patterns/structural/facade.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
Example from https://en.wikipedia.org/wiki/Facade_pattern#Python


*What is this pattern about?
The Facade pattern is a way to provide a simpler unified interface to
a more complex system. It provides an easier way to access functions
of the underlying system by providing a single entry point.
This kind of abstraction is seen in many real life situations. For
example, we can turn on a computer by just pressing a button, but in
fact there are many procedures and operations done when that happens
(e.g., loading programs from disk to memory). In this case, the button
serves as an unified interface to all the underlying procedures to
turn on a computer.

*Where is the pattern used practically?
This pattern can be seen in the Python standard library when we use
the isdir function. Although a user simply uses this function to know
whether a path refers to a directory, the system makes a few
operations and calls other modules (e.g., os.stat) to give the result.

*References:
https://sourcemaking.com/design_patterns/facade
https://fkromer.github.io/python-pattern-references/design/#facade
https://python-3-patterns-idioms-test.readthedocs.io/en/latest/ChangeInterface.html#facade

*TL;DR
Provides a simpler unified interface to a complex system.
"""

from __future__ import print_function


# Complex computer parts
class CPU(object):
    """
    Simple CPU representation.
    """
    def freeze(self):
        print("Freezing processor.")

    def jump(self, position):
        print("Jumping to:", position)

    def execute(self):
        print("Executing.")


class Memory(object):
    """
    Simple memory representation.
    """
    def load(self, position, data):
        print("Loading from {0} data: '{1}'.".format(position, data))


class SolidStateDrive(object):
    """
    Simple solid state drive representation.
    """
    def read(self, lba, size):
        return "Some data from sector {0} with size {1}".format(lba, size)


class ComputerFacade(object):
    """
    Represents a facade for various computer parts.
    """
    def __init__(self):
        self.cpu = CPU()
        self.memory = Memory()
        self.ssd = SolidStateDrive()

    def start(self):
        self.cpu.freeze()
        self.memory.load("0x00", self.ssd.read("100", "1024"))
        self.cpu.jump("0x00")
        self.cpu.execute()


def main():
    """
    >>> computer_facade = ComputerFacade()
    >>> computer_facade.start()
    Freezing processor.
    Loading from 0x00 data: 'Some data from sector 100 with size 1024'.
    Jumping to: 0x00
    Executing.
    """


if __name__ == "__main__":
    import doctest
    doctest.testmod(optionflags=doctest.ELLIPSIS)