Saving uploaded files in Flask

Introduction

Saving uploaded files lets your web app keep files users send, like photos or documents. This helps you use or share those files later.

When users upload profile pictures to your website.

When a form lets users submit documents or reports.

When you want to store images users add to a blog post.

When users upload files to share with others on your app.

Syntax

Flask

from flask import request

file = request.files['file']
file.save('/path/to/save/filename')

Use request.files to get the uploaded file from the form.

Call save() on the file object with the path where you want to store it.

Examples

Saves the uploaded file from the form field named 'photo' into the 'uploads' folder with the name 'profile.jpg'.

Flask

file = request.files['photo']
file.save('uploads/profile.jpg')

Checks if a file was uploaded under 'document' and saves it using its original filename inside 'uploads' folder.

Flask

uploaded_file = request.files.get('document')
if uploaded_file:
    uploaded_file.save(f"uploads/{uploaded_file.filename}")

Sample Program

This Flask app shows a simple form to upload a file. When a file is sent, it saves it in the 'uploads' folder and confirms the filename saved.

Flask

from flask import Flask, request, render_template_string
import os

app = Flask(__name__)

UPLOAD_FOLDER = 'uploads'
os.makedirs(UPLOAD_FOLDER, exist_ok=True)

@app.route('/', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        file = request.files.get('file')
        if file:
            save_path = os.path.join(UPLOAD_FOLDER, file.filename)
            file.save(save_path)
            return f'File saved as {file.filename}'
        return 'No file uploaded'
    return render_template_string('''
        <form method="post" enctype="multipart/form-data">
            <input type="file" name="file">
            <input type="submit" value="Upload">
        </form>
    ''')

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

Important Notes

Always check if the file exists before saving to avoid errors.

Use secure filenames to avoid security risks (Flask has werkzeug.utils.secure_filename for this).

Make sure the folder where you save files exists and your app has permission to write there.

Summary

Use request.files to get uploaded files in Flask.

Call save() on the file object with a path to store it.

Check for file presence and use safe filenames to keep your app secure.

Practice

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

easy

A. request.args

B. request.files

C. request.data

D. request.form

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 files

   request.form is for form fields, request.data is raw data, and request.args is for URL query parameters.

3. Final Answer:

   request.files -> Option B

4. Quick Check:

   Uploaded files = request.files [OK]

Hint: Uploaded files are always in request.files [OK]

Common Mistakes:

• Using request.form to get files
• Trying to access files from request.args
• Confusing request.data with files

2. Which of the following is the correct way to save an uploaded file named file to a folder called uploads in Flask?

easy

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

B. file.save(file.filename)

C. save(file, 'uploads/')

D. request.save(file, 'uploads/')

Solution

1. Step 1: Use the file object's save method

   The uploaded file object has a save() method that takes the full path to save the file.

2. Step 2: Provide correct path including folder and filename

   Concatenate the folder path and the file's filename to save it in the right place.

3. Final Answer:

   file.save('uploads/' + file.filename) -> Option A

4. Quick Check:

   Use file.save(path) with full path [OK]

Hint: Use file.save('folder/' + file.filename) to save files [OK]

Common Mistakes:

• Calling save() without a path
• Using save() as a standalone function
• Not including folder path in save()

3. What will happen when this Flask code runs after a file upload?

file = request.files['photo']
file.save('/tmp/' + file.filename)
return 'Saved!'

medium

A. Nothing is saved because request.files['photo'] is empty by default

B. The file is saved but the filename is lost, causing an error

C. The code raises an error because save() needs a full absolute path

D. The uploaded file is saved in /tmp/ with its original filename and 'Saved!' is returned

Solution

1. Step 1: Access the uploaded file from request.files

   The code gets the file named 'photo' from the uploaded files correctly.

2. Step 2: Save the file to /tmp/ folder with original filename

   Calling save() with '/tmp/' + filename saves the file in that folder.

3. Final Answer:

   The uploaded file is saved in /tmp/ with its original filename and 'Saved!' is returned -> Option D

4. Quick Check:

   save() stores file at given path, returns confirmation [OK]

Hint: save() stores file at given path, returns confirmation [OK]

Common Mistakes:

• Assuming save() needs absolute path always
• Thinking request.files['photo'] is empty without upload
• Confusing save() behavior with form data saving

4. Identify the error in this Flask code snippet for saving an uploaded file:

file = request.files['file']
file.save(file.filename)

medium

A. Missing folder path in save() can cause files to save in wrong location

B. Using request.files instead of request.form causes error

C. file.save() requires a file object, not a filename string

D. file.filename is not a valid attribute of the file object

Solution

1. Step 1: Check the save() method argument

   The code saves the file using only the filename, without specifying a folder path.

2. Step 2: Understand file saving location

   Without a folder path, the file saves in the current working directory, which may be unintended or insecure.

3. Final Answer:

   Missing folder path in save() can cause files to save in wrong location -> Option A

4. Quick Check:

   Always include folder path in save() [OK]

Hint: Always include folder path in save() to control file location [OK]

Common Mistakes:

• Not specifying folder path in save()
• Confusing request.files with request.form
• Assuming file.filename is invalid

5. You want to securely save an uploaded file in Flask to the uploads folder, avoiding unsafe filenames. Which code snippet correctly achieves this?

from werkzeug.utils import secure_filename
file = request.files['file']
filename = secure_filename(file.filename)
file.save(f'uploads/{filename}')

hard

A. The code will fail because secure_filename() returns a file object, not a string

B. secure_filename() is unnecessary; file.save() cleans filenames automatically

C. This code safely saves the file with a cleaned filename in uploads folder

D. You must manually create the uploads folder before saving, or it errors

Solution

1. Step 1: Use secure_filename to clean the filename

   The function secure_filename() removes unsafe characters to prevent security issues.

2. Step 2: Save the file with the cleaned filename in the uploads folder

   Using file.save() with the cleaned filename inside the uploads folder saves the file securely.

3. Final Answer:

   This code safely saves the file with a cleaned filename in uploads folder -> Option C

4. Quick Check:

   Use secure_filename() before saving uploaded files [OK]

Hint: Use secure_filename() to avoid unsafe file names [OK]

Common Mistakes:

• Skipping secure_filename and risking unsafe filenames
• Assuming save() cleans filenames automatically
• Not creating uploads folder before saving