werkzeug.exceptions.BadRequestKeyError While trying to get .csv file from a GET request using Flask

I'm trying to get a .csv file from a form, get it on my server, open it and treat the nunmers to return the valid ones, but when I try to open the file from the request I get "werkzeug.exceptions.BadRequestKeyError"

My html is this:

<!DOCTYPE html>
<html lang="en" dir="ltr">
  <head>
    <meta charset="utf-8">
    <title>Flask Tutorial</title>
  </head>
  <body>
    <form action="/api/listtels" method="get">
      <label for="tels">file:</label>
      <input name="tels" type="file" id="tels"><br><br>
      <input type="submit" value="send">
    </form>
  </body>
</html>

And my python code is this:

import flask
import csv
from flask import Flask, render_template, request, jsonify

app = flask.Flask(__name__)
app.config["DEBUG"] = True

@app.route('/', methods=['GET'])
def home():
    return render_template("home.html")

@app.route('/api/listtels', methods=['GET'])
def ltels():

    tels = "ERROR" 
    ret = "ERROR"
    if 'tels' in request.args:
        tels = request.files['tels']
        ret = "tel\n"
    else:
        return("\'tels\' arg is needed")
    with open(tels, encoding='UTF-8') as f:
        rows = csv.reader(f,delimiter=",",lineterminator="\n")
        next(rows, None)
        for row in rows:
            tel = row[0]
            tel = str(''.join(filter(str.isnumeric, tel)))
            if len(tel) == 11:
                ret = ret + tel + "\n"
            elif len(tel) == 13:
                ret = ret + "+" + tel + "\n"
    return (ret)


app.run()

It's been 3 days since I started to use Python and Flask, so plz, no judgment :D

Solution

The flask request.files machinery assumes that the request is going to be a POST, because it isn't possible to upload a file via a GET request (you can GET the file's data, but that's not quite the same thing^*).

Therefore, the form's method attribute should be POST, and it should also have the attribute enctype set to "multipart/form-data" for absolute correctness.

<form action="/api/listtels" method="POST" enctype="multipart/form-data">

On the server side

The route should accept POST requests only.
The names of file inputs don't form part of request.args, so the membership test should be changed to test request.files.
The uploaded file needs to be decoded and loaded into a buffer so that it can be handled by csv.reader (or it could be written to disk and then read back)

import io

@app.route('/api/listtels', methods=['POST'])
def ltels():

    tels = "ERROR" 
    ret = "ERROR"
    # Check if the file is loaded
    if 'tels' in request.files:
        tels = request.files['tels']
        ret = "tel\n"
    else:
        return("\'tels\' arg is needed")
    # Load decoded file data into a text buffer
    buf = io.StringIO(tels.read().decode('utf-8'))
    rows = csv.reader(buf,delimiter=",",lineterminator="\n")
    next(rows, None)
    for row in rows:
        tel = row[0]
        tel = str(''.join(filter(str.isnumeric, tel)))
        if len(tel) == 11:
            ret = ret + tel + "\n"
        elif len(tel) == 13:
            ret = ret + "+" + tel + "\n"
return (ret)

^* You could get the content via GET either by pasting the file's contents into an <input type="textarea"...> or manipulating the form data with javascript before sending it to the server.