File size limits in Flask - Mini Project: Build & Apply

File Size Limits in Flask

📖 Scenario: You are building a simple Flask web app that allows users to upload files. To keep the server safe and efficient, you want to limit the size of files users can upload.

🎯 Goal: Create a Flask app that sets a file size limit of 1 megabyte (1MB) and handles file uploads with this limit enforced.

📋 What You'll Learn

Create a Flask app instance

Set the maximum allowed file size to 1MB using Flask configuration

Write a route to accept file uploads

Add error handling for files that exceed the size limit

💡 Why This Matters

🌍 Real World

Limiting file upload size is important to protect servers from overload and abuse in real web applications.

💼 Career

Understanding how to configure Flask apps and handle file uploads is a common task for backend web developers.

1

Create the Flask app instance

Create a Flask app instance called app using Flask(__name__).

Flask

# Create the Flask app instance
# Your code here

Hint

Use Flask(__name__) to create the app instance.

2

Set the maximum file size limit

Set the Flask configuration variable MAX_CONTENT_LENGTH to 1 * 1024 * 1024 (which is 1MB) on the app.config dictionary.

Flask

from flask import Flask

app = Flask(__name__)
# Set max file size limit to 1MB
# Your code here

Hint

Use app.config['MAX_CONTENT_LENGTH'] = 1 * 1024 * 1024 to set the limit.

3

Create a file upload route

Create a route /upload that accepts POST requests. Inside the route function called upload_file, get the uploaded file from request.files['file'] and return the string 'File uploaded'.

Flask

from flask import Flask, request

app = Flask(__name__)
app.config['MAX_CONTENT_LENGTH'] = 1 * 1024 * 1024

# Create /upload route to accept POST and get uploaded file
# Your code here

Hint

Use @app.route('/upload', methods=['POST']) and get the file with request.files['file'].

4

Add error handling for large files

Import RequestEntityTooLarge from werkzeug.exceptions. Add an error handler for RequestEntityTooLarge on app.errorhandler that returns the string 'File too large' with status code 413.

Flask

from flask import Flask, request
from werkzeug.exceptions import RequestEntityTooLarge

app = Flask(__name__)
app.config['MAX_CONTENT_LENGTH'] = 1 * 1024 * 1024

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files['file']
    return 'File uploaded'

# Add error handler for RequestEntityTooLarge
# Your code here

Hint

Use @app.errorhandler(RequestEntityTooLarge) to catch large file errors.

Practice

1. What does setting MAX_CONTENT_LENGTH in a Flask app do?

easy

A. Sets the maximum number of files a user can upload.

B. Limits the maximum size of incoming request data, like file uploads.

C. Defines the maximum number of users allowed to connect.

D. Controls the maximum time a request can take to process.

Solution

1. Step 1: Understand the role of MAX_CONTENT_LENGTH

   This setting defines the maximum size in bytes for incoming request data, such as file uploads.

2. Step 2: Identify what it limits

   It limits the size of the request body, preventing large uploads that could harm the app.

3. Final Answer:

   Limits the maximum size of incoming request data, like file uploads. -> Option B

4. Quick Check:

   MAX_CONTENT_LENGTH limits upload size [OK]

Hint: MAX_CONTENT_LENGTH sets max upload size in bytes [OK]

Common Mistakes:

• Confusing size limit with number of files
• Thinking it limits request time
• Assuming it controls user connections

2. Which of the following is the correct way to set a 2MB file size limit in Flask?

easy

A. app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024

B. app.MAX_CONTENT_LENGTH = 2 * 1024 * 1024

C. app.config.MAX_CONTENT_LENGTH = 2 * 1024 * 1024

D. app.set_config('MAX_CONTENT_LENGTH', 2 * 1024 * 1024)

Solution

1. Step 1: Recall Flask config syntax

   Flask config values are set using app.config dictionary syntax.

2. Step 2: Check each option

   app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024 uses app.config['MAX_CONTENT_LENGTH'] correctly. Others use invalid syntax.

