How to debug a Flask app

Question

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

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If you are running it locally and want to be able to step through the code   python -m pdb script py

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Use loggers and print statements in the Development Environment  you can go for sentry in case of production environments

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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

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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

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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

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Quick tip - if you use a PyCharm  go to Edit Configurations    Configurations and enable FLASK DEBUG checkbox  restart the Run

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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    quot app quot     env FLASK ENV    quot development quot

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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

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You can use app run debug True  for the Werkzeug Debugger edit as mentioned below  and I should have known

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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

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To activate debug mode in flask you simply type set FLASK DEBUG 1 on your CMD for windows and export FLASK DEBUG 1 on Linux termial then restart your app and you are good to go

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with virtual env activate export FLASK DEBUG true  you can configure export FLASK APP app py    run py export FLASK ENV    quot development quot   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

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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   lt html gt  lt body gt  lt  body gt  lt  html gt     Start the application as follows   FLASK APP main py FLASK DEBUG 1 flask run

User · Answer

To debug a Flask application you need to use  server run      debug True

[python] How to debug a Flask app

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