, but the database rejects it because the username column is defined with unique=True.

It is important to note that the error page that is presented to the user does not provide much information about the error, and that is good. I definitely do not want users to learn that the crash was caused by a database error, or what database I'm using, or what are some of the table and field names in my database. All that information should be kept internal.

There are a few things that are far from ideal. I have an error page that is very ugly and does not match the application layout. I also have important application stack traces being dumped on a terminal that I need to constantly watch to make sure I don't miss any errors. And of course I have a bug to fix. I'm going to address all these issues, but first, let's talk about Flask's debug mode.

Debug Mode

The way you saw that errors are handled above is great for a system that is running on a production server. If there is an error, the user gets a vague error page (though I'm going to make this error page nicer), and the important details of the error are in the server process output or in a log file.

But when you are developing your application, you can enable debug mode, a mode in which Flask outputs a really nice debugger directly on your browser. To activate debug mode, stop the application, and then set the following environment variable:

(venv) $ export FLASK_ENV=development

If you are on Microsoft Windows, remember to use set instead of export.

After you set FLASK_ENV, restart the server. The output on your terminal is going to be slightly different than what you are used to see:

(venv) microblog2 $ flask run
 * Serving Flask app 'microblog.py' (lazy loading)
 * Environment: development
 * Debug mode: on
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 118-204-854

Now make the application crash one more time to see the interactive debugger in your browser:

Flask Debugger

The debugger allows you expand each stack frame and see the corresponding source code. You can also open a Python prompt on any of the frames and execute any valid Python expressions, for example to check the values of variables.

It is extremely important that you never run a Flask application in debug mode on a production server. The debugger allows the user to remotely execute code in the server, so it can be an unexpected gift to a malicious user who wants to infiltrate your application or your server. As an additional security measure, the debugger running in the browser starts locked, and on first use will ask for a PIN number, which you can see in the output of the flask run command.

Since I am in the topic of debug mode, I should mention the second important feature that is enabled with debug mode, which is the reloader. This is a very useful development feature that automatically restarts the application when a source file is modified. If you run flask run while in debug mode, you can then work on your application and any time you save a file, the application will restart to pick up the new code.

Custom Error Pages

Flask provides a mechanism for an application to install its own error pages, so that your users don't have to see the plain and boring default ones. As an example, let's define custom error pages for the HTTP errors 404 and 500, the two most common ones. Defining pages for other errors works in the same way.

To declare a custom error handler, the @errorhandler decorator is used. I'm going to put my error handlers in a new app/errors.py module.

app/errors.py: Custom error handlers

from flask import render_template
from app import app, db

@app.errorhandler(404)
def not_found_error(error):
    return render_template('404.html'), 404

@app.errorhandler(500)
def internal_error(error):
    db.session.rollback()
    return render_template('500.html'), 500

The error functions work very similarly to view functions. For these two errors, I'm returning the contents of their respective templates. Note that both functions return a second value after the template, which is the error code number. For all the view functions that I created so far, I did not need to add a second return value because the default of 200 (the status code for a successful response) is what I wanted. In this case these are error pages, so I want the status code of the response to reflect that.

The error handler for the 500 errors could be invoked after a database error, which was actually the case with the username duplicate above. To make sure any failed database sessions do not interfere with any database accesses triggered by the template, I issue a session rollback. This resets the session to a clean state.

Here is the template for the 404 error:

app/templates/404.html: Not found error template

{% extends "base.html" %}

{% block content %}
    <h1>File Not Found</h1>
    <p><a href="{{ url_for('index') }}">Back</a></p>
{% endblock %}

And here is the one for the 500 error:

app/templates/500.html: Internal server error template

{% extends "base.html" %}

{% block content %}
    <h1>An unexpected error has occurred</h1>
    <p>The administrator has been notified. Sorry for the inconvenience!</p>
    <p><a href="{{ url_for('index') }}">Back</a></p>
{% endblock %}

Both templates inherit from the base.html template, so that the error page has the same look and feel as the normal pages of the application.

To get these error handlers registered with Flask, I need to import the new app/errors.py module after the application instance is created:

app/__init__.py: Import error handlers

# ...

from app import routes, models, errors

If you set FLASK_ENV=production in your terminal session and then trigger the duplicate username bug one more time, you are going to see a slightly more friendly error page.

Custom 500 Error Page

Sending Errors by Email

The other problem with the default error handling provided by Flask is that there are no notifications, stack trace for errors are printed to the terminal, which means that the output of the server process needs to be monitored to discover errors. When you are running the application during development, this is perfectly fine, but once the application is deployed on a production server, nobody is going to be looking at the output, so a more robust solution needs to be put in place.