3. Final Answer:

   app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024 -> Option A

4. Quick Check:

   Use app.config['KEY'] = value [OK]

Hint: Use app.config['KEY'] to set Flask config [OK]

Common Mistakes:

• Using dot notation instead of dictionary
• Calling non-existent set_config method
• Assigning config directly on app object

3. Given this Flask app code snippet, what happens if a user uploads a 3MB file?

app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024

@app.route('/upload', methods=['POST'])
def upload():
    file = request.files['file']
    return 'File uploaded!'

medium

A. Flask automatically returns a 413 error and blocks the upload.

B. The upload succeeds and returns 'File uploaded!'

C. The app crashes with a runtime error.

D. The file is accepted but truncated to 2MB.

Solution

1. Step 1: Understand MAX_CONTENT_LENGTH effect on uploads

   Flask rejects requests exceeding this size with a 413 error automatically.

2. Step 2: Analyze the 3MB upload against 2MB limit

   3MB is larger than 2MB limit, so Flask blocks the request before reaching the route.

3. Final Answer:

   Flask automatically returns a 413 error and blocks the upload. -> Option A

4. Quick Check:

   Upload > MAX_CONTENT_LENGTH causes 413 error [OK]

Hint: Uploads over limit get 413 error automatically [OK]

Common Mistakes:

• Assuming upload succeeds anyway
• Thinking app crashes instead of 413 error
• Believing file is truncated silently

4. Identify the error in this Flask code that tries to limit upload size:

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

@app.route('/upload', methods=['POST'])
def upload():
    if request.content_length > app.config['MAX_CONTENT_LENGTH']:
        return 'File too large', 413
    file = request.files['file']
    return 'Upload successful'

medium

A. The route should allow GET method to upload files.

B. The content_length attribute does not exist on request.

C. The manual size check is unnecessary; Flask blocks large uploads automatically.

D. The MAX_CONTENT_LENGTH value must be set in bytes, not calculated.

Solution

1. Step 1: Recall Flask's built-in upload size handling

   Flask automatically returns a 413 error if upload exceeds MAX_CONTENT_LENGTH.

2. Step 2: Analyze the manual check in code

   The manual check duplicates Flask's behavior and is redundant.

3. Final Answer:

   The manual size check is unnecessary; Flask blocks large uploads automatically. -> Option C

4. Quick Check:

   Flask auto-blocks large uploads, no manual check needed [OK]

Hint: Flask auto-blocks large uploads; no manual check needed [OK]

Common Mistakes:

• Thinking content_length is missing
• Believing GET method is required for uploads
• Misunderstanding byte calculation for MAX_CONTENT_LENGTH

5. You want to allow file uploads up to 10MB but also want to show a custom error page instead of Flask's default 413 error. Which approach is best?

hard

A. Set MAX_CONTENT_LENGTH to 10MB and catch exceptions in the route to show custom error.

B. Remove MAX_CONTENT_LENGTH and manually check file size in the route.

C. Set MAX_CONTENT_LENGTH to 20MB and reject files over 10MB manually.

D. Set MAX_CONTENT_LENGTH to 10MB and add an error handler for 413 to show a custom page.

Solution

1. Step 1: Set MAX_CONTENT_LENGTH to desired limit

   Setting it to 10MB ensures Flask blocks uploads larger than this automatically.

2. Step 2: Add a Flask error handler for 413 errors

   Define a custom error handler to display a friendly message instead of default error.

3. Step 3: Avoid manual size checks or mismatched limits

   Manual checks or mismatched limits complicate logic and risk errors.

4. Final Answer:

   Set MAX_CONTENT_LENGTH to 10MB and add an error handler for 413 to show a custom page. -> Option D

5. Quick Check:

   Use MAX_CONTENT_LENGTH + 413 error handler for custom message [OK]

Hint: Use MAX_CONTENT_LENGTH + 413 error handler for custom message [OK]

Common Mistakes:

• Removing MAX_CONTENT_LENGTH and checking manually
• Setting limit higher than needed and manual rejection
• Catching exceptions instead of error handlers