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?
15 Answers
153
votes
Running the app in development mode will show an interactive traceback and console in the browser when there is an error. To run in development mode, set the FLASK_ENV=development environment variable then use the flask run command (remember to point FLASK_APP to your app as well).
For Linux, Mac, Linux Subsystem for Windows, Git Bash on Windows, etc.:
export FLASK_APP=myapp
export FLASK_ENV=development
flask run
For Windows CMD, use set instead of export:
set FLASK_ENV=development
For PowerShell, use $env:
$env:FLASK_ENV = "development"
Prior to Flask 1.0, this was controlled by the FLASK_DEBUG=1 environment variable instead.
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.
43
votes
You can use app.run(debug=True) for the Werkzeug Debugger edit as mentioned below, and I should have known.
30
votes
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
20
votes
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
15
votes
12
votes
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.
7
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6
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5
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2
votes
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)
1
votes
0
votes
If you're using pycharm like me and are using the "run" button, you simply have to modify some settings inside pycharm itself. What the IDE does is run a command using your options, just nicely wrapped.
Top right corner > Edit Configurations... > check the box next to "FLASK_DEBUG".
This option enables werkzeug debugger without you having to change your code (good for apps that you need to deploy). See here: https://flask.palletsprojects.com/en/master/debugging/
Works perfectly.
app.run()(either with debug on or off). Flask behaves like any other python application, so you can debug it the same way you debug any Python application. If you want to use logging, use logging. If you want to prints, use prints. You can even use a debugger if you want. - Mark Hildreth