Python Dependency Injection via MRO/super() demo based on Raymong Hettinger talk during PyCon 2015

dependency-injection-demo.py

"""
This example demonstrates a "dependency injection" technique described
by Raymond Hettinger during PyCon 2015:
    https://www.youtube.com/watch?v=EiOglTERPEo

Regular dependency injection requires one to provide a resource to an object
that requires it using the __init__() method (or a class constructor).

This method provides an alternative way to specify a resource using Python's
Method Resolution Order (MRO) and super() magic.
"""

from abc import ABC, abstractmethod
from typing import Iterable

# ----------------------------

class AbstractItemSource(ABC):
    @abstractmethod
    def get_items(self) -> Iterable[str]:
        raise NotImplementedError()


class DefaultItemSource(AbstractItemSource):
    def __init__(self, url: str):
        self.url = url

    def get_items(self) -> Iterable[str]:
        yield f"First default item ({self.url})"
        yield f"Second default item ({self.url})"


class AlternateItemSource(AbstractItemSource):
    def __init__(self, filename: str):
        self.filename = filename

    def get_items(self) -> Iterable[str]:
        yield f"First alt item ({self.filename})"
        yield f"Second alt item ({self.filename})"
        yield f"Third alt item ({self.filename})"


# ----------------------------

class AbstractItemReader:
    """
    AbstractItemReader instance that reads items from an arbitrary
    AbstractItemSource provided using "multiple inheritance" syntax
    (see implementations below)
    """
    def read(self):
        print(f"{self.__class__.__name__}:")

        for item in self.get_items():
            print(f" > {item}")

        print()


class DefaultItemReader(AbstractItemReader, DefaultItemSource):
    """
    AbstractItemReader instance with DefaultItemSource as the 
    item source [get_items()]
    """
    
    def __init__(self, url: str):
        super().__init__(url)


class AltItemReader(AbstractItemReader, AlternateItemSource):
    """
    AbstractItemReader instance with AlternateItemSource as the
    item source [get_items()]
    """
    
    def __init__(self, filename: str):
        super().__init__(filename)

        
# ----------------------------

if __name__ == "__main__":
    DefaultItemReader('http://url.com').read()
    AltItemReader('file.txt').read()


# ----------------------------
# Result
# ----------------------------
#
# DefaultItemReader:
#  > First default item (http://url.com)
#  > Second default item (http://url.com)
# 
# AltItemReader:
#  > First alt item (file.txt)
#  > Second alt item (file.txt)
#  > Third alt item (file.txt)
#