File upload processing in Flask - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when uploading a file with this Flask route?

Consider this Flask route that handles file uploads. What will the response be if a user uploads a file named example.txt?

Flask

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

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files.get('file')
    if file:
        filename = file.filename
        return f'File {filename} uploaded successfully'
    return 'No file uploaded'

A File None uploaded successfully

B No file uploaded

C Internal Server Error

D File example.txt uploaded successfully

💡 Hint

Check how the file is accessed from the request and what attribute is used for the filename.

The request.files.get('file') retrieves the uploaded file object. If it exists, file.filename returns the original filename. So the response includes the uploaded filename.

📝 Syntax

intermediate

Which option correctly saves an uploaded file in Flask?

Given a Flask route receiving a file upload, which code snippet correctly saves the file to the uploads/ folder?

Flask

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

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files.get('file')
    if file:
        # Save file here
        pass
    return 'No file uploaded'

A save(file, 'uploads/' + file.filename)

B file.save('uploads/' + file.filename)

C file.write('uploads/' + file.filename)

D file.store('uploads/' + file.filename)

💡 Hint

Look for the method provided by Flask's FileStorage object to save files.

The save() method is the correct way to save an uploaded file to disk in Flask. Other methods like write or store do not exist on the file object.

🔧 Debug

advanced

Why does this Flask file upload code raise a KeyError?

Examine this Flask route snippet. Why does it raise a KeyError when no file is uploaded?

Flask

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

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files['file']
    if file:
        return f'Uploaded {file.filename}'
    return 'No file uploaded'

A Because <code>request.files['file']</code> raises KeyError if 'file' key is missing

B Because <code>file.filename</code> is None

C Because the route does not allow POST method

D Because <code>request.files</code> is always empty

💡 Hint

Consider how dictionary access works in Python when a key is missing.

Accessing request.files['file'] directly raises a KeyError if the key 'file' is not present. Using request.files.get('file') avoids this error by returning None instead.

❓ state_output

advanced

What is the value of filename after this Flask upload code runs?

Given this Flask route, what will be the value of filename if the user uploads a file named photo.png?

Flask

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

@app.route('/upload', methods=['POST'])
def upload_file():
    file = request.files.get('file')
    filename = file.filename if file else 'No file'
    return filename

A photo.png

B No file

C None

D file

💡 Hint

Check the conditional expression assigning filename.

If a file is uploaded, file is not None, so filename is set to file.filename, which is the uploaded file's name.

🧠 Conceptual

expert

Which Flask configuration is essential to securely handle file uploads?

When building a Flask app that accepts file uploads, which configuration helps prevent security risks by limiting the size of uploaded files?

A app.config['SECRET_KEY'] = 'mysecret'

B app.config['UPLOAD_FOLDER'] = '/tmp/uploads'

C app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024

D app.config['DEBUG'] = True

💡 Hint

Think about how to limit the size of incoming data to avoid large file uploads.

The MAX_CONTENT_LENGTH setting limits the maximum size of incoming request data, preventing users from uploading files larger than the specified size. Other options configure upload folder, debugging, or security keys but do not limit upload size.