title: SQLAlchemy cannot connect to Postgresql on localhost
body: <p>I'm sure this is such an easy error to fix, if I could only find where it is. This is the error from the Flask app:</p>
  
  <pre><code>11:58:18 web.1  | ERROR:xxxxxx.core:Exception on / [GET]
  11:58:18 web.1  | Traceback (most recent call last):
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask/app.py", line 1817, in wsgi_app
  11:58:18 web.1  |     response = self.full_dispatch_request()
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
  11:58:18 web.1  |     rv = self.handle_user_exception(e)
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
  11:58:18 web.1  |     reraise(exc_type, exc_value, tb)
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
  11:58:18 web.1  |     rv = self.dispatch_request()
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask/app.py", line 1461, in dispatch_request
  11:58:18 web.1  |     return self.view_functions[rule.endpoint](**req.view_args)
  11:58:18 web.1  |   File "xxxxxxx/web.py", line 202, in home
  11:58:18 web.1  |     d = {'featured': cached_apps.get_featured_front_page(),
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask_cache/__init__.py", line 245, in decorated_function
  11:58:18 web.1  |     rv = f(*args, **kwargs)
  11:58:18 web.1  |   File "/Users/xxxxxxx/Desktop/PythonProjects/xxxxxx/xxxxxx2/xxxxxxx/cache/apps.py", line 35, in get_featured_front_page
  11:58:18 web.1  |     results = db.engine.execute(sql)
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 780, in engine
  11:58:18 web.1  |     return self.get_engine(self.get_app())
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 797, in get_engine
  11:58:18 web.1  |     return connector.get_engine()
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 470, in get_engine
  11:58:18 web.1  |     self._sa.apply_driver_hacks(self._app, info, options)
  11:58:18 web.1  |   File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 739, in apply_driver_hacks
  11:58:18 web.1  |     if info.drivername.startswith('mysql'):
  11:58:18 web.1  | AttributeError: 'NoneType' object has no attribute 'drivername'
  </code></pre>
  
  <p>From what I've been able to find online, it seems like the problem is that I might not be connecting correctly to the database. The app works fine in Heroku, but not when I run on localhost. </p>
  
  <p><code>which psql</code>: </p>
  
  <pre><code>/Applications/Postgres.app/Contents/MacOS/bin/psql
  </code></pre>
  
  <p><code>which postgres</code>:</p>
  
  <pre><code>/Applications/Postgres.app/Contents/MacOS/bin/postgres
  </code></pre>
  
  <p>Postgres.app is running on 5432. </p>
  
  <p>I don't know what else to check. </p>
  
  <p>If it's supposed to connect to the same postgres DB on heroku regardless, why would it work on heroku, but not from localhost?</p>
  
  <p>Maybe the app on localhost is using a wrong version of Postgres? I've tried uninstalling them (and only leaving Postgres.app), but I'm not sure if there's anything left on my computer that's causing conflicts. How would I check that? I'd appreciate any help. </p>
  
  <p><strong>EDIT: More info</strong><br>
  Segment from the alembic.ini file:</p>
  
  <pre><code>[alembic]
  # path to migration scripts
  script_location = alembic
  
  # template used to generate migration files
  # file_template = %%(rev)s_%%(slug)s
  
  # under Heroku, the line below needs to be inferred from
  # the environment
  sqlalchemy.url = postgres://xxxxxxxxxx:xxxxxxxxxxxx@xxxxxx.compute-1.amazonaws.com:5432/xxxxxxxxx
  
  # Logging configuration
  [loggers]
  keys = root,sqlalchemy,alembic
  </code></pre>
  
  <p>I have a short script that produces the same error:
  I run <code>python cli.py db_create</code>
  on </p>
  
  <pre><code>#!/usr/bin/env python
  import os
  import sys
  import optparse
  import inspect
  
  import xxxxxxx.model as model
  from xxxxxx.core import db
  import xxxxx.web as web
  
  from alembic.config import Config
  from alembic import command    
  def setup_alembic_config():
      if "DATABASE_URL" not in os.environ:
          alembic_cfg = Config("alembic.ini")
      else:
          dynamic_filename = "alembic-heroku.ini"
          with file("alembic.ini.template") as f:
              with file(dynamic_filename, "w") as conf:
                  for line in f.readlines():
                      if line.startswith("sqlalchemy.url"):
                          conf.write("sqlalchemy.url = %s\n" %
                                     os.environ['DATABASE_URL'])
                      else:
                          conf.write(line)
          alembic_cfg = Config(dynamic_filename)
  
      command.stamp(alembic_cfg, "head")
  
  def db_create():
      '''Create the db'''
      db.create_all()
      # then, load the Alembic configuration and generate the
      # version table, "stamping" it with the most recent rev:
      setup_alembic_config()
      # finally, add a minimum set of categories: Volunteer Thinking, Volunteer Sensing, Published and Draft
      categories = []
      categories.append(model.Category(name="Thinking",
                                       short_name='thinking',
                                       description='Volunteer Thinking apps'))
      categories.append(model.Category(name="Volunteer Sensing",
                                       short_name='sensing',
                                       description='Volunteer Sensing apps'))
      db.session.add_all(categories)
      db.session.commit()
  </code></pre>
  
  <p>and I get:</p>
  
  <pre><code>Traceback (most recent call last):
    File "cli.py", line 111, in &lt;module&gt;
      _main(locals())
    File "cli.py", line 106, in _main
      _methods[method](*args[1:])
    File "cli.py", line 33, in db_create
      db.create_all()
    File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 856, in create_all
      self._execute_for_all_tables(app, bind, 'create_all')
    File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 848, in _execute_for_all_tables
      op(bind=self.get_engine(app, bind), tables=tables)
    File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 797, in get_engine
      return connector.get_engine()
    File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 470, in get_engine
      self._sa.apply_driver_hacks(self._app, info, options)
    File "/Library/Python/2.7/site-packages/flask_sqlalchemy/__init__.py", line 739, in apply_driver_hacks
      if info.drivername.startswith('mysql'):
  AttributeError: 'NoneType' object has no attribute 'drivername'
  </code></pre>