Solution
# .env file
UPLOAD_PATH=project/client/static/
# config.py
import os
class Config(object):
UPLOAD_PATH = os.environ.get("UPLOAD_PATH")
# __init__.py
from config import Config
app = Flask(__name__)
app.config.from_object(Config)
import os
@main_blueprint.route("/tasks", methods=["POST"])
def run_task():
# ...
file.save(os.path.join(app.config["UPLOAD_PATH"], filepath)
# ...