Python : SOLID Principles

SOLID principles:

• S Single Responsibility - Should have single reason to change and should be related to its primary objective.
• O Open Closed - add features by extension, not by modification
• L Liskov Substitution
• I Interface Segregation
• D Dependecny Inversion

AntiPattern:
    God Object : Reverse of Single Responsibility ie., 1 class doing everything

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Single Responsibility : A class should have only one job
Example :

class User:

def __init__(self, name: str): self.name = name

def get_name(self) :pass

def save(self, user: User): pass

Problem: User class is taking responsibility of Saving as well .
Solution :

class User:

def __init__(self, name: str): self.name = name

def get_name(self): pass

class UserDB:

def save(self, user: User):pass

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Open Closed : Classes should be open for extension, not for modification.
Example :

class bottle:

def __init__(self, season, price):

self.season = season

self.price = price

def give_discount(self):

if self.season == 'normal': return self.price

if self.season == 'festival': return self.price * 0.1

Problem : Let’s imagine you have a store, and you give a discount of 20% to your favorite customers using this class:
When you decide to offer double the 20% discount to VIP customers. You may modify the class

Solution:

class bottle:

def __init__(self, season, price):

self.season = season

self.price = price

def give_discount(self):return self.price

class Festival_bottle_price(bottle):

def give_discount(self): return self.price * 0.1

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Liskov Substitution Principle:  A sub-class can assume the place of its super-class without errors.
Example :

class Bird():

def fly(self):pass

class Duck(Bird):

def fly(self):pass

class penguin(Bird):

def fly(self):pass

Problem : This breaks LSP.As penguin cannot fly hence this causes error
Solution :

class Bird():pass

class FlyingBirds(Bird) :

def fly(self):pass

class Duck ( FlyingBirds):pass

class Ostrich ( Bird): pass

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Interface Segregation:Make interfaces that are client specific Clients should not be forced to depend upon interfaces that they do not use
Example :

class Machine:

def print(self):raise NotImplementedError()

def call(self):raise NotImplementedError()

class Printer(Machine):

def print(self, document): pass

Problem : Printer object throws exception when Printer().call() is performed as it should not have been exposed in the first place.
Solution:

class Machine:

def perform_operation(self):raise NotImplementedError()

class Printer(Machine):

def perform_operation(self, document): pass

class call_phone(Machine):

def perform_operation(self, document): pass

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Dependency Inversion  :A: High-level modules should not depend on low-level level modules. It should depend upon its abstraction.
Example :

class Http:

def __init__(self, xml_http_service: XMLHttpService):self.xml_http_service = xml_http_service

def get(self, url: str, options: dict):self.xml_http_service.request(url, 'GET')

def post(self, url, options: dict):self.xml_http_service.request(url, 'POST')

class XMLHttpService(XMLHttpRequestService):pass

Problem:Http =high-level component , XMLHttpService= low-level component.
The Http class should care less the type of Http service you are using

Solution:

class Connection:

def request(self, url: str, options: dict):

raise NotImplementedError

class Http:

def __init__(self, http_connection: Connection):

self.http_connection = http_connection

def get(self, url: str, options: dict):self.http_connection.request(url, 'GET')

def post(self, url, options: dict):self.http_connection.request(url, 'POST')

class NodeHttpService(Connection):def request(self, url: str, options:dict):pass

class MockHttpService(Connection):def request(self, url: str, options:dict):pass