The Flask Mega-Tutorial Part VII: Error Handling (2018)

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(Great news! There is a new version of this tutorial!)

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 (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/", in _execute_context
  File "venv/lib/python3.6/site-packages/sqlalchemy/engine/", 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 '' (lazy loading)
 * Environment: development
 * Debug mode: on
 * Running on (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/ module.

app/ Custom error handlers

from flask import render_template
from app import app, db

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

def internal_error(error):
    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/ module after the application instance is created:

app/ 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: 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 = ['']

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/ 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)

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_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/ Logging to a file

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

# ...

if not app.debug:
    # ...

    if not os.path.exists('logs'):
    file_handler = RotatingFileHandler('logs/microblog.log', maxBytes=10240,
        '%(asctime)s %(levelname)s: %(message)s [in %(pathname)s:%(lineno)d]'))

    app.logger.setLevel(logging.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/ 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 != self.original_username:
            user = User.query.filter_by(
            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/ Validate username in edit profile form.

@app.route('/edit_profile', methods=['GET', 'POST'])
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.

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  • #1 David Collins said

    Hi Miguel,

    I'm unclear on two points in this tutorial at this time:
    (i) when you're adding the five email server details to the configuration file, it looks like the same variable that is retrieved with the GET function is the variable that it is being assigned to. Are those actually the same variable being identified twice or different variables being given the same name.

    (ii) the two scripts which reference 'SMTPHandler' and 'RotatingFileHandler,' which module(s) are those being inserted into. I'm sorry I can't find that reference. It appears to be an existing module because we are not modifying an existing module to reference a new module.

  • #2 Miguel Grinberg said

    @David: Regarding #1, you may be confusing class variables in the Config class with environment variables. The Flask configuration is given through the class variables in Config. For many of these, I source the values from environment variables of the same name, so that these values can be set externally. And for #2, the file is app/init,py. When in doubt, you can consult the diff link provided at the top of each article. That will send you to a detailed list of all the changes introduced in the chapter.

  • #3 Serhii said

    Hello Miguel,
    After reading paragraph 'Fixing the Duplicate Username Bug', I have implement it, but it doesn't fix the bug.
    What did i miss? (I think validate_username() method is not used in code)
    And how is better to communicate, for those who bought a book, and will have questions in the future material?

  • #4 Miguel Grinberg said

    @Serhii: compare your code against mine in GitHub, the bug fix does work. If you have a specific question, my suggestion is that you write it on Stack Overflow, which has a "flask-mega-tutorial" tag now. I'm notified when questions with that tag appear.

  • #5 bob said

    Hello Miguel

    Couldn't validate_username use current_user directly instead of passing it through the form's init method?

  • #6 Miguel Grinberg said

    @bob: yes, you could do that, but then your Form class depends on Flask-Login. In general, when there is an easy way to prevent these dependencies I prefer that approach, the more inter-dependencies you have in your project the more complex is to make changes in the future.

  • #7 Ghouse said

    Hi miguel,

    I'm learning flask through your blog and it is very awesome.
    I'm not sure about about how below code works. Could you please explain.
    def init(self, original_username, args, kwargs):
    super(EditProfileForm, self).init(
    args, **kwargs)
    self.original_username = original_username

    using super your are calling again "EditProfileForm" constructor why it requires. if am i missing anything("python concept").Please guide me. Thanks for your wonderful blog.

  • #8 Miguel Grinberg said

    @Ghouse: super is used to invoke a method in the parent class. In this particular case, I have created a class constructor and I want to invoke the class constructor in the parent class, which is the FlaskForm class. Typically when you override a method in a derived class, you want to invoke the original method in the parent class as well.

  • #9 Alexander said

    I get an error when sending an error message

    in line 749 of the module .... Python33 \ Lib \

        if isinstance(msg, str):
            msg = _fix_eols(msg).encode('ascii')
  • #10 Miguel Grinberg said

    @Alexander: Python 3.3 is very old, I have not tested that this application is compatible with it. Do you get the same error on Python 3.5 or 3.6?

  • #11 Alexander said

    It's Python 3.3 that matters to me. I have clients who refuse to migrate from the version of Windows XP. :) Therefore, we have to investigate the health of the modules in this version. I have not tested 3.5. I think that everything is in order there.

    In the end, so far. I corrected the line in the module
    Added by encode ('utf-8')
    def sendmail(self, from_addr, to_addrs, msg, mail_options=[],
    if isinstance(msg, str):
    msg = _fix_eols(msg).encode('ascii')
    msg = _fix_eols(msg).encode('utf-8') # add utf

    Another way I could not think of.

  • #12 Felipe Silveira said

    Hello Miguel,

    Thank you very much for your tutorial! Many of the issues I had while trying to learn python/flask are very well explained here!

    I have a request: If you don't have already talked about that in later chapters, can you explain a little bit about how to prevent/fix the race conditions that you briefly talked about at the end of this topic?

