title: AttributeError: 'NoneType' object has no attribute '_instantiate_plugins' while creating engine
body: <p>I have a python script(list.py) which is used to interact with postgresql database.</p>
  
  <pre><code>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()
  </code></pre>
  
  <p>I have postgresql installed on Ubuntu 16.04 with lecture3 as database.When I execute the code as <code>python list.py</code>,I get the following error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "list.py", line 5, in &lt;module&gt;
      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'
  </code></pre>
  
  <p>postgres is the postgresql username and nehal is the password.
  <strong>How do I correct the error?</strong></p>