Config files example

This example demonstrates the use of config files to instantiated objects.

class DBConnection:
    def connect(self):
        ...

class MySQLConnection(DBConnection):
    def __init__(self, host: str, user: str, password: str) -> None:
        self.host = host
        self.user = user
        self.password = password

    def connect(self) -> None:
        print(f"MySQL connecting to {self.host}")


class PostgreSQLConnection(DBConnection):
    def __init__(self, host: str, user: str, password: str, database: str) -> None:
        self.host = host
        self.user = user
        self.password = password
        self.database = database

    def connect(self) -> None:
        print(f"PostgreSQL connecting to {self.host}")

To support this, we can have a parallel config structure:

conf/
├── config.yaml
└── db
    ├── mysql.yaml
    └── postgresql.yaml

Config files:

db/mysql.yaml

# @package _group_
_target_: my_app.MySQLConnection
host: localhost
user: root
password: 1234

db/postgresql.yaml

# @package _group_
_target_: my_app.PostgreSQLConnection
host: localhost
user: root
password: 1234
database: tutorial

config.yaml

defaults:
  - db: mysql

With this, you can instantiate the object from the configuration with a single line of code:

@hydra.main(config_path="conf", config_name="config")
def my_app(cfg):
    connection = hydra.utils.instantiate(cfg.db)
    connection.connect()