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Editorial Team
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Asked: 2026-06-05T08:49:48+00:00

I’m trying to make upload site that takes request and stores the files in

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I’m trying to make upload site that takes request and stores the files in ‘static’ folder in flask. I always get bad request when I try sending “POST” request on in.The only reason that I need it is to test my drag and drop javascript uploader. Can someone point me in the right direction?

import os
from flask import Flask, request, redirect, url_for
from werkzeug import secure_filename

UPLOAD_FOLDER = 'static'
ALLOWED_EXTENSIONS = set(['txt', 'pdf', 'png', 'jpg', 'jpeg', 'gif'])

app = Flask(__name__)
app.config['UPLOAD_FOLDER'] = UPLOAD_FOLDER


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

@app.route('/', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        file = request.files['xhr2upload'] # [0]
        if file and allowed_file(file.filename):
            filename = secure_filename(file.filename)
            file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))
            return redirect(url_for('uploaded_file',
                                    filename=filename))
    return '''
    <!doctype html>
    <title>Upload new File</title>
    <h1>Upload new File</h1>
    <form action="" method=post enctype=multipart/form-data>
      <p><input type=file name=file>
         <input type=submit value=Upload>
    </form>
    '''

if __name__ == '__main__':
    app.run()
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1 Answer

  1. Editorial Team

    This is a common problem to have with Flask early on – when you attempt to access request.args, request.form, request.values and request.files they will throw a KeyError if the key does not exist in them – just as an ordinary dictionary would in Python (try {}["test"] in any Python interpreter). Flask adds a bit of sugar to the request dictionaries – its KeyError is actually a subclass of HTTPException that raises a 400 Bad Request if it is not caught. (See this part of the quickstart documentation)

    The line that is causing this issue is request.files['xhr2upload']. You do not include any JavaScript on the page and the only <input type="file"> you have on the page has the name of “file”, not “xhr2upload”. Either change request.files['xhr2upload'] to request.files['file'] or load an ajax uploader into your page and have it post your files using the xhr2upload name.

    If you have a form that may have a field and you want to check if a field is defined without raising a KeyError then you can use the .get method:

    request.form["not_here"] # KeyError -> 400 Bad Request

request.form.get("might_be_here") # Value if provided, else None
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