Why is my Python server always returning 404 when the page is found?

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Python

flask

Hello, coding guys,

As you saw in the title, my Python API returns a 404 page when a page is found, like /python/ should return "print("Hello, world!")", but returns 404 instead. This is my code:

from flask import Flask, send_from_directory, abort
import json
from flask_cors import CORS
app = Flask("app")
CORS(app)

@app.route("/")
def main():
	return abort(400)

@app.route("/<string:lang>/")
def hwres(lang):
	if lang.split("/")[0] == lang:
		lang = lang.split("\\")[0]
	else:
		lang = lang.split("/")[0]
	lang = lang.lower()
	with open("examples.json", "r") as exampleIO:
		global exampleJson, exampleJsonStr
		exampleJsonStr = exampleIO.read()
		exampleJson = json.loads(exampleJsonStr)
	try:
		str(exampleJson["hw"][lang])
	except:
		return abort(400)
	with open("hw-code.txt", "r+") as exampleIO:
		exampleIO.truncate()
	with open("hw-code.txt", "a") as exampleIO:
		exampleIO.write(exampleJson["hw"][lang])
	return send_from_directory(".", "hw-code.json")

app.run(host="0.0.0.0", port=8080)

Abou the main site returning Bad Request, that's supposed to happen.

What I have tried: