5.2. OOP Attribute ClassVar

  • Class Variables

  • Instance Variables

  • Type Annotations

5.2.1. Class Variables

  • Fields defined on a class

  • Must have default values

  • Share state

  • Also known as 'static attributes'

Class variables are defined on a class:

>>> class Astronaut:
...     pass
>>>
>>>
>>> Astronaut.firstname = 'Mark'
>>> Astronaut.lastname = 'Watney'

Class variables are defined in a class:

>>> class Astronaut:
...     firstname = 'Mark'
...     lastname = 'Watney'

5.2.2. Instance Variables

  • Fields defined on an instance

  • Do not share state (unless mutable argument in method signature)

  • By convention initialized in __init__()

  • Also known as 'dynamic attributes'

Instance variables are defined on an instance:

>>> class Astronaut:
...     pass
>>>
>>>
>>> astro = Astronaut()
>>> astro.firstname = 'Mark'
>>> astro.lastname = 'Watney'

Instance variables are defined in init:

>>> class Astronaut:
...     def __init__(self):
...         self.firstname = 'Mark'
...         self.lastname = 'Watney'

Instance variables with variable values:

>>> class Astronaut:
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname

5.2.3. Class and Instance Variables

Class and instance variables defined in code:

>>> class Astronaut:
...     pass
>>>
>>>
>>> Astronaut.firstname = 'Mark'
>>> Astronaut.lastname = 'Watney'
>>>
>>> astro = Astronaut()
>>> astro.firstname = 'Melissa'
>>> astro.lastname = 'Lewis'

Class and instance variables defined in class:

>>> class Astronaut:
...     firstname = 'Mark'
...     lastname = 'Watney'
...
...     def __init__(self):
...         self.firstname = 'Mark'
...         self.lastname = 'Watney'

Note, the last example makes not meaningful sense. Instance variables will shadow class variables.

5.2.4. Type Annotations

Type annotations are not variable definition:

>>> x: int
>>>
>>> print(x)
Traceback (most recent call last):
NameError: name 'x' is not defined

Type annotations will only tell, that if there will be an identifier with name x then it should be an int:

>>> x: int
>>> x = 1
>>>
>>> print(x)
1

Typically it is written in shorter form:

>>> x: int = 1
>>>
>>> print(x)
1

These are not attributes at all (sic!). These are type annotations only, and they do not exist before initialization in a code:

>>> class Astronaut:
...     firstname: str
...     lastname: str

Class variables with type annotations:

>>> class Astronaut:
...     firstname: str = 'Mark'
...     lastname: str = 'Watney'

Class variables with proper type annotations:

>>> from typing import ClassVar
>>>
>>>
>>> class Astronaut:
...     firstname: ClassVar[str] = 'Mark'
...     lastname: ClassVar[str] = 'Watney'

Instance variables with type annotations:

>>> class Astronaut:
...     firstname: str
...     lastname: str
...
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname

5.2.5. Dataclasses

  • Dataclass uses class variables notation to create instance fields

  • Dataclass do not validate type annotations, unless ClassVar

>>> from dataclasses import dataclass
>>> from typing import ClassVar

Instance variables:

>>> @dataclass
... class Astronaut:
...     firstname: str
...     lastname: str

Instance variables with default values:

>>> @dataclass
... class Astronaut:
...     firstname: str = 'Mark'
...     lastname: str = 'Watney'

Class variables must have default values:

>>> @dataclass
... class Astronaut:
...     firstname: ClassVar[str] = 'Mark'
...     lastname: ClassVar[str] = 'Watney'

5.2.6. Class vs. Instance Variables

Lets define a class with class variable:

>>> class Astronaut:
...     agency = 'NASA'

Lets create three instances of Astronaut class:

>>> mark = Astronaut()
>>> melissa = Astronaut()
>>> rick = Astronaut()

We will print agency field:

>>> print(mark.agency)
NASA
>>>
>>> print(melissa.agency)
NASA
>>>
>>> print(rick.agency)
NASA
>>>
>>> print(Astronaut.agency)
NASA

Lets change field on a class and print agency field:

>>> Astronaut.agency = 'ESA'
>>>
>>>
>>> print(mark.agency)
ESA
>>>
>>> print(melissa.agency)
ESA
>>>
>>> print(rick.agency)
ESA
>>>
>>> print(Astronaut.agency)
ESA

Lets change field on an instance and print agency field:

>>> mark.agency = 'POLSA'
>>>
>>>
>>> print(mark.agency)
POLSA
>>>
>>> print(melissa.agency)
ESA
>>>
>>> print(rick.agency)
ESA
>>>
>>> print(Astronaut.agency)
ESA

Note, that the class which defined instance variable shadowed the class variable.

Lets change field on a class and print agency field:

>>> Astronaut.agency = 'NASA'
>>>
>>>
>>> print(mark.agency)
POLSA
>>>
>>> print(melissa.agency)
NASA
>>>
>>> print(rick.agency)
NASA
>>>
>>> print(Astronaut.agency)
NASA

Lets delete field from an instance and print agency field:

>>> del mark.agency
>>>
>>>
>>> print(mark.agency)
NASA
>>>
>>> print(melissa.agency)
NASA
>>>
>>> print(rick.agency)
NASA
>>>
>>> print(Astronaut.agency)
NASA

5.2.7. Mechanism

  • vars(obj) is will return obj.__dict__

>>> class Astronaut:
...     firstname = 'Mark'
...     lastname = 'Watney'
...
...     def __init__(self, firstname, lastname):
...         self.firstname = firstname
...         self.lastname = lastname
>>>
>>>
>>> astro = Astronaut('Melissa', 'Lewis')
>>>
>>> vars(astro)
{'firstname': 'Melissa', 'lastname': 'Lewis'}
>>>
>>> vars(Astronaut)  
mappingproxy({'__module__': '__main__',
              'firstname': 'Mark',
              'lastname': 'Watney',
              '__init__': <function Astronaut.__init__ at 0x...>,
              '__dict__': <attribute '__dict__' of 'Astronaut' objects>,
              '__weakref__': <attribute '__weakref__' of 'Astronaut' objects>,
              '__doc__': None})

5.2.8. Use Case - 0x01

>>> from typing import ClassVar
>>>
>>>
>>> class Astronaut:
...     firstname: str
...     lastname: str
...     age: int
...     AGE_MIN: ClassVar[int] = 30
...     AGE_MAX: ClassVar[int] = 50

5.2.9. Use Case - 0x02

>>> from typing import ClassVar
>>>
>>>
>>> class Astronaut:
...     firstname: str
...     lastname: str
...     age: int
...     AGE_MIN: ClassVar[int] = 30
...     AGE_MAX: ClassVar[int] = 50
...
...     def __init__(self, firstname, lastname, age):
...         self.firstname = firstname
...         self.lastname = lastname
...         self.age = age
...
...         if not self.AGE_MIN <= self.age < self.AGE_MAX:
...             raise ValueError('age is invalid')

5.2.10. Use Case - 0x03

>>> from dataclasses import dataclass
>>> from typing import ClassVar
>>>
>>>
>>> @dataclass
... class Astronaut:
...     firstname: str
...     lastname: str
...     age: int
...     AGE_MIN: ClassVar[int] = 30
...     AGE_MAX: ClassVar[int] = 50
...
...     def __post_init__(self):
...         if not self.AGE_MIN <= self.age < self.AGE_MAX:
...             raise ValueError('age is invalid')

5.2.11. Assignments