File size limits in Flask

Introduction

File size limits help keep your app safe and fast by stopping users from uploading very large files.

When you want to stop users from uploading files bigger than a certain size.

When you want to save server space and avoid slow uploads.

When you want to protect your app from accidental or harmful large files.

When you want to give users clear feedback if their file is too big.

Syntax

Flask

app.config['MAX_CONTENT_LENGTH'] = size_in_bytes

Set MAX_CONTENT_LENGTH in your Flask app config to limit upload size.

The size is in bytes. For example, 1 MB = 1_000_000 bytes.

Examples

This sets the max upload size to 1 megabyte.

Flask

app.config['MAX_CONTENT_LENGTH'] = 1_000_000  # 1 MB limit

This sets the max upload size to 5 megabytes using multiplication for clarity.

Flask

app.config['MAX_CONTENT_LENGTH'] = 5 * 1024 * 1024  # 5 MB limit

Sample Program

This Flask app limits file uploads to 2 MB. If a user tries to upload a bigger file, Flask stops the request and returns an error automatically.

Flask

from flask import Flask, request, abort

app = Flask(__name__)

# Limit uploads to 2 MB
app.config['MAX_CONTENT_LENGTH'] = 2 * 1024 * 1024

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files.get('file')
    if not file:
        return 'No file uploaded', 400
    # If file is too large, Flask automatically aborts with 413 error
    return f'File {file.filename} uploaded successfully!'

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

Important Notes

Flask returns a 413 error if the file is too big. You can customize this error by adding an error handler for 413.

Remember to set MAX_CONTENT_LENGTH before running your app.

This limit applies to the whole request size, including all form data.

Summary

Set MAX_CONTENT_LENGTH in Flask config to limit file upload size.

Limits help protect your app and improve user experience.

Flask automatically stops requests that exceed the limit with a 413 error.

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