AttributeError: 'NoneType' object has no attribute '_instantiate_plugins'

Question

I'm trying to get started using Flask with PostgreSQL to create a simple project. For the sake of completeness I will include all terminal commands along with my code. Working in my SQL_Example folder and a local PostgreSQL database "lecture3" with table "flights":

$ python3 -m venv venv
$ . venv/bin/activate
$ pip install Flask

Now I continued with the following code:

import os

from flask import Flask, render_template, request
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker

app = Flask(__name__)

engine = create_engine(os.getenv("DATABASE_URL"))
db = scoped_session(sessionmaker(bind=engine))

@app.route("/")
def index():
    flights = db.execute("SELECT * FROM flights").fetchall()
    return render_template("index.html", flights=flights)

@app.route("/book", methods=["POST"])
def book():
    name = request.form.get("name")
    try:
        flight_id = int(request.form.get("flight_id"))
    except ValueError:
        return render_template("error.html", message="Invalid flight number.")

    if db.execute("SELECT * FROM flights WHERE id = :id", {"id": flight_id}).rowcount == 0:
        return render_template("error.html", message="No such flight with that id.")
    db.execute("INSERT INTO passengers (name, flight_id) VALUES (:name, :flight_id)", {"name": name, "flight_id": flight_id})
    db.commit()
    return render_template("success.html")

Executing the following commands:

$ export FLASK_APP="application.py"
$ flask run

Encountered the following error:

Traceback (most recent call last):
  File "/Users/spencermiller/Desktop/SQL_Example/venv/bin/flask", line 11, in <module>
    sys.exit(main())
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 967, in main
    cli.main(args=sys.argv[1:], prog_name="python -m flask" if as_module else None)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 586, in main
    return super(FlaskGroup, self).main(*args, **kwargs)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/click/core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/click/decorators.py", line 73, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 848, in run_command
    app = DispatchingApp(info.load_app, use_eager_loading=eager_loading)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 305, in __init__
    self._load_unlocked()
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 330, in _load_unlocked
    self._app = rv = self.loader()
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 388, in load_app
    app = locate_app(self, import_name, name)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/flask/cli.py", line 240, in locate_app
    __import__(module_name)
  File "/Users/spencermiller/Desktop/SQL_Example/application.py", line 9, in <module>
    engine = create_engine(os.getenv("DATABASE_URL"))
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/sqlalchemy/engine/__init__.py", line 479, in create_engine
    return strategy.create(*args, **kwargs)
  File "/Users/spencermiller/Desktop/SQL_Example/venv/lib/python3.7/site-packages/sqlalchemy/engine/strategies.py", line 56, in create
    plugins = u._instantiate_plugins(kwargs)
AttributeError: 'NoneType' object has no attribute '_instantiate_plugins'

I have seen some similar issues but none of those solutions have worked for me.

Environment variable:

TERM_PROGRAM=Apple_Terminal
SHELL=/bin/bash
TERM=xterm-256color
TMPDIR=/var/folders/pf/2_4xxn2s0jl7m8r0xv9wg9fr0000gp/T/
Apple_PubSub_Socket_Render=/private/tmp/com.apple.launchd.CyouqBAyRx/Render
TERM_PROGRAM_VERSION=421.1.1
OLDPWD=/Users/spencermiller
TERM_SESSION_ID=3CA9A778-D58A-405E-8EC3-D5F86D87FFFA
USER=spencermiller
SSH_AUTH_SOCK=/private/tmp/com.apple.launchd.aXcWzrdMYX/Listeners
PATH=/Library/Frameworks/Python.framework/Versions/3.7/bin:/Library/Frameworks/Python.framework/Versions/3.7/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin
PWD=/Users/spencermiller/Desktop/SQL_Example
LANG=en_CA.UTF-8
XPC_FLAGS=0x0
XPC_SERVICE_NAME=0
SHLVL=1
HOME=/Users/spencermiller
LOGNAME=spencermiller
DISPLAY=/private/tmp/com.apple.launchd.WkWoFkNc6W/org.macosforge.xquartz:0
_=/usr/bin/printenv

Answer 1

in this piece of code, you are trying to create your flask's engine with a database URL that is on you environment variables.

engine = create_engine(os.getenv("DATABASE_URL"))

My guess is that you didn't set up this environment variable, so you're trying to create an engine with a None url.

You need to set your env variable, or hard code your DATABASE_URL into python.

db_url = 'postgresql+psycopg2://{user}:{password}@{url}/{db}'
engine = create_engine(db_url)

Hope it helps.