dependency_injection.py

"""
Dependency Injection (DI) is a technique whereby one object supplies the dependencies (services)
to another object (client).
It allows to decouple objects: no need to change client code simply because an object it depends on
needs to be changed to a different one. (Open/Closed principle)

Port of the Java example of Dependency Injection" in
"xUnit Test Patterns - Refactoring Test Code" by Gerard Meszaros
(ISBN-10: 0131495054, ISBN-13: 978-0131495050)

In the following example `time_provider` (service) is embedded into TimeDisplay (client).
If such service performed an expensive operation you would like to substitute or mock it in tests.

class TimeDisplay(object):

    def __init__(self):
        self.time_provider = datetime.datetime.now

    def get_current_time_as_html_fragment(self):
        current_time = self.time_provider()
        current_time_as_html_fragment = "<span class=\"tinyBoldText\">{}</span>".format(current_time)
        return current_time_as_html_fragment

"""

import datetime
from typing import Callable


class ConstructorInjection:
    def __init__(self, time_provider: Callable) -> None:
        self.time_provider = time_provider

    def get_current_time_as_html_fragment(self) -> str:
        current_time = self.time_provider()
        current_time_as_html_fragment = '<span class="tinyBoldText">{}</span>'.format(
            current_time
        )
        return current_time_as_html_fragment


class ParameterInjection:
    def __init__(self) -> None:
        pass

    def get_current_time_as_html_fragment(self, time_provider: Callable) -> str:
        current_time = time_provider()
        current_time_as_html_fragment = '<span class="tinyBoldText">{}</span>'.format(
            current_time
        )
        return current_time_as_html_fragment


class SetterInjection:
    """Setter Injection"""

    def __init__(self):
        pass

    def set_time_provider(self, time_provider: Callable):
        self.time_provider = time_provider

    def get_current_time_as_html_fragment(self):
        current_time = self.time_provider()
        current_time_as_html_fragment = '<span class="tinyBoldText">{}</span>'.format(
            current_time
        )
        return current_time_as_html_fragment


def production_code_time_provider() -> str:
    """
    Production code version of the time provider (just a wrapper for formatting
    datetime for this example).
    """
    current_time = datetime.datetime.now()
    current_time_formatted = "{}:{}".format(current_time.hour, current_time.minute)
    return current_time_formatted


def midnight_time_provider() -> str:
    """Hard-coded stub"""
    return "24:01"


def main():
    """
    >>> time_with_ci1 = ConstructorInjection(midnight_time_provider)
    >>> time_with_ci1.get_current_time_as_html_fragment()
    '<span class="tinyBoldText">24:01</span>'

    >>> time_with_ci2 = ConstructorInjection(production_code_time_provider)
    >>> time_with_ci2.get_current_time_as_html_fragment()
    '<span class="tinyBoldText">...</span>'

    >>> time_with_pi = ParameterInjection()
    >>> time_with_pi.get_current_time_as_html_fragment(midnight_time_provider)
    '<span class="tinyBoldText">24:01</span>'

    >>> time_with_si = SetterInjection()

    >>> time_with_si.get_current_time_as_html_fragment()
    Traceback (most recent call last):
    ...
    AttributeError: 'SetterInjection' object has no attribute 'time_provider'

    >>> time_with_si.set_time_provider(midnight_time_provider)
    >>> time_with_si.get_current_time_as_html_fragment()
    '<span class="tinyBoldText">24:01</span>'
    """


if __name__ == "__main__":
    import doctest

    doctest.testmod(optionflags=doctest.ELLIPSIS)