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How are you meant to debug errors in Flask? Print to the console? Flash messages to the page? Or is there a more powerful option available to figure out what's happening when something goes wrong?
Running the app in debug mode will show an interactive traceback and console in the browser when there is an error. As of Flask 2.2, to run in debug mode, pass the --app and --debug options to the flask command.
$ flask --app example --debug run
Prior to Flask 2.2, this was controlled by the FLASK_ENV=development environment variable instead. You can still use FLASK_APP and FLASK_DEBUG=1 instead of the options above.
For Linux, Mac, Linux Subsystem for Windows, Git Bash on Windows, etc.:
$ export FLASK_APP=example
$ export FLASK_DEBUG=1
$ flask run
For Windows CMD, use set instead of export:
set FLASK_DEBUG=1
For PowerShell, use $env:
$env:FLASK_DEBUG = "1"
If you're using the app.run() method instead of the flask run command, pass debug=True to enable debug mode.
Tracebacks are also printed to the terminal running the server, regardless of development mode.
If you're using PyCharm, VS Code, etc., you can take advantage of its debugger to step through the code with breakpoints. The run configuration can point to a script calling app.run(debug=True, use_reloader=False), or point it at the venv/bin/flask script and use it as you would from the command line. You can leave the reloader disabled, but a reload will kill the debugging context and you will have to catch a breakpoint again.
You can also use pdb, pudb, or another terminal debugger by calling set_trace in the view where you want to start debugging.
Be sure not to use too-broad except blocks. Surrounding all your code with a catch-all try... except... will silence the error you want to debug. It's unnecessary in general, since Flask will already handle exceptions by showing the debugger or a 500 error and printing the traceback to the console.
You can use app.run(debug=True) for the Werkzeug Debugger edit as mentioned below, and I should have known.
From the 1.1.x documentation, you can enable debug mode by exporting an environment variable to your shell prompt:
export FLASK_APP=/daemon/api/views.py # path to app
export FLASK_DEBUG=1
python -m flask run --host=0.0.0.0
One can also use the Flask Debug Toolbar extension to get more detailed information embedded in rendered pages.
from flask import Flask
from flask_debugtoolbar import DebugToolbarExtension
import logging
app = Flask(__name__)
app.debug = True
app.secret_key = 'development key'
toolbar = DebugToolbarExtension(app)
#app.route('/')
def index():
logging.warning("See this message in Flask Debug Toolbar!")
return "<html><body></body></html>"
Start the application as follows:
FLASK_APP=main.py FLASK_DEBUG=1 flask run
If you're using Visual Studio Code, replace
app.run(debug=True)
with
app.run()
It appears when turning on the internal debugger disables the VS Code debugger.
If you want to debug your flask app then just go to the folder where flask app is. Don't forget to activate your virtual environment and paste the lines in the console change "mainfilename" to flask main file.
export FLASK_APP="mainfilename.py"
export FLASK_DEBUG=1
python -m flask run --host=0.0.0.0
After you enable your debugger for flask app almost every error will be printed on the console or on the browser window.
If you want to figure out what's happening, you can use simple print statements or you can also use console.log() for javascript code.
To activate debug mode in flask you simply type set FLASK_DEBUG=1 on your CMD for windows, or export FLASK_DEBUG=1 on Linux terminal then restart your app and you are good to go!!
Install python-dotenv in your virtual environment.
Create a .flaskenv in your project root. By project root, I mean the folder which has your app.py file
Inside this file write the following:
FLASK_APP=myapp
FLASK_ENV=development
Now issue the following command:
flask run
When running as python app.py instead of the flask command, you can pass debug=True to app.run.
if __name__ == "__main__":
app.run(debug=True)
$ python app.py
with virtual env activate
export FLASK_DEBUG=true
you can configure
export FLASK_APP=app.py # run.py
export FLASK_ENV = "development"
to start
flask run
the result
* 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: xxx-xxx-xxx
and if you change
export FLASK_DEBUG=false
* Environment: development
* Debug mode: off
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
For Windows users:
Open Powershell and cd into your project directory.
Use these commandos in Powershell, all the other stuff won't work in Powershell.
$env:FLASK_APP = "app"
$env:FLASK_ENV = "development"
If you have PyCharm Professional, you can create a Flask server run configuration and enable the FLASK_DEBUG checkbox. Go to Run > Edit Configurations, select or create a Flask server configuration, and enable the FLASK_DEBUG checkbox. Click OK, then click the run button.
You can install python-dotenv with
pip install python-dotenv then create a .flask_env or a .env file
The contents of the file can be:
FLASK_APP=myapp
FLASK_DEBUG=True
Use loggers and print statements in the Development Environment, you can go for sentry in case of production environments.