    Many years ago, I was hired to fix an issue like that in a rather complex enterprise system (and it happened all the time, since it was a busy server), but I didn't like the approach I used back then. I'd love to know how you would prevent such issue.

    Thanks! :D

  • #13 Peter said

    Hi Miguel, this blog is such a wonderful resource. I'm learning so much. Thank you!

    I wanted to let you know that when I updated my EditProfileForm class in to include the custom validator via the code above, I encountered a bug. Flask successfully runs, but every time I click on "Edit your profile" from the User page, it gave me a 500 error. I ultimately got the form to produce the expected behavior by editing as follows:

    from flask_login import current_user

    <h1>. . .</h1>

    class EditProfileForm(FlaskForm):
    # . . .

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

    Let me know if there's something I'm missing here? I'm very new to all of this, so I'm not sure if I'm just missing an earlier piece of code that makes it work, if it's a Python compatibility thing, if there was a typo, or something else.

  • #14 Miguel Grinberg said

    @Peter: you have not provided the error that you are getting, so I can't really tell you what's wrong. I recommend that you compare your code against mine in GitHub to find if there are differences.

  • #15 Miguel Grinberg said

    @Felipe: The way to prevent the race conditions is by introducing locks, either at the database or application layers. If you lock the users table between the time you check for the existance of the username until the time you use it, then you are guaranteed that the username will still be available when you use it.

  • #16 Alamin said

    Hi Miguel, I'm unable to understand this line.
    fromaddr='no-reply@' + app.config['MAIL_SERVER']

  • #17 Miguel Grinberg said

    @Alamin: that line sets the sender's email address. I'm using a fake email address here which is formed using the "no-reply" name and the email server. You can use any email address here if you prefer, like for example the administrator's email address.

  • #18 Pablo Gianni said

    Hi Miguel,
    First of all, excellent Mega Tutorial! Thank you very much.

    I'll try to do my contribution: When I was tring to reproduce the bug, having exported FLASK_DEBUG=1, I found the following error

    flask run
    * Serving Flask app "microblog"
    * Forcing debug mode on
    * Running on (Press CTRL+C to quit)
    * Restarting with stat
    * Debugger is active!
    * Debugger PIN: 273-561-291 - - [25/Feb/2018 17:21:31] "GET /edit_profile HTTP/1.1" 200 - - - [25/Feb/2018 17:21:40] "POST /edit_profile HTTP/1.1" 500 -
    Error on request:
    Traceback (most recent call last):
    File "/home/my_user/venv/v1/lib/python3.5/site-packages/werkzeug/", line 269, in run_wsgi
    File "/home/my_user/venv/v1/lib/python3.5/site-packages/werkzeug/", line 260, in execute
    File "/home/my_user/venv/v1/lib/python3.5/site-packages/werkzeug/", line 231, in write
    status < 200 or status in (204, 304)):
    TypeError: unorderable types: str() < int()

    It seems that the package werkzeug has kind of problem in his version 0.14. I upgraded it to the 0.14.1

    pip install --upgrade werkzeug

    and it looks like it is working properly. Now I can visualize the debug page in the browser.

    I hope this helps other as newbie as me. Thanks again!

  • #19 Bruno said

    @Alexander Since you're working with the version 3.3 of Python, be careful with the next version of Flask (0.13): "Python 2.6 and 3.3 are no longer supported".

  • #20 Asquared said

    Hi Miguel,
    Any suggestion on overcoming this issue, after exporting app, and "flask run", this appears :
    File "/home/Documents/microblog/", line 1, in <module>
    from app import app, db
    File "/home/Documents/microblog/app/", line 32, in <module>

    fromaddr='no-reply@' + app.config['MAIL_SERVER'],
    TypeError: Can't convert 'NoneType' object to str implicitly

  • #21 Vitalii said

    Hi Miguel, awesome stuff, thank you!
    Could you please explain how come the validation on the EditProfileForm works, but there is no call for the validate_username method? Are all methods declared in the FlaskForm child class being invoked automatically during submit?

  • #22 Miguel Grinberg said

    @Asquared: you need to set a valid value for the MAIL_SERVER configuration variable. Use "localhost" if you have nothing else.

  • #23 Miguel Grinberg said

    @Vitalii: The FlaskForm class uses introspection to find any methods defined in the form class that have the name "validate_<field>", and invokes them during validation.

  • #24 Eliyahu Sternberg said

    What is meant by the line

    secure = ()

    when creating the mailhandler?

    I understand that you are initializing secure to None before the if statement, so is "secure = ()" just to make it not None is using TLS?

  • #25 Miguel Grinberg said

    @Eliyahu: This is documented in the Python standard library documentation. The secure argument is an empty tuple to use TLS without providing a specific certificate and key. A one-element tuple is used when you provide just a private key, and a 2-element tuple is used when you provide a certificate and a private key. Reference:

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