File uploads handling 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 configured with app.config['MAX_CONTENT_LENGTH'] = 1024 (1 KB). What will happen if a user tries to upload a file of 2 KB?

Flask

from flask import Flask, request
app = Flask(__name__)
app.config['MAX_CONTENT_LENGTH'] = 1024

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

A The server silently ignores the file and returns a 200 OK with no content.

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

C The file uploads successfully but is truncated to 1 KB.

D Flask saves the full file but logs a warning about the size.

💡 Hint

Think about what Flask does when the uploaded content exceeds the configured max size.

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

📝 Syntax

intermediate

Which code correctly saves an uploaded file securely?

Given a Flask route handling file uploads, which option correctly saves the uploaded file using a secure filename?

Flask

from flask import Flask, request
from werkzeug.utils import secure_filename
app = Flask(__name__)

@app.route('/upload', methods=['POST'])
def upload():
    file = request.files['file']
    # Save file here
    pass

A

filename = secure_filename(file.filename)
file.save(f"uploads/{filename}")
return 'File saved'

B

filename = file.filename
file.save(f"uploads/{filename}")
return 'File saved'

C

filename = secure_filename(file.filename)
file.save(f"/uploads/{filename}")
return 'File saved'

D

filename = secure_filename(file.filename)
file.save(f"uploads/" + file.filename)
return 'File saved'

💡 Hint

Use the secure_filename function to avoid unsafe filenames.

Option A uses secure_filename to sanitize the filename and saves it in the 'uploads' folder correctly. Option A skips sanitization, which is unsafe. Option A uses an absolute path that may not exist or be writable. Option A mixes secure filename with original filename, which is inconsistent.

🔧 Debug

advanced

Why does this Flask upload code raise a KeyError?

This Flask route raises a KeyError when a POST request without a file is sent. Why?

Flask

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

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

A Because request.files is always empty regardless of the form.

B Because the route does not allow GET method.

C Because 'file' key is missing in request.files when no file is uploaded.

D Because the file object does not have a filename attribute.

💡 Hint

Check what happens if the form does not include a file input named 'file'.

If the client sends a POST request without a file named 'file', accessing request.files['file'] raises a KeyError because that key does not exist.

❓ state_output

advanced

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

Given this Flask code snippet, what will be the value of 'filename' if the uploaded file is named '../../secret.txt'?

Flask

from werkzeug.utils import secure_filename
filename = secure_filename('../../secret.txt')

A 'secret_txt'

B '.._.._secret.txt'

C '../../secret.txt'

D 'secret.txt'

💡 Hint

secure_filename removes directory traversal parts for safety.

secure_filename strips directory paths and returns only the base filename, so '../../secret.txt' becomes 'secret.txt'.

🧠 Conceptual

expert

Which statement about Flask file upload handling is true?

Select the correct statement about handling file uploads in Flask.

A The request.files object contains FileStorage objects representing uploaded files.

B Flask automatically saves uploaded files to disk without calling save().

C You must manually parse multipart form data to access uploaded files in Flask.

D Flask limits file upload size by default to 1 MB.

💡 Hint

Think about how Flask exposes uploaded files in the request object.

Flask provides uploaded files as FileStorage objects inside request.files. You must call save() to store them. Flask does not limit upload size by default and handles multipart parsing internally.