How to connect to posgres in Heroku from Python? - heroku

Question

I have followed the documentation and in my Python code I've added:
import os
import psycopg2
DATABASE_URL = os.environ['postgres://sficoXXX:842YYY#ec2-54-165-184-219.compute-1.amazonaws.com:5432/database-name']
conn = psycopg2.connect(DATABASE_URL, sslmode='require')
The URL I took from the session Config Vars (I used: heroku pg:credentials:url).
When I check the connection to the database using heroku pg:psql everything seems to be working fine.
But after deploying it shows the following error:
Failed to create session:
'postgres://sficoXXX:842YYY#ec2-54-165-184-219.compute-1.amazonaws.com:5432/database-name'
Traceback (most recent call last): File
"/Users/spoleto/.pyenv/versions/3.8.13/lib/python3.8/site-packages/otree/session.py",
line 447, in create_session_traceback_wrapper
return create_session(**kwargs) File "/Users/spoleto/.pyenv/versions/3.8.13/lib/python3.8/site-packages/otree/session.py",
line 418, in create_session
func(subsession) File "/Users/spoleto/PycharmProjects/upstream-reciprocity/prototypes/Consent/init.py",
line 35, in creating_session
DATABASE_URL = os.environ[postgres://sficoXXX:842YYY#ec2-54-165-184-219.compute-1.amazonaws.com:5432/database-name']
File "/Users/spoleto/.pyenv/versions/3.8.13/lib/python3.8/os.py", line
675, in getitem
raise KeyError(key) from None
Am I doing this right? Where does the error come from? How is the connection supposed to be established?
This is almost a copy/paste from the documentation.

Answer 1

The whole point of putting a connection string into an environment variable is so it doesn't need to be in your source code.
Instead of looking up the value of DATABASE_URL manually and pasting it into your source code, use the name of the environment variable:
DATABASE_URL = os.environ['DATABASE_URL']
Now your code will look for an environment variable with that name dynamically every time it runs, and set your DATABASE_URL variable to the value of the DATABASE_URL environment variable.
(The code you show in your question looks for an environment variable named postgres://..., which is very unlikely to exist.)
Note that this will fail with an IndexError if an environment variable named DATABASE_URL cannot be found. A safer way of doing this might be to use the .get() method (make sure to use round parentheses instead of square brackets):
DATABASE_URL = os.environ.get('DATABASE_URL')
Now you can even provide a fallback, e.g. for local development:
DATABASE_URL = os.environ.get('DATABASE_URL') or "postgres://postgres:postgres#localhost:5432/postgres"
Note: You didn't leak your whole connection string in your question, but I suggest you rotate your credentials anyway:
heroku pg:credentials:rotate
This will invalidate the old connection string and generate a new one.
The good news is that your DATABASE_URL environment variable will automatically be updated, and since your code now reads that value at runtime, it will continue to work!