Class anatomy: the __init__ constructor

Object-Oriented Programming in Python

Alex Yarosh

Content Quality Analyst @ DataCamp

Methods and attributes

  • Methods are function definitions within a class
  • self as the first argument
  • Define attributes by assignment
  • Refer to attributes in class via self.___

   

class MyClass:
    # function definition in class
    # first argument is self
    def my_method1(self, other_args...):
        # do things here

    def my_method2(self, my_attr):        
        # attribute created by assignment
        self.my_attr = my_attr
        ...
Object-Oriented Programming in Python

Constructor

  • Add data to object when creating it?
  • Constructor __init__() method is called every time an object is created.
class Customer:
       def __init__(self, name):     
        self.name = name           # <--- Create the .name attribute and set it to name parameter
        print("The __init__ method was called")


cust = Customer("Lara de Silva")   #<--- __init__ is implicitly called 
print(cust.name)

The __init__ method was called
Lara de Silva
Object-Oriented Programming in Python
class Customer:
       def __init__(self, name, balance):  # <-- balance parameter added

        self.name = name
        self.balance = balance             # <-- balance attribute added
        print("The __init__ method was called")

cust = Customer("Lara de Silva", 1000)     # <-- __init__ is called
print(cust.name)
print(cust.balance)

The __init__ method was called
Lara de Silva
1000
Object-Oriented Programming in Python
class Customer:
       def __init__(self, name, balance=0):  #<--set default value for balance

        self.name = name
        self.balance = balance
        print("The __init__ method was called")


cust = Customer("Lara de Silva") # <-- don't specify balance explicitly

print(cust.name)
print(cust.balance) # <-- attribute is created anyway

The __init__ method was called
Lara de Silva
0
Object-Oriented Programming in Python

Attributes in methods

class MyClass:
    def my_method1(self, attr1):
        self.attr1 = attr1
        ...

    def my_method2(self, attr2):        
        self.attr2 = attr2
        ...

obj = MyClass()
obj.my_method1(val1) # <-- attr1 created
obj.my_method2(val2) # <-- attr2 created

Attributes in the constructor

class MyClass:
    def __init__(self, attr1, attr2):
        self.attr1 = attr1
        self.attr2 = attr2
        ...

# All attributes are created
obj = MyClass(val1, val2)
  • easier to know all the attributes
  • attributes are created when the object is created
  • more usable and maintainable code
Object-Oriented Programming in Python

Best practices

1. Initialize attributes in __init__()
Object-Oriented Programming in Python

Best practices

1. Initialize attributes in __init__()
2. Naming

CamelCase for classes, lower_snake_case for functions and attributes

Object-Oriented Programming in Python

Best practices

1. Initialize attributes in __init__()
2. Naming

CamelCase for class, lower_snake_case for functions and attributes

3. Keep self as self
class MyClass:
    # This works but isn't recommended
    def my_method(kitty, attr):
       kitty.attr = attr
Object-Oriented Programming in Python

Best practices

1. Initialize attributes in __init__()
2. Naming

CamelCase for class, lower_snake_case for functions and attributes

3. self is self
4. Use docstrings
class MyClass:
    """This class does nothing"""
    pass
Object-Oriented Programming in Python

Let's practice!

Object-Oriented Programming in Python

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