File uploads handling in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What Flask object is used to access uploaded files in a request?

The request.files object holds all uploaded files in a Flask request. You can access files by their form field names.

beginner

How do you save an uploaded file to the server in Flask?

Use the save() method on the uploaded file object, passing the destination path as a string, e.g., file.save('/path/to/save').

intermediate

Why should you check the file extension or content type before saving an uploaded file?

To avoid security risks like uploading harmful files. Checking extensions or MIME types helps ensure only allowed file types are saved.

intermediate

What Flask configuration helps limit the maximum size of uploaded files?

The MAX_CONTENT_LENGTH configuration sets the maximum size (in bytes) for incoming request data, preventing very large uploads.

intermediate

How can you safely generate a filename for saving an uploaded file in Flask?

Use werkzeug.utils.secure_filename() to clean the filename, removing unsafe characters and preventing directory traversal attacks.

Which Flask object holds uploaded files from a form?

A request.files

B request.form

C request.data

D request.args

request.files contains uploaded files. request.form has text fields.

What method do you call to save an uploaded file in Flask?

A save()

B upload()

C store()

D write()

The save() method saves the file to disk.

Which Flask config limits upload size?

A MAX_UPLOAD_SIZE

B UPLOAD_LIMIT

C MAX_FILE_SIZE

D MAX_CONTENT_LENGTH

MAX_CONTENT_LENGTH sets max request size.

Why use secure_filename() on uploaded files?

A To compress the file

B To encrypt the file

C To clean the filename for safety

D To rename the file randomly

secure_filename() removes unsafe characters from filenames.

What happens if an uploaded file exceeds MAX_CONTENT_LENGTH?

A File uploads normally

B Flask raises a RequestEntityTooLarge error

C File is truncated

D File is saved partially

Flask blocks the request and raises an error if size is too big.

Explain the steps to handle a file upload in Flask from receiving the file to saving it safely.

Think about how to get the file, check it, and save it securely.

Describe how Flask's MAX_CONTENT_LENGTH setting helps with file uploads.

Consider why limiting upload size is important.

Practice

1. In Flask, which object do you use to access files uploaded by a user in a form?

easy

A. request.files

B. request.form

C. request.args

D. request.data

Solution

1. Step 1: Understand Flask request objects

   Flask provides different objects to access data sent by the client. request.files holds uploaded files.

2. Step 2: Identify correct object for file uploads

   Files sent via form with enctype='multipart/form-data' are accessed through request.files.

3. Final Answer:

   request.files -> Option A

4. Quick Check:

   File uploads = request.files [OK]

Hint: Uploaded files are in request.files, not request.form [OK]

Common Mistakes:

• Using request.form to get files
• Trying to access files via request.args
• Not using multipart/form-data in form

2. Which of the following is the correct way to check if a file was uploaded before saving it in Flask?

easy

A. if request.args.get('file')

B. if request.files['file'] is None

C. if request.form['file'] != None

D. if 'file' in request.files and request.files['file'].filename != ''

Solution

1. Step 1: Check file presence in request.files

   Use 'file' in request.files to confirm the file key exists.

2. Step 2: Ensure filename is not empty

   Check request.files['file'].filename != '' to confirm a file was selected.

3. Final Answer:

   if 'file' in request.files and request.files['file'].filename != '' -> Option D

4. Quick Check:

   Check key and filename before saving [OK]

Hint: Check both key and filename before saving file [OK]

Common Mistakes:

• Checking request.form instead of request.files
• Not verifying filename is not empty
• Using request.args for file data

3. What will be the output of this Flask route when a file named 'photo.png' is uploaded?

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

@app.route('/upload', methods=['POST'])
def upload():
    f = request.files['file']
    filename = f.filename
    return f"Uploaded: {filename}"

medium

A. Uploaded: None

B. KeyError: 'file'

C. Uploaded: photo.png

D. Uploaded: ''

Solution

1. Step 1: Access file from request.files

   The code gets the file object from request.files['file'], which holds the uploaded file.

2. Step 2: Return the filename string

   The filename attribute contains the original file name, here 'photo.png', which is returned in the response.

3. Final Answer:

   Uploaded: photo.png -> Option C

4. Quick Check:

   Filename returned matches uploaded file [OK]

Hint: Filename attribute holds the uploaded file's name [OK]

Common Mistakes:

• Assuming filename is None if file exists
• Not using request.files to get file
• Forgetting to use POST method

4. Identify the error in this Flask file upload code snippet:

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

medium

A. Using POST method instead of GET

B. Saving file without checking if 'file' key exists in request.files

C. Missing return statement

D. Incorrect filename attribute usage

Solution

1. Step 1: Check for 'file' key in request.files

   The code accesses request.files['file'] directly without verifying if the key exists, risking a KeyError.

2. Step 2: Properly check key before accessing

   Use if 'file' in request.files before accessing to avoid errors.

3. Final Answer:

   Saving file without checking if 'file' key exists in request.files -> Option B

4. Quick Check:

   Always check key exists before accessing [OK]

Hint: Check 'file' key in request.files before accessing [OK]

Common Mistakes:

• Directly accessing request.files['file'] without check
• Assuming filename always exists
• Ignoring form enctype for file uploads

5. You want to securely save an uploaded file in Flask to a folder named 'uploads'. Which of the following is the best practice to avoid security risks?

hard

A. Use secure_filename() from werkzeug.utils before saving the file

B. Save the file directly with file.filename without changes

C. Rename the file to a fixed name like 'upload.txt' always

D. Save files outside the Flask app folder without any checks

Solution

1. Step 1: Understand filename security risks

   Using raw filenames can allow directory traversal or unsafe characters causing security issues.

2. Step 2: Use secure_filename() to sanitize filenames

   The secure_filename() function cleans the filename to a safe format before saving.

3. Final Answer:

   Use secure_filename() from werkzeug.utils before saving the file -> Option A

4. Quick Check:

   Sanitize filenames with secure_filename() [OK]

Hint: Always sanitize filenames with secure_filename() before saving [OK]

Common Mistakes:

• Saving files with user-provided names directly
• Ignoring filename sanitization
• Using fixed names causing overwrites