How to Create Named Constructors with Class Methods in Python

The aim of this page📝 is to describe the implementation and benefits of named constructors in Python — aka factory function.

• In python, it s possible for a class to support named constructors
• Originally, this is a C++ idiom / design pattern
• A so-called factory method returns an instance of a class
• The method has a name other than the default __init__(), which allows callers to express intent
• Therefore, it also allows object construction to be performed with different combinations of arguments
• The following class method

@classmethod
def create_empty(cls, owner_code):
    return cls(owner_code, contents=[])

… accepts owner_code and calls the class to create the (object) instance

• The whole code contains also a regular initializer, see

class ShippingContainer:

    next_serial = 1337

    @classmethod
    def _generate_serial(cls):
        result = cls.next_serial
        cls.next_serial += 1
        return result

    @classmethod
    def create_empty(cls, owner_code):
        return cls(owner_code, contents=[])

    def __init__(self, owner_code, contents):
        self.owner_code = owner_code
        self.contents = contents
        self.serial = ShippingContainer._generate_serial()

• You can, however, create an object by calling the create_empty "constructor" / "factory function"
• Of course, if you try to create an instance without any method and with missing arguments you’ll get a TypeError

>>> c = ShippingContainer()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 2 required positional arguments: 'owner_code' and 'contents'

• To create a regular instance, you provide all the args defined in the __init__()

>>> c = ShippingContainer(owner_code="ABC",contents=['Apples','Oranges'])

• By calling create_empty(cls, owner_code), you return the class object providing it with

1. passed owner_code
2. empty list for contents parameter

• This would be synonymous with calling

c = ShippingContainer(owner_code="ABC", contents=[])

• This technique allows defining multiple constructors with different behaviors
• → adds an extra layer between the class and the __init__() method
• → avoids adding unnecessary complexity to the __init__() itself
• → possible to pass different forms of an argument list