12.3. OOP Attribute About¶

12.3.1. Rationale¶

Attributes are also known as "Properties" or "Fields"
Attributes store information for instances
Access field values using dot (.) notation

attribute¶: Variable inside the class. In Python, methods also can be described as attributes, but justification for that is a bit more complex which will be introduced later in a book.
field¶: Variable inside the class. Can be used as a synonym of property or state.
property¶: Variable inside the class. Should not change during lifetime of an object.
state¶: Variable inside the class. Changes during lifetime of an object. Represents current state of an object.
namespace¶: Container for storing related data

Class example with distinction of properties and state attributes.

An example "Glass with Water" can illustrate the distinction of properties and state attributes:

Properties:

color
width
height
radius
capacity
net mass (without water)

State:

volume (how much water is currently in the glass)
gross mass = net mass + water mass (water mass depends on its volume used))

12.3.2. What are attributes?¶

Identifiers and scalars creates values:

>>> point_x: int
>>> point_y: int
>>> point_z: int

Values with relations creates structure:

>>> point: tuple[int,int,int]

Data with meaning creates data:

>>> point: dict[str,int]

Information with context and relations creates information:

>>> class Point:
...     x: int
...     y: int
...     z: int

12.3.3. Namespace¶

Unrelated values:

>>> x = 1
>>> y = 2
>>> z = 3

Class creates space, in which names has meaning:

>>> class Point:
...     x: int
...     y: int
...     z: int
>>>
>>>
>>> point = Point()
>>> point.x = 1
>>> point.y = 2
>>> point.z = 3

12.3.4. Use Case - 0x01¶

>>> class Laptop:
...     cpu: float
...     ram: int
...     ssd: int
>>>
>>>
>>> macbook = Laptop()
>>> lenovo = Laptop()
>>> hp = Laptop()
>>> asus = Laptop()

12.3.5. Use Case - 0x02¶

>>> class Date:
...     year: int
...     month: int
...     day: int
>>>
>>>
>>> # Gagarin - first man to leave Earth
>>> gagarin = Date()
>>> gagarin.year = 1961
>>> gagarin.month = 4
>>> gagarin.day = 12
>>>
>>> # Armstrong - first man to walk on the Moon
>>> armstrong = Date()
>>> armstrong.year = 1969
>>> armstrong.month = 7
>>> armstrong.day = 21

12.3.6. Use Case - 0x03¶

Relation

>>> class Date:
...     year: int
...     month: int
...     day: int
>>>
>>>
>>> class Person:
...     firstname: str
...     lastname: str
...     date_of_birth: Date
...     height: float
...     weight: float

12.3.7. Good Practices¶

snake_case name convention
Attributes should be defined only in __init__() method
More information in OOP Init Method

12.3.8. References¶

glassimg: https://media.istockphoto.com/vectors/glasses-set-for-water-glasses-full-empty-halffilled-with-water-vector-vector-id905957960?k=6&m=905957960&s=612x612&w=0&h=DE0uCDCehEA_eDHzHW38jvhl3pYjNuoqXZ_6ZzHbz0M=