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

  • #101 Miguel Grinberg said 2019-01-03T09:26:59Z

    @Tri: The first method just saves the original username as an attribute of the form object. This attribute is used in the second method to ensure that the validation of the username works as expected. The rules are: If the username is the same as before, then it is valid, else, it is only valid if it is a username that is not taken by any other account.

  • #102 Daniel said 2019-01-13T02:39:58Z

    Hi Miguel, I'm loving this tutorial, so thank you.

    Now that I have added your fix to the Edit Profile issue, I keep getting an error. I'm pretty sure my code matches yours, but maybe I've missed something.

    forms.py

    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): #automatically invoked upon validation if username.data != self.orginal_username: user = User.query.filter_by(username=self.username.data).first() if user is not None: raise ValidationError('Please use a different username.')

    routes.py

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

    The error is: File "/Users/#my name/Development/microblog/app/forms.py", line 43, in validate_username if username.data != self.orginal_username: AttributeError: 'EditProfileForm' object has no attribute 'orginal_username'

    Is the def init in the EditProfileForm(FlaskForm) failing to actually assign the self.original username attribute somehow?

  • #103 Miguel Grinberg said 2019-01-13T17:53:26Z

    @Daniel: It's a typo, I think. The attribute is "self.original_username", you have "self.orginal_username" (there's a missing i in there).

  • #104 Aleksander said 2019-01-24T22:42:06Z

    Excellent guide so far.

    I have one question regarding your fix for the edit profile page. I find the behavior of the page itself somewhat strange as the way it is now the edit page allows anyone logged to create a user with no password (or a default one, haven't been able to check what it actually does).

    Wouldn't it be better to have some form of access control where you can't change the about me part for other people than your own user?

  • #105 Miguel Grinberg said 2019-01-24T23:43:57Z

    @Aleksander: I'm not sure I understand the problem you are reporting. The /edit-profile route only allows you to edit your own profile page, you cannot edit another user's information.

  • #106 Valerii said 2019-02-08T10:13:01Z

    Hello Miguel, I am running a web application structured very much like in this tutorial. I am using MySQL however, and the connection to the DB server can be unavailable. Which results in the connection related exception not being handled by the @app.errorhandler(500) decorated function (also built very much like in this article). I assume this happens because the db connection is established before the 'errors' module import is made in the config.py (please correct me if I'm wrong). If that is indeed the case, Is there a short elegant way to handle such exception same way the 'errors' module does without breaking current config structure too much? Or if the cause is different?

    Thanks!

    P.S. Your tutorial is absolutely awesome!

  • #107 Miguel Grinberg said 2019-02-08T14:08:37Z

    @Valerii: do you have a stack trace for this error that I can see?

  • #108 Miguel Grinberg said 2019-02-12T17:37:07Z

    @Valerii: the error is "Can't connect to MySQL server on 'localhost'". Your database connection URL is likely incorrect.

  • #109 Valerii said 2019-02-13T18:46:34Z

    @Miguel, that's right, it happens if the MySQL server on my machine is not running (the app works fine when it does). I assume that's what would be happening if the DB server on an external machine becomes unavailable which is a possibility I guess. It then displays a generic "Internal Server Error" page instead of the one that is served by the error handler function (that works for other uncaught exceptions I've encountered so far). Hence my question, am I doing something wrong or is there something else that needs to be done to handle such an exception like others?

  • #110 Miguel Grinberg said 2019-02-13T20:06:30Z

    @Valerii: I suppose if the connection problem occurs while a route is running, then you will be redirected to the 500 handler. I suspect in this case the error occurs when the application is starting up, not while a route is being handled. So the correct way to test this would be to start the app, make sure everything works, then kill MySQL and see if the errors are handled properly.

  • #111 Valerii said 2019-02-18T19:34:50Z

    @Miguel, I did just that and the it doesn’t seem to depend much on whether the DB is unavailable on startup or goes down some time after that. It turns out, however, that it depends more on whether the user is logged in. Anonymous’ users routes that raise uncaught errors (both those that rely on DB somehow and those that don’t) are handled correctly. But if the user is logged in, the handler just fails, even if it itself seemingly doesn’t have to do anything with the DB (I made sure the route method is not decorated with @login_required; that it doesn’t reference the db, current_user, etc.; I made the error pages’ templates not extend the ‘base.html’ and therefore not reference the ‘current_user’ variable either)

    If I kill the server after the app startup and user loading at least one page successfully, I get following error in the stack trace (even when invoking routes that seemingly don’t reference the user as mentioned above): MySQLdb._exceptions.OperationalError: (2013, 'Lost connection to MySQL server during query')

    The above exception was the direct cause of the following exception: <a lot of less informative stuff in the middle> sqlalchemy.exc.OperationalError: (MySQLdb._exceptions.OperationalError) (2013, 'Lost connection to MySQL server during query') [SQL: 'SELECT user.id AS user_id, user.username AS user_username, user.email AS user_email, user.password_hash AS user_password_hash \nFROM user \nWHERE user.id = %s'] [parameters: (1,)] (Background on this error at: http://sqlalche.me/e/e3q8)

    2019-02-18 20:18:53,473 INFO: Employees startup [in D:\pathtoapp\app__init__.py:35]

    So SQLALchemy’s IntegrityError (that can be caused by duplicate usernames) is handled correctly, but its OperationalError (that can be caused by the absence of MySQL DB connection) is not.