I think it is very important that I take a proactive approach regarding errors. If an error occurs on the production version of the application, I want to know right away. So my first solution is going to be to configure Flask to send me an email immediately after an error, with the stack trace of the error in the email body.

The first step is to add the email server details to the configuration file:

config.py: Email configuration

class Config(object):
    # ...
    MAIL_SERVER = os.environ.get('MAIL_SERVER')
    MAIL_PORT = int(os.environ.get('MAIL_PORT') or 25)
    MAIL_USE_TLS = os.environ.get('MAIL_USE_TLS') is not None
    MAIL_USERNAME = os.environ.get('MAIL_USERNAME')
    MAIL_PASSWORD = os.environ.get('MAIL_PASSWORD')
    ADMINS = ['your-email@example.com']

The configuration variables for email include the server and port, a boolean flag to enable encrypted connections, and optional username and password. The five configuration variables are sourced from their environment variable counterparts. If the email server is not set in the environment, then I will use that as a sign that emailing errors needs to be disabled. The email server port can also be given in an environment variable, but if not set, the standard port 25 is used. Email server credentials are by default not used, but can be provided if needed. The ADMINS configuration variable is a list of the email addresses that will receive error reports, so your own email address should be in that list.

Flask uses Python's logging package to write its logs, and this package already has the ability to send logs by email. All I need to do to get emails sent out on errors is to add a SMTPHandler instance to the Flask logger object, which is app.logger:

app/__init__.py: Log errors by email

import logging
from logging.handlers import SMTPHandler

# ...

if not app.debug:
    if app.config['MAIL_SERVER']:
        auth = None
        if app.config['MAIL_USERNAME'] or app.config['MAIL_PASSWORD']:
            auth = (app.config['MAIL_USERNAME'], app.config['MAIL_PASSWORD'])
        secure = None
        if app.config['MAIL_USE_TLS']:
            secure = ()
        mail_handler = SMTPHandler(
            mailhost=(app.config['MAIL_SERVER'], app.config['MAIL_PORT']),
            fromaddr='no-reply@' + app.config['MAIL_SERVER'],
            toaddrs=app.config['ADMINS'], subject='Microblog Failure',
            credentials=auth, secure=secure)
        mail_handler.setLevel(logging.ERROR)
        app.logger.addHandler(mail_handler)

As you can see, I'm only going to enable the email logger when the application is running without debug mode, which is indicated by app.debug being True, and also when the email server exists in the configuration.

Setting up the email logger is somewhat tedious due to having to handle optional security options that are present in many email servers. But in essence, the code above creates a SMTPHandler instance, sets its level so that it only reports errors and not warnings, informational or debugging messages, and finally attaches it to the app.logger object from Flask.

There are two approaches to test this feature. The easiest one is to use the SMTP debugging server from Python. This is a fake email server that accepts emails, but instead of sending them, it prints them to the console. To run this server, open a second terminal session and run the following command on it:

(venv) $ python -m smtpd -n -c DebuggingServer localhost:8025

Leave the debugging SMTP server running and go back to your first terminal and set export MAIL_SERVER=localhost and MAIL_PORT=8025 in the environment (use set instead of export if you are using Microsoft Windows). Make sure the FLASK_ENV variable is set to production or not set at all, since the application will not send emails in debug mode. Run the application and trigger the SQLAlchemy error one more time to see how the terminal session running the fake email server shows an email with the full stack trace of the error.

A second testing approach for this feature is to configure a real email server. Below is the configuration to use your Gmail account's email server:

export MAIL_SERVER=smtp.googlemail.com
export MAIL_PORT=587
export MAIL_USE_TLS=1
export MAIL_USERNAME=<your-gmail-username>
export MAIL_PASSWORD=<your-gmail-password>

If you are using Microsoft Windows, remember to use set instead of export in each of the statements above.

The security features in your Gmail account may prevent the application from sending emails through it unless you explicitly allow "less secure apps" access to your Gmail account. You can read about this here, and if you are concerned about the security of your account, you can create a secondary account that you configure just for testing emails, or you can enable less secure apps only temporarily to run this test and then revert back to the default.

Yet another alternative is to use a dedicated email service such as SendGrid, which allows you to send up to 100 emails per day on a free account. The SendGrid blog has a detailed tutorial on using the service in a Flask application.

