File upload forms in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What HTML element is used to allow users to select files for upload in a form?

The <input> element with type="file" is used to let users pick files to upload.

beginner

In Flask, which object holds the uploaded file data after a form submission?

Flask stores uploaded files in the request.files dictionary, accessible by the file input's name.

beginner

Why must the form tag include enctype="multipart/form-data" when uploading files?

This encoding type tells the browser to send files and form data properly so the server can read the files.

intermediate

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

Use the save() method on the uploaded file object, providing a safe file path to store it.

intermediate

What is a common security practice when saving uploaded files in Flask?

Use werkzeug.utils.secure_filename() to clean the filename and avoid unsafe characters or paths.

Which attribute must a form have to upload files correctly?

A type="file"

B method="get"

C action="upload"

D enctype="multipart/form-data"

The form must have enctype="multipart/form-data" to send files properly.

In Flask, where do uploaded files appear after form submission?

A request.files

B request.args

C request.form

D request.data

Uploaded files are stored in request.files, not in form or args.

What method do you call on an uploaded file object to save it?

A write()

B save()

C store()

D upload()

The save() method saves the uploaded file to disk.

Which function helps secure filenames before saving in Flask?

A secure_filename()

B sanitize_path()

C clean_name()

D safe_path()

secure_filename() from werkzeug.utils cleans filenames safely.

What input type allows users to select multiple files at once?

A <input type="files">

B <input type="upload">

C <input type="file" multiple>

D <input type="file" multiple="false">

Adding the multiple attribute lets users pick several files.

Explain the steps to create a file upload form in Flask and handle the uploaded file safely.

Think about both the HTML form setup and the Flask backend handling.

Why is it important to use secure_filename() when saving uploaded files in Flask?

Consider what could happen if filenames are not cleaned.

Practice

1. What must you set in the HTML form tag to allow file uploads in a Flask app?

easy

A. method="get"

B. enctype="multipart/form-data"

C. action="upload"

D. type="file"

Solution

1. Step 1: Understand form encoding for file uploads

   File uploads require the form to send data in a special format that supports files, which is "multipart/form-data".

2. Step 2: Identify the correct form attribute

   The form tag must have the attribute enctype="multipart/form-data" to handle file uploads correctly.

3. Final Answer:

   enctype="multipart/form-data" -> Option B

4. Quick Check:

   Form enctype = multipart/form-data [OK]

Hint: File uploads always need enctype multipart/form-data [OK]

Common Mistakes:

• Using method="get" instead of post
• Forgetting enctype attribute
• Setting type="file" on form tag instead of input
• Confusing action URL with encoding

2. Which Flask code correctly accesses an uploaded file named 'photo' from a form?

easy

A. file = request.form['photo']

B. file = request.args['photo']

C. file = request.data['photo']

D. file = request.files['photo']

Solution

1. Step 1: Identify where uploaded files are stored in Flask

   Uploaded files are stored in request.files, not in request.form or request.args.

2. Step 2: Access the file by its form field name

   Use request.files['photo'] to get the uploaded file object named 'photo'.

3. Final Answer:

   file = request.files['photo'] -> Option D

4. Quick Check:

   Uploaded files = request.files [OK]

Hint: Use request.files to get uploaded files, not request.form [OK]

Common Mistakes:

• Using request.form to get files
• Using request.args which is for URL params
• Trying to access request.data as a dict
• Misspelling 'files' or field name

3. What will the following Flask code print if no file is uploaded?

file = request.files.get('document')
if file and file.filename != '':
    print('File received')
else:
    print('No file uploaded')

medium

A. File received

B. KeyError exception

C. No file uploaded

D. Empty string printed

Solution

1. Step 1: Understand request.files.get behavior when no file sent

   request.files.get('document') returns None if no file was uploaded with that name.

2. Step 2: Evaluate the if condition

   Since file is None, the condition 'file and file.filename != ""' is False, so the else block runs.

3. Final Answer:

   No file uploaded -> Option C

4. Quick Check:

   No file means else prints [OK]

Hint: Check file is not None and filename not empty to confirm upload [OK]

Common Mistakes:

• Assuming request.files['document'] raises error if missing
• Not checking filename for empty string
• Expecting 'File received' even if no file sent
• Confusing request.files.get with request.files[]

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

file = request.files['file']
filename = secure_filename(file)
save_path = os.path.join('uploads', filename)
file.save(save_path)

medium

A. secure_filename should be called on file.filename, not file

B. file.save() requires a file object, not a path

C. os.path.join cannot join strings

D. request.files['file'] returns a string, not a file object

Solution

1. Step 1: Check usage of secure_filename

   secure_filename expects a string filename, but file is a file object, so file.filename should be passed.

2. Step 2: Confirm correct file saving steps

   After getting a safe filename, join path and call file.save(path) correctly.

3. Final Answer:

   secure_filename should be called on file.filename, not file -> Option A

4. Quick Check:

   Pass filename string to secure_filename [OK]

Hint: Use secure_filename(file.filename), not file itself [OK]

Common Mistakes:

• Passing file object to secure_filename
• Confusing file.save argument type
• Misusing os.path.join with non-string
• Assuming request.files returns string

5. You want to allow users to upload multiple images at once and save only PNG files securely. What issue does this Flask code snippet have?

files = request.files.getlist('images')
for file in files:
    if file and file.filename.endswith('.png'):
        filename = secure_filename(file.filename)
        file.save(os.path.join('uploads', filename))

hard

A. file.filename.endswith('.png') is case-sensitive and may miss '.PNG'

B. getlist() returns a single file, so loop will fail

C. secure_filename should be called after saving the file

D. This code correctly filters PNG files and saves them securely

Solution

1. Step 1: Understand getlist() usage

   request.files.getlist('images') correctly returns a list of uploaded files for the field 'images'.

2. Step 2: Check file extension filtering

   Using file.filename.endswith('.png') is case-sensitive and will miss files with uppercase extensions like '.PNG'.

3. Step 3: Confirm secure_filename usage

   secure_filename is correctly called before saving the file.

4. Final Answer:

   file.filename.endswith('.png') is case-sensitive and may miss '.PNG' -> Option A

5. Quick Check:

   Check file extension case-insensitively [OK]

Hint: Check file extensions case-insensitively for uploads [OK]

Common Mistakes:

• Assuming getlist returns one file
• Calling secure_filename after saving
• Ignoring uppercase file extensions
• Not checking if file is None before saving