8 Design Patterns Every Programmer Should Know

GeorgiLukanov87

Mar 7th, 2025 (edited)

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# 8 Design Patterns Every Programmer Should Know
# Creational Patterns
=========================================================================================================
Factory
class Burger:
def __init__(self, ingredients):
self.ingredients = ingredients
def print(self):
print(self.ingredients)
class BurgerFactory:
def createCheeseBurger(self):
ingredients = ["bun", "cheese", "beef-patty"]
return Burger(ingredients)
def createDeluxeCheeseBurger(self):
ingredients = ["bun", "tomatoe", "lettuce", "cheese", "beef-patty"]
return Burger(ingredients)
def createVeganBurger(self):
ingredients = ["bun", "special-sauce", "veggie-patty"]
return Burger(ingredients)
burgerFactory = BurgerFactory()
burgerFactory.createCheeseBurger().print()
burgerFactory.createDeluxeCheeseBurger().print()
burgerFactory.createVeganBurger().print()
Output:
['bun', 'cheese', 'beef-patty'] ['bun', 'tomatoe', 'lettuce', 'cheese', 'beef-patty'] ['bun', 'special-sauce', 'veggie-patty']
=========================================================================================================
Builder
class Burger:
def __init__(self):
self.buns = None
self.patty = None
self.cheese = None
def setBuns(self, bunStyle):
self.buns = bunStyle
def setPatty(self, pattyStyle):
self.patty = pattyStyle
def setCheese(self, cheeseStyle):
self.cheese = cheeseStyle
class BurgerBuilder:
def __init__(self):
self.burger = Burger()
def addBuns(self, bunStyle):
self.burger.setBuns(bunStyle)
return self
def addPatty(self, pattyStyle):
self.burger.setPatty(pattyStyle)
return self
def addCheese(self, cheeseStyle):
self.burger.setCheese(cheeseStyle)
return self
def build(self):
return self.burger
burger = BurgerBuilder() \
.addBuns("sesame") \
.addPatty("fish-patty") \
.addCheese("swiss cheese") \
.build()
=========================================================================================================
Singleton
class ApplicationState:
instance = None
def __init__(self):
self.isLoggedIn = False
@staticmethod
def getAppState():
if not ApplicationState.instance:
ApplicationState.instance = ApplicationState()
return ApplicationState.instance
appState1 = ApplicationState.getAppState()
print(appState1.isLoggedIn)
appState2 = ApplicationState.getAppState()
appState1.isLoggedIn = True
print(appState1.isLoggedIn)
print(appState2.isLoggedIn)
Output:
False True True
=========================================================================================================
Behavioural Patterns
"""Observer / PubSub
It's common for different components of an app to respond to events or state changes, but how can we communicate these events?
The Observer pattern is a popular solution. We have a Subject (aka Publisher) which will be the source of events. And we could have multiple Observers (aka Subscribers) which will recieve events from the Subject in realtime."""
class YoutubeChannel:
def __init__(self, name):
self.name = name
self.subscribers = []
def subscribe(self, sub):
self.subscribers.append(sub)
def notify(self, event):
for sub in self.subscribers:
sub.sendNotification(self.name, event)
from abc import ABC, abstractmethod
class YoutubeSubscriber(ABC):
@abstractmethod
def sendNotification(self, event):
pass
class YoutubeUser(YoutubeSubscriber):
def __init__(self, name):
self.name = name
def sendNotification(self, channel, event):
print(f"User {self.name} received notification from {channel}: {event}")
channel = YoutubeChannel("neetcode")
channel.subscribe(YoutubeUser("sub1"))
channel.subscribe(YoutubeUser("sub2"))
channel.subscribe(YoutubeUser("sub3"))
channel.notify("A new video released")
"""In this case we have multiple Subscribers listening to a single published. But users could also be subscribed to multiple channels.
Since the Publishers & Subscribers don't have to worry about each others' implementations, they are loosely coupled."""
User sub1 received notification from neetcode: A new video released User sub2 received notification from neetcode: A new video released User sub3 received notification from neetcode: A new video released
=========================================================================================================
Iterator
"""Many objects in python have built-in iterators. That's why we can conveniently iterate through an array using the key word in."""
myList = [1, 2, 3]
for n in myList:
print(n)
Output:
1 2 3
