标签：#lowleveldesign

Guidelines, not rules. Here's the difference — and why it matters. What is SOLID? SOLID is a set of software design guidelines — not hard rules, but principles that guide how we organize our code. The goal is simple: as your codebase grows and your team scales, things should get easier to change, not harder. SOLID is what makes that possible. Five principles. One goal. Let's walk through each one with real code. S — Single Responsibility Principle A class, function, or method should have one and only one reason to change. The Violation class Bird : def __init__ ( self , name : str , bird_type : str ): self . name = name self . bird_type = bird_type def make_sound ( self ): # two jobs — deciding the type AND making the sound if self . bird_type == " parrot " : print ( " Squawk! " ) elif self . bird_type == " eagle " : print ( " Screech! " ) elif self . bird_type == " owl " : print ( " Hoot! " ) else : print ( " ... " ) make_sound() has two responsibilities — deciding which bird type it is AND making the sound. That's two reasons to change. Add a new bird? Touch make_sound() . Change how sounds work? Touch make_sound() again. Two different reasons, one method. SRP violated. The Fix from abc import ABC , abstractmethod class Bird ( ABC ): def __init__ ( self , name : str ): self . name = name @abstractmethod def make_sound ( self ): pass class Parrot ( Bird ): def make_sound ( self ): print ( " Squawk! " ) class Eagle ( Bird ): def make_sound ( self ): print ( " Screech! " ) class Owl ( Bird ): def make_sound ( self ): print ( " Hoot! " ) # Usage birds = [ Parrot ( " Polly " ), Eagle ( " Sam " ), Owl ( " Oliver " )] for bird in birds : bird . make_sound () Now each class has one responsibility. Parrot.make_sound() only changes if parrots change how they sound. Nothing else touches it. O — Open/Closed Principle A class should be open for extension but closed for modification. SRP and OCP go hand in hand. When you fixed SRP in the Bird example above — you also fixed OCP.