python - Error while connecting to postgresql using sqlalchemy

Question

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?

Answer 1

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")