5.5. OOP Self

Calling method on an instance
Calling function on a class and passing instance as an argument
Both are equivalent

5.5.1. Instance Name

Usually first parameter of a method is named self (PEP8)
But it can be named differently

>>> class User:
...     def __init__(instance, firstname, lastname):
...         instance.firstname = firstname
...         instance.lastname = lastname
...
...     def login(instance):
...         print(f'User login: {instance.firstname} {instance.lastname}')
>>>
>>> obj = User('Alice', 'Apricot')
>>> obj.login()
User login: Alice Apricot

5.5.2. With Self

>>> class User:
...     def login(self):
...         print('ok')

Calling function on a class:

>>> User.login()
Traceback (most recent call last):
TypeError: User.login() missing 1 required positional argument: 'self'

Calling method on an instance:

>>> User().login()
ok

5.5.3. Without Self

>>> class User:
...     def login():
...         print('ok')

Calling function on a class:

>>> User.login()
ok

Calling method on an instance:

>>> User().login()
Traceback (most recent call last):
TypeError: User.login() takes 0 positional arguments but 1 was given

5.5.4. Passing Instance

Calling method on an instance is equivalent to calling function on a class and passing instance as an argument

Calling method on an instance is equivalent to calling function on a class and passing instance as an argument.

SetUp:

>>> class User:
...     def login(self):
...         print('ok')

Calling method on an instance:

>>> obj = User()
>>> obj.login()
ok

Or:

>>> User().login()
ok

Calling function on a class and passing instance as an argument:

>>> User.login(obj)
ok

5.5.5. Use Case - 1

Using instance method:

>>> str('hello').upper()
'HELLO'

Using class function:

>>> str.upper('hello')
'HELLO'

5.5.6. Use Case - 2

SetUp:

>>> data = ['Alice', 'Bob', 'Carol']

Using instance method:

>>> ','.join(data)
'Alice,Bob,Carol'

Using class function:

>>> str.join(',', data)
'Alice,Bob,Carol'

5.5.7. Use Case - 3

SetUp:

>>> data = ['Alice', 'Bob', 'Carol']

Using list comprehension and instance method:

>>> result = [x.upper() for x in data]
>>> list(result)
['ALICE', 'BOB', 'CAROL']

Using list comprehension and class function:

>>> result = [str.upper(x) for x in data]
>>> list(result)
['ALICE', 'BOB', 'CAROL']

Using map and class function:

>>> result = map(str.upper, data)
>>> list(result)
['ALICE', 'BOB', 'CAROL']

5.5.8. Use Case - 4

>>> class User:
...     def __init__(self, username):
...         self.username = username
...
...     def login(self):
...         print(f'User login: {self.username}')

Create users:

>>> users = [
...     User('alice'),
...     User('bob'),
...     User('carol'),
... ]

Login all users:

>>> users = map(User.login, users)
>>> result = list(users)
User login: alice
User login: bob
User login: carol