I'm having a similar issue. I've seen it recommended to use request.files.getlist("file[]") or request.files.getlist("file") << this worked for me for 2 files but when I started upload 3 separate files via 3 form elements I would only get 2. So here's the code I'm using right now.

for file in request.files:
    files[file].save(out_filename)

You can use method getlist of flask.request.files, for example:

@app.route("/upload", methods=["POST"])
def upload():
    uploaded_files = flask.request.files.getlist("file[]")
    print uploaded_files
    return ""

Return after iterating through all files, you are returning after first iteration

if request.method == 'POST':
     for f in request.files.getlist('file')
         f.save(os.path.join(app.config.get('UPLOAD_FOLDER'), f.filename))
     return 'upload'

The docs describe the attributes available on the request object (from flask import request) during a request. In most common cases request.data will be empty because it's used as a fallback:

request.data Contains the incoming request data as string in case it came with a mimetype Flask does not handle.

• request.args: the key/value pairs in the URL query string
• request.form: the key/value pairs in the body, from a HTML post form, or JavaScript request that isn't JSON encoded
• request.files: the files in the body, which Flask keeps separate from form. HTML forms must use enctype=multipart/form-data or files will not be uploaded.
• request.values: combined args and form, preferring args if keys overlap
• request.json: parsed JSON data. The request must have the application/json content type, or use request.get_json(force=True) to ignore the content type.

All of these are MultiDict instances (except for json). You can access values using:

• request.form['name']: use indexing if you know the key exists
• request.form.get('name'): use get if the key might not exist
• request.form.getlist('name'): use getlist if the key is sent multiple times and you want a list of values. get only returns the first value.

Once you have a complete list of files that you want to upload, you can make a POST request from JavaScript to your Flask endpoint to store the files.

With a minimal form

<form id="form-files">
    <input id="file-input" type="file" multiple=true />
    <button type="submit">Submit</button>
</form>

You can intercept the form submit action and send the files to a Flask endpoint. The fileList array comes from your file upload/remove logic.

var formFiles = document.getElementById('form-files');

formFiles.addEventListener('submit', function(e) {
    e.preventDefault();
    var formData = new FormData();
    var request = new XMLHttpRequest();
    for (var i = 0; i < fileList.length; i++) {
        formData.append('files[]', fileList[i]);
    }
    request.open('POST', '/upload');
    request.send(formData);
});

Finally write a flask upload endpoint that processes (stores) the files.

@app.route("/upload", methods=["POST"])
def upload():
    uploaded_files = request.files.getlist("file[]")
    # Store files

request.files is populated with whatever the client submitted. Unfortunately, browsers submit file inputs even when no file is selected, which looks like a file with no name and no data. Neither a name or data is required to have a valid empty file, so it's left up to the app to decide what to do.

FileStorage will be considered False if it has no name. Flask-WTF considers a FileStorage with no name to be empty for validation.

photo = request.files["photo"]

if not photo:
    # no photo

photos = request.files["photo"].getlist()

if not photos or not any(f for f in photos):
    # no photos

If I've read this correctly...

request.args.getlist only grabs data posted as URL parameters, hence the KeyError when trying to access plants[0] during a POST request. Notice in the server log, print(thisPlant) during the POST request gives an empty list: []

Seems like you just need to restructure that route, so the problematic line is only executed within a GET request.

Something like:

def editFunction():    

    # Force flash() to get the messages on the same page as the redirect.
    get_flashed_messages()

    if request.method = 'GET':
        # Get arguments from url_for in administration
        thisPlant = request.args.getlist("plants")
        print(thisPlant)

        # Get ID on edit plant
        plant_id = thisPlant[0]
        print(plant_id)

        # Do return here, rather than in 'else' statement at bottom of file
        return render_template("edit.html", name=getUserName(), picture=getUserPicture(), role=getUserRole(), plants=thisPlant)

    elif request.method == "POST":

        # code to handle POST Request

        # ...

        flash("Plant edited")
        return redirect("/administration")