Class methods

Introduction to Object-Oriented Programming in Python

George Boorman

Curriculum Manager, DataCamp

Methods

class Employee:
    def __init__(self, name, salary):
        self.name = name
        self.salary = salary

    def give_raise(self, amount):
        self.salary += amount


# Unique attribute values
emp_one = Employee("John", 40000)

emp_one.give_raise(5000)
print(emp_one.salary)










# Unique attribute values
emp_two = Employee("Jane", 60000)

emp_two.give_raise(5000)
print(emp_two.salary)

Class methods

Possible to define class methods
Must have a narrow scope because they can't use object-level data

class MyClass:
    # Use a decorator to declare a class method
    @classmethod

    # cls argument refers to the class
    def my_awesome_method(cls, args...): 
        # Do stuff here
        # Can't use any instance attributes

# Call the class, not the object
MyClass.my_awesome_method(args...)

As with self, cls is a convention but any word will work

Alternative constructors

class Employee:
  def __init__(self, name, salary):
      self.name = name
      self.salary = salary

  @classmethod
  def from_file(cls, filename):
      with open(filename, "r") as f:
          # Read the first line
          name = f.readline().strip()
          # Read the second line as integer
          salary = int(f.readline().strip())

      return cls(name, salary)

Allow alternative constructors
Can only have one __init__()

Use class methods to create objects
Use return to return an object
cls(...) will call __init__(...)

Alternative constructors

class Employee:
  def __init__(self, name, salary):
      self.name = name
      self.salary = salary
  @classmethod
  def from_file(cls, filename):
      with open(filename, "r") as f:
          name = f.readline().strip()
          salary = int(f.readline().strip())
      return cls(name, salary)

Employee.txt

Text file containing the name John Smith and salary 40000

# Create an employee without calling Employee()
emp = Employee.from_file("employee_data.txt")
print(emp.name)

John Smith

When to use class methods

Alternative constructors
Methods that don't require instance-level attributes
Restricting to a single instance (object) of a class
- Database connections
- Configuration settings

Let's practice!

Introduction to Object-Oriented Programming in Python