[Python] Instance/Static/Class/Abstract methods

Rex Chiang

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Feb 20, 2022

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Instance method

class User:
  static_var = "Static Variable"

  def func(self):
    print("Func")
    print(self.static_var)

if __name__ == "__main__":
  User.func()   # Error: TypeError
  user = User()
  user.func()   # Func
                # Static Variable

• Can only use instance of class to call.
• Need to use instance(self) as parameter.
• Can use instance(self) to access static variables outside of function.

Static method

class User:
  def func1():
    print("Func1")

  @staticmethod
  def func2():
    print("Func2")

if __name__ == "__main__":
  User.func1()   # Func1
  User.func2()   # Func2
  user = User()
  user.func1()   # Error: TypeError
  user.func2()   # Func2

• Can use both class or instance to call.
• No need and can’t use instance as parameter.
• Can’t access static variables outside of function.
• Would load in memory in class initialization stage instead of instance Initialization stage.
• Can use same function within each instances, and let relationship of members more clearly.
• Since it only load memory in class initialization stage, it saves memory when there have lots of instances.

Class method

class User:
  static_var = "Static Variable"

  @classmethod
  def func(cls):
    print("Func")
    print(cls.static_var)

  if __name__ == "__main__":
    User.func()   # Func
                  # Static Variable
    user = User()
    user.func()   # Func
                  # Static Variable

• Can use both class or instance to call.
• Need to use class(cls) as parameter.
• Can use class(cls) to access static variables outside of function.

Abstract method

import abc

class User(metaclass = abc.ABCMeta):
  @abc.abstractmethod
  def func(self):
    return NotImplemented

class Admin(User):
  pass

if __name__ == "__main__":
  Admin() # Error: TypeError

• Can only inherit from subclass.
• Subclass need to override the abstract methods.

import abc

class User(metaclass = abc.ABCMeta):
  @abc.abstractmethod
  def func(self):
    return NotImplemented

class Admin(User):
  def func(self):
    print("Func")

if __name__ == "__main__":
  Admin().func() # Func

• Can define the protocol for inheritance.
• Reduce the dependency between objects, and improve code quality with polymorphism.