docker confusion about port - postgresql

Question

I created a docker postgres container by using a compose file:
postgres:
image: postgres:12-alpine
volumes:
- ./init.sql:/docker-entrypoint-initdb.d/init.sql
ports:
- "5432:5432"
init.sql:
CREATE USER test WITH PASSWORD '123456';
ALTER USER test WITH SUPERUSER;
CREATE DATABASE test;
I also created a pgAdmin4 container by running:
docker pull dpage/pgadmin4
docker run -p 5050:80 \
-e "PGADMIN_DEFAULT_EMAIL=user#domain.com" \
-e "PGADMIN_DEFAULT_PASSWORD=SuperSecret" \
-d dpage/pgadmin4
when setting up the connection in pgAdmin. I need to set up the server connection to 172.17.0.2 by looking at the postgres container IPAddress: docker container inspect 02e83f5f39d6 | grep IPAddress, using localhost as connection string will fail in pgAdmin4.
However, when I use python sqlalchemy, I could connect to localhost:5432.
import pandas as pd
from sqlalchemy import create_engine, MetaData
eng = create_engine('postgresql://test:123456#localhost:5432/test', echo=True)
meta = MetaData()
meta.reflect(eng, schema='public')
This is confusing. Why pgAdmin didn't recognize the port mapping? Is it because it is running from another container? What should be the correct set up then?

Answer 1

localhost is relative to the container / host.
For the pgAdmin container, localhost is itself. So you need to specify the IP to make pgAdmin point to the correct host.
Similarly for the python script running on your machine, localhost is the host machine (your PC). But since you specified the port mapping 5432:5432 the port on localhost:5432 is forwarded by docker the postgres containers port 5432. So if the script connects to localhost:5432 - it is forwarded to the container by docker.
There is nothing wrong for with this setup. Better would be have the pgAdmin container as part of the same compose file. With that you can utilize the name of the service as a DNS mapping to the container IP for easier management.