What Type of Design Pattern Am I?

Question 1

Abstract Factory

Answer 1

Creational
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“The Factory of Factories”

Lets you produce a family of related products w/o specifying their concrete classes.

Concrete factories inherit from abstract factory and instantiate a concrete familial product (inherits from abstract product).
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Example: 
Abstract Furniture Factory
Concrete Traditional Factory
Concrete Modern Factory
Concrete Couch Product
Concrete Bed Product

Question 2

Builder

Answer 2

Creational
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Solves Telescoping Constructor Anti-Pattern

Builds complex objects by separating the common CONSTRUCTION steps (abstract Builder) from the various possible REPRESENTATION - aka concrete Builders. These concrete Builders implement the same steps in a different manner, instead of constructor overloading.

Optional director to have a reusable definition for the order of the construction methods, or you can call from client code.
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Example:
Director
Abstract Car Builder
Concrete Manual Car Builder
Concrete Automatic Car Builder

Question 3

Factory Method

Answer 3

Creational
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Creates an interface for creating objects, but allows subclasses to determine which class to instantiate.
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Example:
Abstract Travel
Abstract Transport
Concrete Ship
Concrete Plane
Concrete WaterTravel (creates Ship)
Concrete AerialTravel (creates Plane)

Question 4

Prototype

Answer 4

Creational
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By having a copy() method in your class (making it a Prototype class), you can create exact copies easily.
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In Python, simply use:

import copy
prototype = Prototype(specific attrs)
copy_1 = copy.deepcopy(prototype)

Question 5

Singleton

Answer 5

Creational
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Restrict a class to one instance.
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Borg implementation - for any class that inherits from Borg, instances retain the same shared state.

In Python, simply define code in a module instead of a class as only one copy is imported

class Borg:
    _shared_state = {}
    def \_\_init\_\_(self):
        self.\_\_dict\_\_ = self._shared_state

class Single(Borg):
def __init__(self, **kwargs):
Borg.__init__(self)
self._shared_state.update(kwargs)
def __str__(self):
return ‘{}’.format(self._shared_state)

Question 6

Adapter

Answer 6

Structural
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Allows objects with incompatible interfaces to collaborate.
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Wrap one interface with an adapter that is compatible with the client/other interface, which handles the conversion.

Question 7

Bridge

Answer 7

Structural
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Separate an abstraction from its implementation so the two can vary independently.
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Take a Shape abstraction that has subclasses Rectangle, Square, etc.

As you add other dimensions - color, size, etc. - and other kinds of shapes - triangles, circles, etc. - your subclasses will become exponential if you create one for each permutation.

Instead allow the varied implementations - different kinds of shapes - to vary independently of their abstraction - the shape class - by creating bridges from the abstraction to classes that define features as a field reference.

Basic delegation/has-a logic in the abstract interface.

Question 8

Composite

Answer 8

Structural
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Composes objects into tree structures, and treats individual and composite objects uniformly.
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Like a directory is-a directory that has-a set of files, any of which can be a directory.

Question 9

Decorator

Answer 9

Structural
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Add functionality to a function/class by wrapping it with a decorator.

In Python, a callable that returns a callable.
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@ my_decorator
def my_fxn(): pass

my_fxn = my_decorator(my_fxn) => what @ does

def my_decorator(func):
    @wraps(func) # inspection shows func, not wrap
    def wrapper(*args, **kwargs):
        # decorate away
        return func(*args, **kwargs)
    return wrapper

Question 10

Facade

Answer 10

Structural
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A simplified interface for a more complex object/interface/library.

When you need limited functionality from the more complex source

Question 11

Flyweight

Answer 11

Structural
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A flyweight object has a shared intrinsic state across instances (constant data), and only needs to differ by the extrinsic state (changing data).

This speeds performance and saves memory.

Question 12

Proxy

Answer 12

Structural
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A placeholder for an object to control access to the potentially resource-heavy object, and only as needed.

A proxy has-a service, and they share an interface.
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A credit card is a proxy for a bank account/cash. They have the same interface - make payments. But for both the client and server it is safer/easier than cash.

Question 13

Chain of Responsibility

Answer 13

Behavioral
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A request is sent through a linked list of handlers with recursive traversal: a handler either handles the request if they can, or passes to the next handler if they cannot.

Question 14

Command

Answer 14

Behavioral
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Encapsulates a request as an object, separating the requestor from the service that performs the request.
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A restaurant order (check) represents a command - it is an object that represents the customer’s specific request for the cook to make.

Question 15

Interpreter

Answer 15

Behavioral
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Implements a specialized language.

Translating source code into an abstract syntax tree, a composite tree structure representing the code with

• operations stored as compound nodes of the tree that hold the grammar functionality
• and terminal nodes that represent operands/variables

Often a parser is used to traverse the tree.
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The Python Interpreter uses this pattern.

Question 16

Iterator

Answer 16

Behavioral
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Provide a way to traverse elements of an aggregate object without exposing the underlying representation
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In Python, not really necessary as iterable objects create iterators when they go through iteration contexts (loops, comprehensions)

And if you’re defining a class, you can use operator overloading on the __iter__ and __next__ methods to define the iteration context. (Iter returns the iterable, next returns the next, raise StopIteration to end the iteration context).

Question 17

Mediator

Answer 17

Behavioral
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Define an object that controls how a set of objects interact, so that they do not interact directly, but always through the mediator.
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We want to design reusable components without complicated dependencies between one another.

Single responsibility principle - extract the component communication to a single space, making it easier to comprehend and maintain

Open/closed principle - you can introduce new mediators w/o altering the components

Question 18

Memento

Answer 18

Behavioral
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A way to restore an object to a previous state w/o breaking encapsulation.

Using a snapshot object that stores the state and only the originator object accesses, and a caretaker that knows how and when the originator should restore to a previous state that stores a stack of mementos.

Question 19

Observer

Answer 19

Behavioral
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Design a subscription mechanism that notifies subscribers when a change is made by the object they observe.

The observer will have a subscriber list that subscribers can unsubscribe themselves from, and a notify method that loops over the subscriber list and calls their update methods appropriately.

Question 20

State

Answer 20

Behavioral
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Allow an object to alter its behavior when its internal state changes by storing a state object in the original context object that defines all state-specific behavior

This is identical in implementation to a Bridge, but the purpose is not to decouple abstraction from implementation, but to alter the object’s behavior.

Similarly, similar in form to Strategy, but with different intent.
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Your phone’s state is unlocked, so pressing buttons executes different functions.

If your phone’s state is unlocked, you are prompted to unlock the phone.

Question 21

Strategy

Answer 21

Behavioral
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Define a family of algorithms, encapsulate each one, and make them interchangeable.

Store a strategy object in the original context object that houses the preferred methodology.

Basically the same as State in design (and therefore Bridge), but differ in intent.
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You have to get to the airport, but depending on factors such as time and money, you might choose a different strategy: walking, bus, driving.

Question 22

Template Method

Answer 22

Behavioral
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Create an abstract template interface that represents the skeleton of an algorithm, that concrete subclasses will inherit and overload the steps as necessary.

Related to Strategy in intent, both about algorithms.

• Template Method uses inheritance to change part of the algorithm and the logic of an entire class
• Strategy uses delegation to change the an entire algorithm and the logic of a single object.

Question 23

Visitor

Answer 23

Behavioral
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Abstract functionality/an operation to be performed on the elements of an object structure/aggregate hierarchy (often tree).

Like a taxi company might have an object structure of customers, and dispatch a taxi (visitor) that will perform the operation, instead of the customer driving themselves.