9.3. Attributes¶

9.3.1. Rationale¶

Attributes are also known as "Properties" or "Fields"

snake_case name convention

Attributes store information (state) for instances

Access field values using dot (.) notation

Attributes should be defined only in __init__() method

More information in Init Method

attribute
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

Listing 9.12. Class example with distinction of properties and state attributes¶

Bucket with Water

    Properties:
        - color
        - width
        - height
        - radius
        - capacity
        - net mass (without water)

    State:
        - volume  (how much water is currently in bucket)
        - gross mass = net mass + water mass (water mass depends on its volume used))

9.3.2. Syntax¶

class Astronaut:
    pass


astro = Astronaut()

astro.firstname = 'Mark'
astro.lastname = 'Watney'

print(astro.firstname)
print(astro.lastname)

9.3.3. Dynamic Attributes¶

Listing 9.13. Dynamic attributes¶

class Astronaut:
    pass


jose = Astronaut()
jose.firstname = 'José'
jose.lastname = 'Jiménez'

print(f'My name... {jose.firstname} {jose.lastname}')
# My name... José Jiménez

Listing 9.14. Accessing not existing attributes¶

class Astronaut:
    pass


astro = Astronaut()

print(astro.missions)
# Traceback (most recent call last):
#     ...
# AttributeError: 'Astronaut' object has no attribute 'missions'

class Astronaut:
    pass


jose = Astronaut()
mark = Astronaut()

jose.name = 'José Jiménez'

print(f'My name... {jose.name}')
# My name... José Jiménez

print(f'My name... {mark.name}')
# Traceback (most recent call last):
#     ...
# AttributeError: 'Astronaut' object has no attribute 'name'

9.3.4. Namespace¶

point_x = 1
point_y = 2
point_z = 3

print(point_x)
print(point_y)
print(point_z)

class Point:
    pass

point = Point()
point.x = 1
point.y = 2
point.z = 3

print(point.x)
print(point.y)
print(point.z)

9.3.5. Different Types¶

Listing 9.15. Dynamic attributes¶

class Iris:
    pass


setosa = Iris()
setosa.features = [5.1, 3.5, 1.4, 0.2]
setosa.label = 'setosa'

print(setosa.label)
# setosa

print(setosa.features)
# [5.1, 3.5, 1.4, 0.2]

sum(setosa.features)
# 10.2

class Astronaut:
    pass


jose = Astronaut()
jose.age = 36

mark = Astronaut()
mark.age = 42.1

9.3.6. Get All Dynamic Attributes and Values¶

obj.__dict__

Listing 9.16. __dict__ - Getting dynamic fields and values¶

class Iris:
    pass


flower = Iris()
flower.sepal_length = 5.1
flower.sepal_width = 3.5
flower.petal_length = 1.4
flower.petal_width = 0.2
flower.species = 'setosa'

print(flower.__dict__)
# {'sepal_length': 5.1,
#  'sepal_width': 3.5,
#  'petal_length': 1.4,
#  'petal_width': 0.2,
#  'species': 'setosa'}

9.3.7. Assignments¶

9.3.7.1. OOP Attribute Model¶

Assignment name: OOP Attribute Model
Last update: 2020-10-01
Complexity level: easy
Lines of code to write: 15 lines
Estimated time of completion: 8 min
Solution: solution/oop_attribute_model.py

English

Use data from "Input" section (see below)
Model the data using classes
Create instances for each record
How many classes are there?
How many instances are there?
Print all fields from each instance

Polish

Użyj danych z sekcji "Input" (patrz poniżej)
Zamodeluj dane za pomocą klas
Stwórz instancje dla każdego wpisu
Ile jest klas?
Ile jest instancji?
Wypisz wszystkie pola każdej z instancji

Input

Mark Watney, USA, 1969-07-21
National Aeronautics and Space Administration, USA, 1958-07-29

Jan Twardowski, Poland, 1961-04-12
Polish Space Agency, Poland, 2014-09-26

The whys and wherefores