factory.py

"""*What is this pattern about?
A Factory is an object for creating other objects.

*What does this example do?
The code shows a way to localize words in two languages: English and
Greek. "get_localizer" is the factory function that constructs a
localizer depending on the language chosen. The localizer object will
be an instance from a different class according to the language
localized. However, the main code does not have to worry about which
localizer will be instantiated, since the method "localize" will be called
in the same way independently of the language.

*Where can the pattern be used practically?
The Factory Method can be seen in the popular web framework Django:
https://docs.djangoproject.com/en/4.0/topics/forms/formsets/
For example, different types of forms are created using a formset_factory

*References:
http://ginstrom.com/scribbles/2007/10/08/design-patterns-python-style/

*TL;DR
Creates objects without having to specify the exact class.
"""

from typing import Dict, Protocol, Type


class Localizer(Protocol):
    def localize(self, msg: str) -> str:
        pass


class GreekLocalizer:
    """A simple localizer a la gettext"""

    def __init__(self) -> None:
        self.translations = {"dog": "σκύλος", "cat": "γάτα"}

    def localize(self, msg: str) -> str:
        """We'll punt if we don't have a translation"""
        return self.translations.get(msg, msg)


class EnglishLocalizer:
    """Simply echoes the message"""

    def localize(self, msg: str) -> str:
        return msg


def get_localizer(language: str = "English") -> Localizer:
    """Factory"""
    localizers: Dict[str, Type[Localizer]] = {
        "English": EnglishLocalizer,
        "Greek": GreekLocalizer,
    }

    return localizers[language]()


def main():
    """
    # Create our localizers
    >>> e, g = get_localizer(language="English"), get_localizer(language="Greek")

    # Localize some text
    >>> for msg in "dog parrot cat bear".split():
    ...     print(e.localize(msg), g.localize(msg))
    dog σκύλος
    parrot parrot
    cat γάτα
    bear bear
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod()