TypeError: teardown_appcontext wrong signature

TypeError: teardown_appcontext wrong signature

$ flask --app app.py run
Traceback (most recent call last):
  File "app.py", line 18, in <module>
    @app.teardown_appcontext
TypeError: teardown_appcontext handlers must accept (exception)

Why this happens

teardown_appcontext functions receive an exception argument (which may be None) indicating whether an error occurred. If the handler signature doesn’t accept this parameter, Flask raises TypeError when calling it.

Fix

Define the handler to accept a single argument, even if you don’t use it.

Wrong code

from flask import Flask
app = Flask(__name__)

@app.teardown_appcontext
def cleanup():
    pass

Fixed code

from flask import Flask
app = Flask(__name__)

@app.teardown_appcontext
def cleanup(exception):
    # free resources regardless of exception
    pass

Tip

Use teardown_appcontext for app-wide cleanup (DB connections, caches). For per-request cleanup, use teardown_request with a similar signature.