How are you meant to debug errors in Flask? Print to the console? Flash messages to the page? Or is there a more powerful option available to figure out what's happening when something goes wrong?
Running the app in debug mode will show an interactive traceback and console in the browser when there is an error. As of Flask 2.2, to run in debug mode, pass the --app and --debug options to the flask command.
$ flask --app example --debug run
Prior to Flask 2.2, this was controlled by the FLASK_ENV=development environment variable instead. You can still use FLASK_APP and FLASK_DEBUG=1 instead of the options above.
For Linux, Mac, Linux Subsystem for Windows, Git Bash on Windows, etc.:
$ export FLASK_APP=example
$ export FLASK_DEBUG=1
$ flask run
For Windows CMD, use set instead of export:
set FLASK_DEBUG=1
For PowerShell, use $env:
$env:FLASK_DEBUG = "1"
If you're using the app.run() method instead of the flask run command, pass debug=True to enable debug mode.
Tracebacks are also printed to the terminal running the server, regardless of development mode.
If you're using PyCharm, VS Code, etc., you can take advantage of its debugger to step through the code with breakpoints. The run configuration can point to a script calling app.run(debug=True, use_reloader=False), or point it at the venv/bin/flask script and use it as you would from the command line. You can leave the reloader disabled, but a reload will kill the debugging context and you will have to catch a breakpoint again.
You can also use pdb, pudb, or another terminal debugger by calling set_trace in the view where you want to start debugging.
Be sure not to use too-broad except blocks. Surrounding all your code with a catch-all try... except... will silence the error you want to debug. It's unnecessary in general, since Flask will already handle exceptions by showing the debugger or a 500 error and printing the traceback to the console.
You can use app.run(debug=True) for the Werkzeug Debugger edit as mentioned below, and I should have known.
From the 1.1.x documentation, you can enable debug mode by exporting an environment variable to your shell prompt:
export FLASK_APP=/daemon/api/views.py # path to app
export FLASK_DEBUG=1
python -m flask run --host=0.0.0.0
One can also use the Flask Debug Toolbar extension to get more detailed information embedded in rendered pages.
from flask import Flask
from flask_debugtoolbar import DebugToolbarExtension
import logging
app = Flask(__name__)
app.debug = True
app.secret_key = 'development key'
toolbar = DebugToolbarExtension(app)
#app.route('/')
def index():
logging.warning("See this message in Flask Debug Toolbar!")
return "<html><body></body></html>"
Start the application as follows:
FLASK_APP=main.py FLASK_DEBUG=1 flask run
If you're using Visual Studio Code, replace
app.run(debug=True)
with
app.run()
It appears when turning on the internal debugger disables the VS Code debugger.
If you want to debug your flask app then just go to the folder where flask app is. Don't forget to activate your virtual environment and paste the lines in the console change "mainfilename" to flask main file.
export FLASK_APP="mainfilename.py"
export FLASK_DEBUG=1
python -m flask run --host=0.0.0.0
After you enable your debugger for flask app almost every error will be printed on the console or on the browser window.
If you want to figure out what's happening, you can use simple print statements or you can also use console.log() for javascript code.
To activate debug mode in flask you simply type set FLASK_DEBUG=1 on your CMD for windows, or export FLASK_DEBUG=1 on Linux terminal then restart your app and you are good to go!!
Install python-dotenv in your virtual environment.
Create a .flaskenv in your project root. By project root, I mean the folder which has your app.py file
Inside this file write the following:
FLASK_APP=myapp
FLASK_ENV=development
Now issue the following command:
flask run
When running as python app.py instead of the flask command, you can pass debug=True to app.run.
if __name__ == "__main__":
app.run(debug=True)
$ python app.py
with virtual env activate
export FLASK_DEBUG=true
you can configure
export FLASK_APP=app.py # run.py
export FLASK_ENV = "development"
to start
flask run
the result
* 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: xxx-xxx-xxx
and if you change
export FLASK_DEBUG=false
* Environment: development
* Debug mode: off
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
For Windows users:
Open Powershell and cd into your project directory.
Use these commandos in Powershell, all the other stuff won't work in Powershell.
$env:FLASK_APP = "app"
$env:FLASK_ENV = "development"
If you have PyCharm Professional, you can create a Flask server run configuration and enable the FLASK_DEBUG checkbox. Go to Run > Edit Configurations, select or create a Flask server configuration, and enable the FLASK_DEBUG checkbox. Click OK, then click the run button.
You can install python-dotenv with
pip install python-dotenv then create a .flask_env or a .env file
The contents of the file can be:
FLASK_APP=myapp
FLASK_DEBUG=True
Use loggers and print statements in the Development Environment, you can go for sentry in case of production environments.
I am trying to learn Flask using VScode.
