Teardown hooks in Flask - Step-by-Step Execution

Concept Flow - Teardown hooks

Request starts

↓

Before request handlers

↓

View function runs

↓

Response generated

↓

Teardown hooks run

↓

Request ends

This flow shows how Flask runs teardown hooks after the response is ready, to clean up resources.

Execution Sample

Flask

from flask import Flask, g
app = Flask(__name__)

@app.teardown_request
def teardown(exception=None):
    print('Teardown hook runs')

This code registers a teardown hook that runs after each request to perform cleanup.

Execution Table

Step	Event	Exception	Action	Output
1	Request starts	None	Start processing request	No output
2	View function runs	None	Process request logic	Response created
3	Response ready	None	Prepare to send response	Response sent
4	Teardown hook runs	None	Run teardown_request function	Prints 'Teardown hook runs'
5	Request ends	None	Cleanup complete	No output

💡 Request processing finished, teardown hooks executed after response sent

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
exception	None	None	None	None
g	Empty	May hold request data	Cleared or unchanged	Empty or reset

Key Moments - 2 Insights

Why does the teardown hook run even if the view function raises an error?

Because teardown hooks run after the response or error handling, as shown in step 4 of the execution_table, ensuring cleanup always happens.

Can the teardown hook access the exception that caused the error?

Yes, the exception parameter in the teardown function receives the error if any, as shown in the variable 'exception' in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed during the teardown hook step?

A 'Teardown hook runs'

B 'Request starts'

C 'Response created'

D 'Cleanup complete'

💡 Hint

Check the 'Output' column at step 4 in the execution_table.

At which step does the response get sent to the client?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Event' and 'Action' columns in the execution_table for when the response is sent.

If an exception occurs in the view function, what value does the 'exception' variable have in the teardown hook?

A A string 'error'

B None

C The exception object

D It is undefined

💡 Hint

Refer to the 'exception' variable in variable_tracker and the teardown function parameter.

Concept Snapshot

Flask teardown hooks run after each request finishes.
Use @app.teardown_request decorator.
They receive an optional exception argument.
Useful for cleaning resources like DB connections.
Run even if errors occur in the view.
Executed after response is sent.

Full Transcript

In Flask, teardown hooks are functions that run after each request finishes processing. They are registered using the @app.teardown_request decorator. These hooks run after the response is sent to the client, allowing you to clean up resources such as database connections. The teardown function receives an optional exception argument that contains any error raised during the request. This ensures cleanup happens even if the view function raises an error. The execution flow starts with the request, runs the view, sends the response, then runs teardown hooks before ending the request.