Logging to a File

Receiving errors via email is nice, but sometimes this isn't enough. There are some failure conditions that do not end in a Python exception and are not a major problem, but they may still be interesting enough to save for debugging purposes. For this reason, I'm also going to maintain a log file for the application.

To enable a file based log another handler, this time of type RotatingFileHandler, needs to be attached to the application logger, in a similar way to the email handler.

app/__init__.py: Logging to a file

# ...
from logging.handlers import RotatingFileHandler
import os

# ...

if not app.debug:
    # ...

    if not os.path.exists('logs'):
        os.mkdir('logs')
    file_handler = RotatingFileHandler('logs/microblog.log', maxBytes=10240,
                                       backupCount=10)
    file_handler.setFormatter(logging.Formatter(
        '%(asctime)s %(levelname)s: %(message)s [in %(pathname)s:%(lineno)d]'))
    file_handler.setLevel(logging.INFO)
    app.logger.addHandler(file_handler)

    app.logger.setLevel(logging.INFO)
    app.logger.info('Microblog startup')

I'm writing the log file with name microblog.log in a logs directory, which I create if it doesn't already exist.

The RotatingFileHandler class is nice because it rotates the logs, ensuring that the log files do not grow too large when the application runs for a long time. In this case I'm limiting the size of the log file to 10KB, and I'm keeping the last ten log files as backup.

The logging.Formatter class provides custom formatting for the log messages. Since these messages are going to a file, I want them to have as much information as possible. So I'm using a format that includes the timestamp, the logging level, the message and the source file and line number from where the log entry originated.

To make the logging more useful, I'm also lowering the logging level to the INFO category, both in the application logger and the file logger handler. In case you are not familiar with the logging categories, they are DEBUG, INFO, WARNING, ERROR and CRITICAL in increasing order of severity.

As a first interesting use of the log file, the server writes a line to the logs each time it starts. When this application runs on a production server, these log entries will tell you when the server was restarted.

Fixing the Duplicate Username Bug

I have exploited the username duplication bug for too long. Now that I have showed you how to prepare the application to handle this type of errors, I can go ahead and fix it.

If you recall, the RegistrationForm already implements validation for usernames, but the requirements of the edit form are slightly different. During registration, I need to make sure the username entered in the form does not exist in the database. On the edit profile form I have to do the same check, but with one exception. If the user leaves the original username untouched, then the validation should allow it, since that username is already assigned to that user. Below you can see how I implemented the username validation for this form:

app/forms.py: Validate username in edit profile form.

class EditProfileForm(FlaskForm):
    username = StringField('Username', validators=[DataRequired()])
    about_me = TextAreaField('About me', validators=[Length(min=0, max=140)])
    submit = SubmitField('Submit')

    def __init__(self, original_username, *args, **kwargs):
        super(EditProfileForm, self).__init__(*args, **kwargs)
        self.original_username = original_username

    def validate_username(self, username):
        if username.data != self.original_username:
            user = User.query.filter_by(username=self.username.data).first()
            if user is not None:
                raise ValidationError('Please use a different username.')

The implementation is in a custom validation method, but there is an overloaded constructor that accepts the original username as an argument. This username is saved as an instance variable, and checked in the validate_username() method. If the username entered in the form is the same as the original username, then there is no reason to check the database for duplicates.

To use this new validation method, I need to add the original username argument in the view function, where the form object is created:

app/routes.py: Validate username in edit profile form.

@app.route('/edit_profile', methods=['GET', 'POST'])
@login_required
def edit_profile():
    form = EditProfileForm(current_user.username)
    # ...

Now the bug is fixed and duplicates in the edit profile form will be prevented in most cases. This is not a perfect solution, because it may not work when two or more processes are accessing the database at the same time. In that situation, a race condition could cause the validation to pass, but a moment later when the rename is attempted the database was already changed by another process and cannot rename the user. This is somewhat unlikely except for very busy applications that have a lot of server processes, so I'm not going to worry about it for now.

At this point you can try to reproduce the error one more time to see how the new form validation method prevents it.

