Pydantic Example

app.py

from __future__ import annotations

from pydantic import BaseModel, FilePath
import logging

from loguru import logger
import abc
from typing import Literal, Union
from pathlib import Path
import json


class AbstractLogger(abc.ABC):

    @abc.abstractmethod
    def print(self, x: str) -> None:
        ...


class AbstractLoggerConfig(BaseModel, abc.ABC):

    def create(self) -> AbstractLogger:
        ...


class PrintLogger(AbstractLogger):

    def __init__(self, config: PrintLoggerConfig):
        self.config = config

    def print(self, x: str) -> None:
        print(x)


class PrintLoggerConfig(AbstractLoggerConfig):
    type: Literal["print"] = "print"

    def create(self) -> PrintLogger:
        return PrintLogger(config=self)


class FileLoggerConfig(AbstractLoggerConfig):
    type: Literal["file"] = "file"
    path: FilePath = "output.txt"

    def create(self) -> AbstractLogger:
        return FileLogger(config=self)


class FileLogger(AbstractLogger):

    def __init__(self, config: FileLoggerConfig):
        self.config = config

    def print(self, x: str) -> None:
        with open(self.config.path, 'w') as f:
            f.write(x)


class Application:

    def __init__(self, config: ApplicationConfig):
        self.config = config
        self.logger = self.config.log_class.create()
        self.logger.print("Application starting")


class ApplicationConfig(BaseModel):
    log_class: Union[PrintLoggerConfig, FileLoggerConfig]

    def create(self):
        return Application(config=self)


config_path = Path("config.json")
app_config = ApplicationConfig.parse_file(config_path)
app = app_config.create()

config.json

{
  "log_class": {
    "type": "file",
    "path": "output.txt"
  }
}

output.txt

Application starting