Ytukyg

I have a python script(list.py) which is used to interact with postgresql database.

import os

from sqlalchemy import create_engine

from sqlalchemy.orm import scoped_session, sessionmaker



engine = create_engine(os.getenv("postgresql://postgres:nehal@localhost:5432/lecture3"))

db = scoped_session(sessionmaker(bind=engine))



def main():

    flights = db.execute("SELECT origin, destination, duration FROM flights").fetchall()

    for flight in flights:

        print(f"{flight.origin} to {flight.destination}, {flight.duration} minutes.")



if __name__ == "__main__":

    main()

I have postgresql installed on Ubuntu 16.04 with lecture3 as database.When I execute the code as python list.py,I get the following error:

Traceback (most recent call last):

  File "list.py", line 5, in <module>

    engine = create_engine(os.getenv("postgresql://postgres:nehal@localhost:5432/lecture3"))

  File "/home/nehal/anaconda3/lib/python3.6/site-packages/sqlalchemy/engine/__init__.py", line 424, in create_engine

    return strategy.create(*args, **kwargs)

  File "/home/nehal/anaconda3/lib/python3.6/site-packages/sqlalchemy/engine/strategies.py", line 52, in create

    plugins = u._instantiate_plugins(kwargs)

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

postgres is the postgresql username and nehal is the password.
How do I correct the error?

os.getenv is used to get the value of an environment variable, and returns None by default if that variable doesn't exist. You're passing it your connection string, which (almost certainly) doesn't exist as an environment variable. So it's returning None, which is given to create_engine, which fails because it's expecting a connection string. Just pass your connection string in directly:

engine = create_engine("postgresql://postgres:nehal@localhost:5432/lecture3") 

I would try to run this without the getenv which seems useless and might return None

create_engine("postgresql://postgres:nehal@localhost:5432/lecture3")

This work for me:

export DATABASE_URL="postgres://localhost/lecture3"

export DATABASE_URL="postgres://localhost:5432/lecture3"

export DATABASE_URL="postgres:///lecture3"

You should use this command in your ENV.

Do this before running $ python list.py
where

 username        your psql username

 password        your psql password

 server          localhost or remote server

 port            5432

 database        your psql database

(flask) $ export DATABASE_URL="postgresql://username:password@localhost:5432/database"

Verify:

(flask) $ echo $DATABASE_URL

Run:

$ python list.py

For me the below worked perfectly without any issues

engine = create_engine("postgres://postgres:password@localhost:5432")

where "password" shall be substituted for your PostgreSQL password you used while installing PostgreSQL.