How Can I Effectively Debug My Flask Application?

Debugging Flask Applications

To troubleshoot errors effectively in Flask, there are several approaches beyond console printing and page flash messages.

Running in Debug Mode

In debug mode, Flask displays an interactive traceback and console in the browser upon an error. To enable debug mode as of Flask 2.2:

flask --app example --debug run

Alternatively, you can use the following environment variables (pre-Flask 2.2):

# Linux, Mac, etc.
export FLASK_APP=example
export FLASK_DEBUG=1
flask run

# Windows (set instead of export)
set FLASK_DEBUG=1

For the app.run() method, pass debug=True:

app.run(debug=True)

Using External Debuggers

IDEs and editors such as PyCharm and VS Code provide built-in debuggers that allow you to step through code with breakpoints. Configure the run configuration to call app.run(debug=True, use_reloader=False) or use the venv/bin/flask script.

Terminal Debuggers

To use terminal debuggers like pdb or pudb, call set_trace in the desired view:

import pdb

@app.route('/debug')
def debug():
    pdb.set_trace()
    return 'Debug breakpoint'

Avoiding Catch-All Except Blocks

Excessive use of catch-all except blocks can suppress critical error messages. Flask handles exceptions by displaying the debugger or a 500 error, making try... except... redundant.

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