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Flask – File Uploading

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Handling file upload in Flask is very easy. It needs an HTML form with its enctype attribute set to ‘multipart/form-data’, posting the file to a URL. The URL handler fetches file from request.files[] object and saves it to the desired location.

Each uploaded file is first saved in a temporary location on the server, before it is actually saved to its ultimate location. Name of destination file can be hard-coded or can be obtained from filename property of request.files[file] object. However, it is recommended to obtain a secure version of it using the secure_filename() function.

It is possible to define the path of default upload folder and maximum size of uploaded file in configuration settings of Flask object.

app.config[‘UPLOAD_FOLDER’] Defines path for upload folder
app.config[‘MAX_CONTENT_PATH’] Specifies maximum size of file yo be uploaded – in bytes

The following code has ‘/upload’ URL rule that displays ‘upload.html’ from the templates folder, and ‘/upload-file’ URL rule that calls uploader() function handling upload process.

‘upload.html’ has a file chooser button and a submit button.

<html>
   <body>
      <form action = "http://localhost:5000/uploader" method = "POST" 
         enctype = "multipart/form-data">
         <input type = "file" name = "file" />
         <input type = "submit"/>
      </form>
   </body>
</html>

You will see the screen as shown below.

Flask File Uploading

Click Submit after choosing file. Form’s post method invokes ‘/upload_file’ URL. The underlying function uploader() does the save operation.

Following is the Python code of Flask application.

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

@app.route('/upload')
def upload_file():
   return render_template('upload.html')
	
@app.route('/uploader', methods = ['GET', 'POST'])
def upload_file():
   if request.method == 'POST':
      f = request.files['file']
      f.save(secure_filename(f.filename))
      return 'file uploaded successfully'
		
if __name__ == '__main__':
   app.run(debug = True)
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