275 comments

  • #251 krojas said 2021-05-02T12:10:46Z

    Hi. Thank you for the guide. It is really helpful. I would like to ask a few questions, I am a bit confused with the 2nd approach of testing the error mail (google approach). Do I just set the export commands and run flask run ? Because when I do this, I am not getting any emails I also get some error such as

    Traceback (most recent call last):
      File "/usr/lib/python3.8/logging/handlers.py", line 1008, in emit
        smtp = smtplib.SMTP(self.mailhost, port, timeout=self.timeout)
      File "/usr/lib/python3.8/smtplib.py", line 253, in __init__
        (code, msg) = self.connect(host, port)
      File "/usr/lib/python3.8/smtplib.py", line 339, in connect
        self.sock = self._get_socket(host, port, self.timeout)
      File "/usr/lib/python3.8/smtplib.py", line 308, in _get_socket
        return socket.create_connection((host, port), timeout,
      File "/usr/lib/python3.8/socket.py", line 808, in create_connection
        raise err
      File "/usr/lib/python3.8/socket.py", line 796, in create_connection
        sock.connect(sa)
    ConnectionRefusedError: [Errno 111] Connection refused
    

    THank you for your response

  • #252 Miguel Grinberg said 2021-05-02T14:14:15Z

    @krojas: the error indicates that a connection could not be made to the email server. My guess is that your MAIL_SERVER variable is not set correctly.

  • #253 Samiul Islam Shibly said 2021-06-13T01:56:35Z

    Hey Miguel! thanks for your great tutorial. My flask learning is going with you. As a newbie, many things still are not clear to me. I cannot understand the below line:

    user = User.query.filter_by(username=self.username.data).first()

    Please make it easy for me. Especially why do you use .first() here?

    Thanks again.

  • #254 Miguel Grinberg said 2021-06-13T11:51:43Z

    @Samiul: A database query returns a list of results. You always have to terminate the query indicating what you want the query to return. If you end it with all(), you get a list of all the results that match the query filters. If you use first(), you get the first matching entry.

  • #255 Elaine said 2021-06-16T13:43:32Z

    Hi,

    I am trying to add the following function to my edit profile form in my forms.py file:

    def validate_username(self, username): if username.data != self.original_username: user = User.query.filter_by(username=self.username.data).first() if user is not None: raise ValidationError('Please use a different username.')

    As my User class is defined in my app.py which imports this forms.py file, how do I access the User class in the forms.py file ? I have tried importing it from app.py but got an error due to a circular import.

    Many thanks!

  • #256 Miguel Grinberg said 2021-06-16T16:07:26Z

    @Elaine: Did you have any problems with using the structure that I use in this tutorial? Your approach of having a bunch of things all in an app.py file is not really viable, unless everything is in that one file. I suggest you adopt a structure similar to mine, where the models are in a separate module that can be imported where necessary.

  • #257 Miguel Novelo said 2021-07-07T02:05:07Z

    I have to say: Awesome tutorial, thank you so much for giving the world this gift.

    One suggestion, I was testing email with DebuggingServer but I was having timeouts, and nothing printed in the DebuggingServer (But using -d I can notice that I was receiving connections) so seems like SMTPHandler now have a timeout argument in its constructor which is defaulted to 1.0, I added timeout=10 and fixed the issue. Usually this is what I see when trying to use the DebuggingServer:

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last): File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/smtplib.py", line 391, in getreply line = self.file.readline(_MAXLINE + 1) File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/socket.py", line 669, in readinto return self._sock.recv_into(b) socket.timeout: timed out

  • #258 Alok said 2021-07-15T20:28:37Z

    I have an issue while implementing logging in file. Windows 10. PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'blog.log' -> 'blog.log.1'

  • #259 Miguel Grinberg said 2021-07-16T11:01:45Z

    @Alok: You seem to have a lock on the log file. Try rebooting the machine, that should clear any locks.

  • #260 rghv said 2021-08-01T15:06:14Z

    Hi Miguel, what happens if the race condition is triggered? Does it lead us back to 500 error?

  • #261 Miguel Grinberg said 2021-08-01T18:39:30Z

    @rghv: Yes, it will be a 500 error. You can catch the SQLAlchemy exception in the route and return a better response to the user if you like.

  • #262 Dayle said 2021-08-13T17:57:56Z

    Thanks a ton for this series - super helpful.

    I'm trying to write an error message to logger in app/main/routes.py using:

    app.logger.error("Encountered an error here!")

    I couldn't find an example where error messages were being logged in the main app.

    How would you import app.logger in to app/main/routes.py that was defined in app/init.py?

    Thanks again.

    BTW: the tag flask-mega-tutorial no longer exists in SO.

  • #263 Miguel Grinberg said 2021-08-15T21:49:58Z

    @Dayle: you can access the Flask logger from a route as current_app.logger. No need to import anything.

