Code Explanation:
1. Define Class D
class D:
A class named D is created.
This class is going to behave as a descriptor because it defines the special method __get__().
A descriptor is an object that can control what happens when an attribute is accessed.
2. Define __set_name__()
def __set_name__(self, owner, name):
__set_name__() is automatically called by Python when a descriptor is assigned to a class attribute during class creation.
It receives three important values:
self → the descriptor object
owner → the class containing the descriptor
name → the name of the attribute
Later, when Python creates class A, this will effectively become:
__set_name__(A, "value")
So:
owner → A
name → "value"
3. Store the Attribute Name
self.name = name
The value of name is:
"value"
Therefore:
self.name = "value"
The descriptor now remembers the name under which it was assigned.
Conceptually:
D object
↓
name = "value"
4. Define __get__()
def __get__(self, obj, owner):
__get__() controls what Python returns when the descriptor is accessed.
The parameters are:
self → descriptor object
obj → instance through which the attribute is accessed
owner → class that owns the descriptor
In this question, we access:
A.value
through the class, not through an instance.
Therefore:
obj → None
owner → A
5. Build the Return Value
return f"{owner.__name__}:{self.name}"
This is the most important line.
Let's break it into two parts.
owner.__name__
Here:
owner → A
Therefore:
owner.__name__ → "A"
self.name
Earlier, we stored:
self.name → "value"
So the f-string becomes:
"A:value"
The __get__() method therefore returns:
A:value
6. Create Class A
class A:
value = D()
First:
D()
creates a descriptor object.
That object is assigned to:
A.value
During class creation, Python automatically calls:
D.__set_name__(A, "value")
Therefore:
self.name = "value"
7. Access A.value
print(A.value)
This is where the descriptor mechanism is triggered.
Python sees that value is a descriptor because the D object has a __get__() method.
So instead of simply returning the D object, Python calls:
D.__get__(descriptor, None, A)
Therefore:
obj = None
owner = A
8. Execute __get__()
Inside __get__():
return f"{owner.__name__}:{self.name}"
we have:
owner.__name__ → "A"
self.name → "value"
Therefore:
"A" + ":" + "value"
produces:
"A:value"
9. print() Displays the Result
print(A.value)
The descriptor returns:
A:value
So Python prints:
A:value
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