"""For more complex objects, like Linked Lists or Binary Search Trees, we can define our own iterators."""
class ListNode:
def __init__(self, val):
self.val = val
self.next = None
class LinkedList:
def __init__(self, head):
self.head = head
self.cur = None
# Define Iterator
def __iter__(self):
self.cur = self.head
return self
# Iterate
def __next__(self):
if self.cur:
val = self.cur.val
self.cur = self.cur.next
return val
else:
raise StopIteration
# Initialize LinkedList
head = ListNode(1)
head.next = ListNode(2)
head.next.next = ListNode(3)
myList = LinkedList(head)
# Iterate through LinkedList
for n in myList:
print(n)
Output:
1 2 3
=========================================================================================================
Strategy
"""A Class may have different behaviour, or invoke a different method based on something we define (i.e. a Strategy). For example, we can filter an array by removing positive values; or we could filter it by removing all odd values. These are two filtering strategies we could implement, but we could add many more."""
from abc import ABC, abstractmethod
class FilterStrategy(ABC):
@abstractmethod
def removeValue(self, val):
pass
class RemoveNegativeStrategy(FilterStrategy):
def removeValue(self, val):
return val < 0
class RemoveOddStrategy(FilterStrategy):
def removeValue(self, val):
return abs(val) % 2
class Values:
def __init__(self, vals):
self.vals = vals
def filter(self, strategy):
res = []
for n in self.vals:
if not strategy.removeValue(n):
res.append(n)
return res
values = Values([-7, -4, -1, 0, 2, 6, 9])
print(values.filter(RemoveNegativeStrategy()))
print(values.filter(RemoveOddStrategy()))
Output:
[0, 2, 6, 9] [-4, 0, 2, 6]
A common alternative to this pattern is to simply pass in an inline / lambda function, which allows us to extend the behaviour of a method or class.
=========================================================================================================
Structural Patterns
Facade
"""According to Oxford Languages, a Facade is
an outward appearance that is maintained to conceal a less pleasant or creditable reality.
In the programming world, the "outward appearance" is the class or interface we interact with as programmers. And the "less pleasant reality" is the complexity that is hidden from us.
So a Facade, is simply a wrapper class that can be used to abstract lower-level details that we don't want to worry about."""
# Python arrays are dynamic by default, but this is an example of resizing.
class Array:
def __init__(self):
self.capacity = 2
self.length = 0
self.arr = [0] * 2 # Array of capacity = 2
# Insert n in the last position of the array
def pushback(self, n):
if self.length == self.capacity:
self.resize()
# insert at next empty position
self.arr[self.length] = n
self.length += 1
def resize(self):
# Create new array of double capacity
self.capacity = 2 * self.capacity
newArr = [0] * self.capacity
# Copy elements to newArr
for i in range(self.length):
newArr[i] = self.arr[i]
self.arr = newArr
# Remove the last element in the array
def popback(self):
if self.length > 0:
self.length -= 1
=========================================================================================================
Adapter
"""Adapters allow incompatible objects to be used together. Following the Open-Closed principle, we can extend class behaviour without modifying the class itself.
If a MicroUsbCable class is initially incompatible with UsbPort, we can create a wrapper class (i.e. an Adapter), which makes them compatible. In this case, a MicroToUsbAdapter makes them compatible, similar to how we use adapters in the real-world."""
class UsbCable:
def __init__(self):
self.isPlugged = False
def plugUsb(self):
self.isPlugged = True
class UsbPort:
def __init__(self):
self.portAvailable = True
def plug(self, usb):
if self.portAvailable:
usb.plugUsb()
self.portAvailable = False
# UsbCables can plug directly into Usb ports
usbCable = UsbCable()
usbPort1 = UsbPort()
usbPort1.plug(usbCable)
class MicroUsbCable:
def __init__(self):
self.isPlugged = False
def plugMicroUsb(self):
self.isPlugged = True
class MicroToUsbAdapter(UsbCable):
def __init__(self, microUsbCable):
self.microUsbCable = microUsbCable
self.microUsbCable.plugMicroUsb()
# can override UsbCable.plugUsb() if needed
# MicroUsbCables can plug into Usb ports via an adapter
microToUsbAdapter = MicroToUsbAdapter(MicroUsbCable())
usbPort2 = UsbPort()
usbPort2.plug(microToUsbAdapter)

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