  • #112 George Damaskos said 2019-02-19T12:18:47Z

    Hello Miguel,
    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.')
    
    
    

    Is the ValidationError caught and handled in an exception block at some point in the application? Thank you very much, George

  • #113 Miguel Grinberg said 2019-02-20T06:51:00Z

    @Valerii: you need to look at all that "less informative" stuff that you omitted to see where the error is being raised. It can be. If the 500 handler isn't invoked, then it is not while your view function is running, must be before or after.

  • #114 Miguel Grinberg said 2019-02-20T06:53:40Z

    @George: the exception is handled by WTForms.

  • #115 Ummar Abbas said 2019-02-27T09:58:21Z

    Hi Miguel, Thanks a lot for the tutorial. I am having trouble with the errorhandler functions following your REST API tutorial using blueprints.

    When my model raises a ValidationError, the @api.errorhanlder(ValidationError) is not tiggered at all.

    I am using ValidationError to report all kinds of db error such as PK violation, unique constraint violation etc.

    Can you please help?

  • #116 Miguel Grinberg said 2019-02-27T15:10:22Z

    @Ummar: are the errors coming from the api blueprint? If you need the handler to trigger on any routes, then you need to use the @api.app_errorhandler decorator.

  • #117 Gareth Kear said 2019-04-03T03:01:21Z

    Hi Miguel,

    SECURITY QUESTION. The line " self.original_username" in EditProfileForm is a mystery to us. When the form posts back, where does it get the instantiated object for EditProfileForm from to look at "self.original_username" ? Is it somehow in that cookie? That object can't be sitting on the server "waiting" for that browser user - right? How would it find it ? All I see is a session value in the cookie when viewing. We need to know for security reasons. In my own Framework, I store sessions and all the values for a session in a DB - which is required by our DHS client. I would like to use Flask for a different client that will not require DB sesssions,

  • #118 simanacci said 2019-04-03T10:33:29Z

    Hi, Using RotatingFileHandler code only creates /logs/file.log and logs app.logger.info only. What I'm I missing? My code is not housed under if not app.debug

  • #119 Miguel Grinberg said 2019-04-04T08:48:01Z

    @Gareth: a new form object is created when form is posted. See the first line of the edit_profile() function, which does:

    form = EditProfileForm(current_user.username)

    This happens at the start of the handling of the POST request. The current_user variable is populated from the contents of the user session, this is something that Flask-Login does for us.

  • #120 Miguel Grinberg said 2019-04-04T08:48:56Z

    @simanacci: did you want long enough for the logs to rotate? You are going to see more log files when the first log file fills.

  • #121 Paul Ryan said 2019-05-04T03:09:09Z

    Hi Miguel,

    When setting up the 'errors by email', why do you use:

    if app.config['MAIL_USERNAME'] or app.config['MAIL_PASSWORD']:

    as opposed to:

    if app.config['MAIL_USERNAME'] and app.config['MAIL_PASSWORD']:

    Seems like you create a tuple 'auth' that expects both variables, so why not ensure both are defined prior to updating auth?

    Thanks! Paul R.

  • #122 Miguel Grinberg said 2019-05-04T10:43:40Z

    @Paul: It's not really a strong argument, but I thought that depending on the authentication style used by the server you may not need both, but just one of them. It's totally fine to use AND in this condition.

  • #123 Paul said 2019-07-21T01:17:06Z

    Miguel - I appreciate your clarity and the attention to detail you present here...as an educator, it really is pleasing to find a resource I can learn from so effectively.

    I'm having a bit of trouble utilizing debug mode in my Windows environment (Python 3.7.4, 64 bit). When I execute "flask run" with FLASK_DEBUG=0, everything works fine. When I set FLASK_DEBUG=1, an error terminates the run (see below). Any suggestions? Probably a simple fix, but I'm a bit stumped as a noob...

    (venv) C:\Users\Paul\PycharmProjects\microblog>set FLASK_DEBUG=1

    (venv) C:\Users\Paul\PycharmProjects\microblog>flask run * Serving Flask app "microblog.py" (lazy loading) * Environment: production WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead. * Debug mode: on * Restarting with stat c:\users\paul\pycharmprojects\microblog\venv\scripts\python.exe: No module named C:\Users\Paul\PycharmProjects\microblog\venv\Scripts\flask

    (venv) C:\Users\Paul\PycharmProjects\microblog>

  • #124 Miguel Grinberg said 2019-07-21T09:57:26Z

    @Paul: I'm not sure exactly what the problem is, but in general when these type of errors occur it is caused by an incorrectly set up virtual environment. I suggest that you trash your venv directory and recreate it from scratch, making sure that you activate it before you install Flask and other dependencies.

  • #125 sunny said 2019-07-22T23:34:34Z

    'flask run' does not work after 'set FLASK_DEBUG=1' but when I set 'set FLASK_DEBUG=0' it works again. when 'set FLASK_DEBUG=1', it gives an error:

    Serving Flask app "microblog.py" (lazy loading) Environment: production WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead. Debug mode: on Restarting with stat c:\users\UserName\desktop\temp\venv\scripts\python.exe: No module named C:\Users\UserName\Desktop\temp\venv\Scripts\flask

Leave a Comment