How to Upload File in Python - Flask

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Introduction

Uploading files is a common requirement in web applications. With Python's Flask framework, handling file uploads becomes straightforward. This guide walks you through the steps to create a simple Flask app that accepts file uploads securely and efficiently.

Project Structure

To get started, set up your project folder with the following structure:

app.py – main Flask application
templates/ – contains your HTML files
uploads/ – directory where uploaded files will be saved

Configuring Flask for File Upload

First, import the necessary modules and configure your app to specify the upload folder and allowed file types.

from flask import Flask, request, render_template, redirect, url_for
from werkzeug.utils import secure_filename
import os

app = Flask(__name__)
app.config['UPLOAD_FOLDER'] = 'uploads/'
app.config['MAX_CONTENT_LENGTH'] = 16 * 1024 * 1024  # Max upload size: 16MB

ALLOWED_EXTENSIONS = {'txt', 'pdf', 'png', 'jpg', 'jpeg', 'gif'}

def allowed_file(filename):
    return '.' in filename and \
           filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

Creating the Upload Route

The upload route handles both displaying the upload form and processing the uploaded file when the form is submitted.

@app.route('/', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        if 'file' not in request.files:
            return 'No file part in the request'
        file = request.files['file']
        if file.filename == '':
            return 'No file selected for uploading'
        if file and allowed_file(file.filename):
            filename = secure_filename(file.filename)
            file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))
            return f'File successfully uploaded: {filename}'
        else:
            return 'Allowed file types are txt, pdf, png, jpg, jpeg, gif'
    return render_template('upload.html')

Upload Form Template

Create a simple HTML form to select and upload a file:

<!-- templates/upload.html -->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Upload File</title>
</head>
<body>
    <h1>Upload a File</h1>
    <form method="POST" enctype="multipart/form-data">
        <input type="file" name="file" required>
        <button type="submit">Upload</button>
    </form>
</body>
</html>

Important Points to Remember

Use secure_filename from Werkzeug to avoid unsafe filenames.
Check if the uploaded file has an allowed extension for security.
Set a maximum upload size to prevent denial-of-service attacks.
Always include enctype="multipart/form-data" in your form for file uploads.

Running the Application

Run your Flask app with the following command:

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

Open http://127.0.0.1:5000/ in your browser, select a file, and hit upload to test the functionality.

Conclusion

Handling file uploads in Flask is simple and secure when you follow best practices like validating file extensions and sanitizing filenames. This tutorial provides a solid foundation to integrate file uploading features into your Python web apps using Flask.

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