Instance Attributes and Class Attributes | Ucode

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Instance Attributes and Class Attributes ​

Since Python is a dynamic language, instances created from a class can bind attributes arbitrarily.

Binding Attributes to Instances ​

The method to bind attributes to an instance is through instance variables or the self variable:

python
class Student(object):
    def __init__(self, name):
        self.name = name

s = Student('Bob')
s.score = 90

Binding Attributes to the Class ​

But what if the Student class itself needs to bind an attribute? You can define the attribute directly within the class. This kind of attribute is a class attribute, owned by the Student class:

python
class Student(object):
    name = 'Student'

After defining a class attribute, although it belongs to the class, all instances of the class can access it. Let's test it:

python
>>> class Student(object):
...     name = 'Student'
...
>>> s = Student()  # Create instance s
>>> print(s.name)  # Print the name attribute. Since the instance doesn't have a name attribute, it looks up the class's name attribute
Student
>>> print(Student.name)  # Print the class's name attribute
Student
>>> s.name = 'Michael'  # Bind a name attribute to the instance
>>> print(s.name)  # Since the instance attribute has a higher priority than the class attribute, it overrides the class's name attribute
Michael
>>> print(Student.name)  # However, the class attribute still exists and can be accessed using Student.name
Student
>>> del s.name  # If you delete the instance's name attribute
>>> print(s.name)  # Calling s.name again. Since the instance's name attribute is not found, the class's name attribute is displayed
Student

From the above example, it is clear that when writing programs, you should never use the same name for both instance attributes and class attributes. This is because an instance attribute with the same name will override the class attribute. However, if you delete the instance attribute and then use the same name, you will access the class attribute.

Exercise ​

To count the number of students, you can add a class attribute to the Student class that automatically increments each time an instance is created:

python
class Student(object):
    count = 0

    def __init__(self, name):
        self.name = name
        Student.count += 1

# Test:
if Student.count != 0:
    print('Test failed!')
else:
    bart = Student('Bart')
    if Student.count != 1:
        print('Test failed!')
    else:
        lisa = Student('Lisa')
        if Student.count != 2:
            print('Test failed!')
        else:
            print('Students:', Student.count)
            print('Test succeeded!')

Summary ​

  • Instance Attributes belong to each individual instance and do not interfere with each other.

  • Class Attributes belong to the class itself, and all instances share the same attribute.

  • Avoid using the same name for both instance attributes and class attributes to prevent hard-to-find errors.