"""

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 = f"{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)