Teardown hooks in Flask

Introduction

Teardown hooks let you run code after a web request finishes. This helps clean up resources like database connections.

Closing a database connection after handling a web request

Releasing file handles or network connections used during a request

Logging information once a request is fully processed

Cleaning up temporary data created during a request

Syntax

Flask

@app.teardown_request
def teardown_func(error=None):
    # cleanup code here

The function runs after each request, even if there was an error.

The optional error parameter tells if the request ended with an error.

Examples

This closes the database connection stored in Flask's g object after each request.

Flask

@app.teardown_request
def close_db(error=None):
    if hasattr(g, 'db'):
        g.db.close()

This logs whether the request finished with or without an error.

Flask

@app.teardown_request
def log_request(error=None):
    if error:
        print(f"Request ended with error: {error}")
    else:
        print("Request finished successfully")

Sample Program

This Flask app opens a dummy database connection before each request and closes it after the request finishes. The teardown hook prints a message when closing.

Flask

from flask import Flask, g

app = Flask(__name__)

@app.before_request
def open_db():
    g.db = open('dummy_db_connection', 'w')

@app.route('/')
def index():
    g.db.write('Hello')
    return 'Hello World'

@app.teardown_request
def close_db(error=None):
    if hasattr(g, 'db'):
        g.db.close()
        print('Database connection closed')

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Teardown hooks run even if your view function raises an error.

Use teardown hooks to avoid resource leaks in your app.

Summary

Teardown hooks run code after each request ends.

They help clean up resources like files or database connections.

The function can check if the request had an error using the optional parameter.

Practice

1. What is the main purpose of a teardown_request function in Flask?

easy

A. To handle incoming HTTP requests

B. To define URL routes

C. To start a new request

D. To run cleanup code after each request finishes

Solution

1. Step 1: Understand the role of teardown hooks

   Teardown hooks run after a request ends to clean up resources like files or database connections.

2. Step 2: Match the purpose with options

   Only To run cleanup code after each request finishes describes running cleanup code after each request finishes, which matches the teardown hook purpose.

3. Final Answer:

   To run cleanup code after each request finishes -> Option D

4. Quick Check:

   Teardown hooks = cleanup after request [OK]

Hint: Teardown hooks always run after request ends [OK]

Common Mistakes:

• Confusing teardown with request handling
• Thinking teardown runs before request
• Assuming teardown defines routes

2. Which of the following is the correct way to register a teardown function in Flask?

easy

A. @app.teardown_request
def cleanup(error=None): pass

B. @app.after_request
def cleanup(): pass

C. @app.route('/teardown')
def cleanup(): pass

D. @app.before_request
def cleanup(): pass

Solution

1. Step 1: Identify the correct decorator for teardown

   The correct decorator to register a teardown function is @app.teardown_request, which optionally accepts an error parameter.

2. Step 2: Check function signature

   The function should accept an optional error=None parameter to detect errors during the request.

3. Final Answer:

   @app.teardown_request\ndef cleanup(error=None): pass -> Option A

4. Quick Check:

   Teardown decorator = @app.teardown_request [OK]

Hint: Teardown functions use @app.teardown_request with error param [OK]

Common Mistakes:

• Using @app.before_request instead
• Missing the optional error parameter
• Confusing with @app.after_request

3. Given this Flask code snippet, what will be printed after a request completes successfully (no error)?

@app.teardown_request
def cleanup(error=None):
    if error:
        print('Error occurred')
    else:
        print('Request finished cleanly')

medium

A. Error occurred

B. Request finished cleanly

C. No output

D. SyntaxError

Solution

1. Step 1: Understand the error parameter in teardown

   The error parameter is None if the request finished without errors.

2. Step 2: Trace the if-else logic

   Since error is None, the else branch runs, printing 'Request finished cleanly'.

3. Final Answer:

   Request finished cleanly -> Option B

4. Quick Check:

   error=None means clean finish [OK]

Hint: error=None means no error, so else branch runs [OK]

Common Mistakes:

• Assuming error is always True
• Expecting no output
• Confusing teardown with before_request

4. What is wrong with this teardown function?

@app.teardown_request
def cleanup():
    print('Cleaning up')

medium

A. Decorator should be @app.before_request

B. Using print inside teardown is not allowed

C. Missing the optional error parameter in the function

D. Function name must be 'teardown' exactly

Solution

1. Step 1: Check teardown function signature

   Teardown functions must accept an optional error=None parameter to receive error info.

2. Step 2: Identify missing parameter issue

   The function cleanup has no parameters, so Flask will raise an error when calling it.

3. Final Answer:

   Missing the optional error parameter in the function -> Option C

4. Quick Check:

   Teardown needs error param or it fails [OK]

Hint: Teardown functions must accept error=None parameter [OK]

Common Mistakes:

• Omitting error parameter
• Using wrong decorator
• Thinking function name is fixed

5. You want to close a database connection after every request, but only if it was opened during that request. Which is the best way to do this using a teardown hook?

hard

A. Use @app.teardown_request with a function that checks if the connection exists and closes it

B. Use @app.before_request to close the connection before the request starts

C. Use @app.route to close the connection on a specific URL

D. Use @app.after_request to close the connection without checking if it exists

Solution

1. Step 1: Understand when to close resources

   Resources like database connections should be closed after the request ends, not before or on specific routes.

2. Step 2: Use teardown_request with conditional check

   The teardown function can check if the connection was opened during the request and close it safely.

3. Final Answer:

   Use @app.teardown_request with a function that checks if the connection exists and closes it -> Option A

4. Quick Check:

   Teardown + check connection before closing = best practice [OK]

Hint: Close resources in teardown only if they exist [OK]

Common Mistakes:

• Closing connection before request starts
• Closing without checking if connection exists
• Trying to close on specific route only