The tutorial that I am following is: Python Flask Tutorial: Full-Featured Web App Part 1 - Getting Started.
I did the following things:
Created a new virtualenv in a folder using: virtualenv venv
activated it as: venv\Scripts\activate (I am on Windows 10)
After that, I created a new directory named Flask_Blog using mkdir Flask_Blog and in it, I created a new flaskblog.py file containing the following code:
from flask import Flask
app = Flask(__name__)
#app.route('/')
def hello():
return 'Hello'
Then, in the terminal of VScode, I changed my working directory in order to be in the Flask_Blog directory using cd Flask_Blog.
Now, when I am doing set FLASK_APP=flaskblog.py followed by flask run, I am getting the following error:
(venv) PS C:\Users\kashy\OneDrive\Desktop\Flask\Flask_Blog> flask run
* Environment: production
WARNING: This is a development server. Do not use it in a production deployment.
Use a production WSGI server instead.
* Debug mode: off
Usage: flask run [OPTIONS]
Error: Could not locate a Flask application. You did not provide the "FLASK_APP" environment variable, and a "wsgi.py" or "app.py" module was not found in the current directory.
But
When I do the same in the cmd prompt, the code runs and I get to see the output.
I am completely new to this. Can anyone please tell me what is the mistake I am doing in VSCode and why is it working in the cmd?
Issue raised in VsCode
Under Powershell, you have to set the FLASK_APP environment variable as follows:
$env:FLASK_APP = "webapp"
Then you should be able to run "python -m flask run" inside the hello_app folder. In other words, PowerShell manages environment variables differently, so the standard command-line "set FLASK_APP=webapp" won't work.
Try Set FLASK_APP = Full path of the folder/filename.py.
This worked for me
This worked for me on the VSCode:
$env:FLASK_APP= 'C:\Python\ex003\main:app'
I'm trying to install OSQA, a Django application on Linux Red Hat (Python 2.7 and Djando 1.6). I synchronized the databases and I restarted Apache without get any error.
export DJANGO_SETTINGS_MODULE=settings
python2.7 manage.py syncdb --settings=settings
python2.7 manage.py migrate --settings=settings --fake
$APACHE/restart
After I opened the site from the browser I get HTTP status "200" Error, and in the logs I get this error:
ImproperlyConfigured: Error importing module forum.middleware.extended_user: "cannot import name get_svn_revision"
(See complete Logs)
Any help is greatly appreciated. Thank you!
I was also having this issue with OSQA, and restarting Apache solved it for me. I'm not sure why this happened though; sorry!
I'm developing a flask.app that uses web socket functionality and installed flask-socket to provide that. So the flask-socket developer recommends gunicorn as web server. My question is now how to connect the remove debugger of pycharm with gunicorn to intercept the execution with breakpoints.
Settings > Project Settings > Python Debugger
There's an option in there to enable "gevent compatible debugging".
Then, go into your debugger settings (shortcut is through the toolbar, click the dropdown near the play/debug icons and select "Edit Configurations"
Set the "Script" to your virtualenv's isntallation of gunicorn, something like:
/Users/iandouglas/.virtualenvs/defaultenv/bin/gunicorn
Set the "Script Parameters" to something like
-b 192.168.1.1:9000 app:yourappname (assuming your primary starting script is called app.py and you're refering to it as 'yourappname'
the "Working directory" will be automatically set otherwise set it to wherever your code lives: /Users/iandouglas/PycharmProjects/MyExampleApp
I have a separate config file for my gunicorn settings, which specifies a host/port but I still had to specify the -b 0.0.0.0:5001 parameter to force gunicorn to bind to all IPs on my machine on port 5001.
p.s.
One important step is to add this envvar as pointed out here
PYDEVD_USE_CYTHON=NO
My case for PyCharm 2018.1.3 Professional:
Go to run/debug configurations
creating-and-editing-run-debug-configurations
Choose new "Python" config
Script path: your_path_to_/venv/bin/gunicorn
Parameters(for my case): -b :5001 --access-logfile - --error-logfile - "run:create_application()"
Python interpreter: your venv python version for project
Working directory: path to your project
Save and press DEBUG (Shift+F9)
be happy!
I was trying to debug on Pycharm 2020.2.1 and the break points weren't correctly working even though the Gevent compatible debugging was enabled. It turned out that I had to disable Cython for my run configuration via setting the environment variable as described here to make it work.
PYDEVD_USE_CYTHON=NO
edit your flask launch python file
$ vim manage.py
remove debug option setting
from web import app
import sys
if __name__ == '__main__':
# app.run(host='0.0.0.0', port=app.config["PORT"], debug=app.config["DEBUG"])
app.run(host='0.0.0.0', port=9998, debug=False)