File size limits in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a file larger than the limit is uploaded?

Consider a Flask app with this setting:
app.config['MAX_CONTENT_LENGTH'] = 1024 * 1024
This limits uploads to 1MB.
What happens if a user tries to upload a 2MB file?

A Flask accepts the file but truncates it to 1MB before saving.

B Flask raises a RequestEntityTooLarge exception and returns a 413 error automatically.

C Flask silently ignores the file and processes the request as if no file was sent.

D Flask crashes and stops the server due to memory overflow.

💡 Hint

Think about how Flask handles requests exceeding the max content length.

Flask automatically raises a RequestEntityTooLarge exception when the uploaded file size exceeds MAX_CONTENT_LENGTH. This results in a 413 HTTP error response.

📝 Syntax

intermediate

Which code correctly sets a 5MB upload limit in Flask?

You want to limit file uploads to 5 megabytes in your Flask app.
Which code snippet correctly sets this limit?

A app.config['MAX_CONTENT_LENGTH'] = 5 ** 1024 ** 1024

B app.config['MAX_CONTENT_LENGTH'] = 5 * 1000 * 1000

C app.config['MAX_CONTENT_LENGTH'] = '5MB'

D app.config['MAX_CONTENT_LENGTH'] = 5 * 1024 * 1024

💡 Hint

Remember that the limit is in bytes and 1MB = 1024 * 1024 bytes.

The MAX_CONTENT_LENGTH config expects an integer number of bytes. 5MB is 5 times 1024 * 1024 bytes.

🔧 Debug

advanced

Why does this Flask app not block large uploads?

Given this Flask app snippet:

app = Flask(__name__)
app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024

@app.route('/upload', methods=['POST'])
def upload():
    file = request.files['file']
    file.save('/tmp/' + file.filename)
    return 'File saved'

Users report uploading files larger than 2MB without errors.
What is the most likely reason?

A The MAX_CONTENT_LENGTH config is set correctly but the server is behind a proxy that allows larger uploads.

B The app does not handle the RequestEntityTooLarge exception, so uploads proceed anyway.

C The client is not sending the Content-Length header, so Flask cannot enforce the limit.

D The MAX_CONTENT_LENGTH setting must be set before creating the Flask app instance.

💡 Hint

Consider how proxies or web servers might affect upload limits.

Flask enforces MAX_CONTENT_LENGTH on the request size it receives. If a proxy or web server in front allows larger uploads, Flask may never see the limit exceeded.

❓ state_output

advanced

What is the value of request.content_length for a 3MB upload?

In a Flask route handling a POST file upload, the user uploads a file exactly 3 megabytes in size.
What will request.content_length contain?

A None, because content_length is only set for GET requests

B 3000000 (which is 3 * 1000 * 1000 bytes)

C 3145728 (which is 3 * 1024 * 1024 bytes)

D Raises an AttributeError because content_length is not available

💡 Hint

Content length is the size in bytes of the request body.

request.content_length returns the size of the request body in bytes, which for 3MB is 3 * 1024 * 1024 = 3145728 bytes.

🧠 Conceptual

expert

How to customize the error response for large file uploads in Flask?

You want your Flask app to return a custom JSON message when a user uploads a file larger than the limit.
Which approach correctly achieves this?

A Use an errorhandler for RequestEntityTooLarge to return a JSON response with status 413.

B Catch the exception inside the route function and return JSON manually.

C Override the MAX_CONTENT_LENGTH setting with a function that returns JSON on error.

D Set a custom response in app.config['MAX_CONTENT_LENGTH_RESPONSE'] with the JSON message.

💡 Hint

Flask allows custom error handlers for exceptions.

Flask lets you register an error handler for RequestEntityTooLarge that can return any response, including JSON with a 413 status.