  • #264 Imprevisible said 2021-08-30T13:42:24Z

    I finished "Sending Errors by Email" chapter, but i got an error and idk how to repair, so my error is at https://pastebin.com/MWZVYkjy

    I have discord or you can contact myself by mail

    discord : <h1>Impre'visible</h1>#0144 email : impr.visible@gmail.com

  • #265 Miguel Grinberg said 2021-08-30T22:04:32Z

    @Imprevisible: the EditProfileForm class takes the original_username as a required argument. You are creating an instance of this class without passing any arguments. This is in line 84 of app/routes.py.

  • #266 Yogesh Bisht said 2021-10-03T06:46:29Z

    Hi Miguel,

    Thanks again for this excellent tutorial.

    I am trying to send test emails using the Sendgrid API as instructed in the following blog: https://sendgrid.com/blog/sending-emails-from-python-flask-applications-with-twilio-sendgrid/ However, I am getting the following error: UnicodeEncodeError: 'ascii' codec can't encode character '\u201d' in position 8: ordinal not in range.

    Kindly help.

  • #267 Miguel Grinberg said 2021-10-03T09:54:05Z

    @Yogesh: My guess is that this is related to using non-English characters, but you haven't provided any context to the error, so I don't really know. If that's the problem, maybe you'll need to write a bug on the SendGrid library's GitHub project and get their help you address it.

  • #268 Hamid Allaoui said 2021-10-24T07:29:11Z

    Thank you for this nice tutorial.

  • #269 Izcoatl Avila said 2021-10-25T03:04:29Z

    Hey! I'm new to Flask and i'm learning through your article. I'm using Python 3.10 and i'm getting this when i try to run the local server, haven't been able to check mails in any scenario both for gmail and the localhost: C:\Users\my-user>python -m smtpd -n -c DebuggingServer localhost:8025 C:\Users\my-user\AppData\Local\Programs\Python\Python310\lib\smtpd.py:104: DeprecationWarning: The asyncore module is deprecated. The recommended replacement is asyncio import asyncore C:\Users\my-user\AppData\Local\Programs\Python\Python310\lib\smtpd.py:105: DeprecationWarning: The asynchat module is deprecated. The recommended replacement is asyncio import asynchat

  • #270 Miguel Grinberg said 2021-10-25T17:49:18Z

    @Izcoatl: you are getting warnings, not errors. These warnings indicate some components of the Python standard library are deprecated, but this shouldn't prevent the SMTP server from being used.

  • #271 Manuel said 2021-11-17T07:22:06Z

    Hi Miguel, I am having a question regarding the last part of this tutorial, when you fix the bug, in the 'class EditProfileForm' you are defining two function, one is <validate_username> which is clear, but previous to that you define <init>. Here it is not clear for me why, could you explain a little? And additionally, could you please give me a hint on when this 'def init()' should be used inside a class or some other object in python?

    Thanks!

  • #272 Miguel Grinberg said 2021-11-17T10:07:20Z

    @Manuel: the __init__() method is a special method, it is called a constructor. It is invoked automatically when the object is created, with the arguments passed to the class. In this usage it is added so that the current user can be passed as an argument when creating the form object.

  • #273 pmcl77 said 2021-12-30T21:32:45Z

    Hi Miguel,

    First, thanks for the great tutorial... I am learning a lot step by step!

    Can it be, that the debug mode does not work when not using the Flask development server, but a UWISG webserver? I have setup a docker image / container to run uwisg and the flask app. The 404 error gets handled correctly, but the username unique constraint error from the database just throws an endless stacktrace output from sqlalchemy in the console and the browser just reports ERR_EMPTY_RESPONSE.

    I did set the FLASK_ENV=development in my docker-compose.yml as well.

    Thanks

  • #274 Miguel Grinberg said 2021-12-30T23:26:23Z

    @pmcl77: If you were expecting to see the debugger page on uWSGI, then no, that does not work out of the box, because this is something that is configured on the development web server. To use the debugger on another web server it has to be manually configured. I don't think it is worth the effort, the development web server is designed to be used for this purpose.

  • #275 Towfiq said 2022-03-18T10:12:50Z

    For the username bug I took a different approach. Also tried to follow the DRY principle and added the _validate_username method to RegistrationForm class.

    class EditProfileForm(FlaskForm):
        ...
    
        @staticmethod
        def validate_username(self, username):
            _validate_username(username)
    
    
    def _validate_username(username):
        user = User.query.filter_by(username=username.data).first()
        if user and user != current_user:
            raise ValidationError('Please use a different username.')
    

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