gistfile1.txt

# Taken DIRECTLY from William's article https://realpython.com/python-interface/
import abc

class FormalParserInterface(metaclass=abc.ABCMeta):
  @classmethod
  def __subclasshook__(cls, subclass):
    print("FormalParserInterface.__subclasshook__")
    return (hasattr(subclass, 'load_data_source') and 
        callable(subclass.load_data_source) and 
        hasattr(subclass, 'extract_text') and 
        callable(subclass.extract_text))

class CsvParserNew:
  def load_data_src(self, path: str, file_name: str) -> str:
    print("CsvParserNew.load_data_source")

  def extract_text(self, full_file_path: str) -> dict:
    print("CsvParserNew.extract_text")

class PdfParserNew(FormalParserInterface):
  def load_data_source(self, path: str, file_name: str) -> str:
    print("PdfParserNew.load_data_source")

  def extract_text(self, full_file_path: str) -> dict:
    print("PdfParserNew.extract_text")

@FormalParserInterface.register # This will trigger FormalParserInterface.__subclasshook__ when you declare the class (i.e. BEFORE you even instantiate it)
class EmlParserNew(FormalParserInterface):
  def load_data_source(self, path: str, file_name: str) -> str:
    print("EmlParserNew.load_data_source")

  def extract_text(self, full_file_path: str) -> dict:
    print("EmlParserNew.extract_text")

pdf = PdfParserNew()
eml = EmlParserNew()

print(issubclass(CsvParserNew, FormalParserInterface))  # False. issubclass will trigger FormalParserInterface.__subclasshook__ 
print(issubclass(PdfParserNew, FormalParserInterface))  # True.
print(issubclass(EmlParserNew, FormalParserInterface))  # True