[Python] Python in a Docker Container refused connection to Postgres in a networked Docker...

I am trying to connect from a python script to a postgres database. Here's my docker-compose:

networks:
backend:
name: value_tracker_backend
external: true
services:
db:
build:
context: ./sql
dockerfile: db.dockerfile
environment:
POSTGRES_PASSWORD: postgres
POSTGRES_DB: EntitiesAndValues
ports:
- "5431:5432"
healthcheck:
test: ["CMD-SHELL", "pg_isready -U postgres -d EntitiesAndValues"]
interval: 10s
retries: 5
start_period: 30s
timeout: 10s
networks:
- backend
dataseeder:
build:
context: .
dockerfile: dataseeder/dataseeder.dockerfile
depends_on:
db:
condition: service_healthy
restart: true
networks:
- backend


And here's a snippet of the python code that's attempting to connect via dockers internal network:

import psycopg2
from enum import Enum

class DatabaseConnector:
def __init__(self, db_name, db_user):
self.db_name = db_name
self.db_user = db_user

def __enter__(self):
self.conn = psycopg2.connect(user = self.db_user,
database = self.db_name,
password = "data",
host = "db",
port = 5432)
return self.conn

def __exit__(self, type, value, tb):
self.conn.close()


I keep getting the following error:

0.394 dsn = 'user=data_seeder password=data host=db port=5432 dbname=EntitiesAndValues'
0.394 connection_factory = None, cursor_factory = None
0.394 kwargs = {'database': 'EntitiesAndValues', 'host': 'db', 'password': 'data', 'port': 5432, ...}
0.394 kwasync = {}
0.394
0.394 def connect(dsn=None, connection_factory=None, cursor_factory=None, **kwargs):
0.394 """
0.394 Create a new database connection.
0.394
0.394 The connection parameters can be specified as a string:
0.394
0.394 conn = psycopg2.connect("dbname=test user=postgres password=secret")
0.394
0.394 or using a set of keyword arguments:
0.394
0.394 conn = psycopg2.connect(database="test", user="postgres", password="secret")
0.394
0.394 Or as a mix of both. The basic connection parameters are:
0.394
0.394 - *dbname*: the database name
0.394 - *database*: the database name (only as keyword argument)
0.394 - *user*: user name used to authenticate
0.394 - *password*: password used to authenticate
0.394 - *host*: database host address (defaults to UNIX socket if not provided)
0.394 - *port*: connection port number (defaults to 5432 if not provided)
0.394
0.394 Using the *connection_factory* parameter a different class or connections
0.394 factory can be specified. It should be a callable object taking a dsn
0.394 argument.
0.394
0.394 Using the *cursor_factory* parameter, a new default cursor factory will be
0.394 used by cursor().
0.394
0.394 Using *async*=True an asynchronous connection will be created. *async_* is
0.394 a valid alias (for Python versions where ``async`` is a keyword).
0.394
0.394 Any other keyword parameter will be passed to the underlying client
0.394 library: the list of supported parameters depends on the library version.
0.394
0.394 """
0.394 kwasync = {}
0.394 if 'async' in kwargs:
0.394 kwasync['async'] = kwargs.pop('async')
0.394 if 'async_' in kwargs:
0.394 kwasync['async_'] = kwargs.pop('async_')
0.394
0.394 dsn = _ext.make_dsn(dsn, **kwargs)
0.394 > conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
0.394 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
0.394 E psycopg2.OperationalError: could not translate host name "db" to address: Name or service not known
0.394
0.394 usr/local/lib/python3.13/site-packages/psycopg2/__init__.py:122: OperationalError


I've tried a number of things to get this to work, leading up the the pre-created external network. This seems to be useful because i'm starting the database first with docker compose up -d db. Then once i know its running i start the dataseeder image as well. Its running some tests (RUN pytest ...) before building and starting the 'release' version.

The run pytest is started in the dataseeder.dockerfile:

FROM python:latest
# Create integration testing environment
RUN apt-get update
COPY dataseeder/ dataseeder/
COPY packages/ packages/
COPY tests/ tests/
COPY localtest.txt .
RUN pip install -r localtest.txt
RUN pytest -v tests/test_EntitiesValues.py


pinging from dataseeder to db:

# ping db
PING db (172.18.0.2) 56(84) bytes of data.
64 bytes from valuetrackersampleproject-db-1.value_tracker_backend (172.18.0.2): icmp_seq=1 ttl=64 time=0.166 ms
64 bytes from valuetrackersampleproject-db-1.value_tracker_backend (172.18.0.2): icmp_seq=2 ttl=64 time=0.093 ms
64 bytes from valuetrackersampleproject-db-1.value_tracker_backend (172.18.0.2): icmp_seq=3 ttl=64 time=0.095 ms
64 bytes from valuetrackersampleproject-db-1.value_tracker_backend (172.18.0.2): icmp_seq=4 ttl=64 time=0.095 ms
64 bytes from valuetrackersampleproject-db-1.value_tracker_backend (172.18.0.2): icmp_seq=5 ttl=64 time=0.079 ms
64 bytes from valuetrackersampleproject-db-1.value_tracker_backend (172.18.0.2): icmp_seq=6 ttl=64 time=0.094 ms


What am I missing here?

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