3.8 class vs instance attributes

class Insan:
    ethnicity = "Balochi"


x = Insan()
y = Insan()

print(x.ethnicity)

Balochi

print(y.ethnicity)

Balochi

print(Insan.ethnicity)

Balochi

x.ethnicity = "muhajir"
y.ethnicity

'Balochi'

Insan.ethnicity

'Balochi'

Insan.ethnicity = "Insaniyat"
Insan.ethnicity

'Insaniyat'

y.ethnicity

'Insaniyat'

x.ethnicity

'muhajir'

Because the attribute was changed for class and instance x was created before this change so x still has old attribute value.

x.__dict__

{'ethnicity': 'muhajir'}

print(y.__dict__)

{}

Because y itself does not have attribute ethnicity so it looked for the attribute value in class Insan and used that value for itself.

print(Insan.__dict__)

{'__module__': '__main__', 'ethnicity': 'Insaniyat', '__dict__': <attribute '__dict__' of 'Insan' objects>, '__weakref__': <attribute '__weakref__' of 'Insan' objects>, '__doc__': None}

So class attributes and instance attributes can have different values. We can find out attributes of class of an instance from the instance using following code

print(x.__class__.__dict__)

{'__module__': '__main__', 'ethnicity': 'Insaniyat', '__dict__': <attribute '__dict__' of 'Insan' objects>, '__weakref__': <attribute '__weakref__' of 'Insan' objects>, '__doc__': None}

class philosopher:
    Quotes = (
        """What cannot be imagined cannot even be talked about.""",
        """Philosophy is a battle against the bewitchment of our intelligence by means of language.,""",
        """The limits of my language means the limits of my world."""
    )

    def __init__(self, name='Wittgenstein', dob=1889):
        self.name = name
        self.build_year = build_year

    # other methods as usual

for number, quote in enumerate(philosopher.Quotes):
    print(str(number + 1) + ":\n" + quote)

1:
What cannot be imagined cannot even be talked about.
2:
Philosophy is a battle against the bewitchment of our intelligence by means of language.,
3:
The limits of my language means the limits of my world.

Counting instances of a class

class Insan:
    counter = 0

    def __init__(self):
        # every time a new instance is created the attribute 'counter' of 'Insan' increases by 1
        type(self).counter += 1

    def __del__(self):
        # every time an instance of `Insan` is deleted, the attribute 'counter' of 'Insan' decreases by 1
        type(self).counter -= 1


x = Insan()
print("Population count is: : " + str(Insan.counter))
y = Insan()
print("Population count is: : " + str(Insan.counter))
del x
print("Population count is: : " + str(Insan.counter))
del y
print("Population count is: : " + str(Insan.counter))

Population count is: : 1
Population count is: : 2
Population count is: : 1
Population count is: : 0

type(self) is evaluated back to Insan