Python
Abstract property
Learn abstract properties with @abstractproperty.
Abstract property
To make an attribute abstract, you stack @property with @abstractmethod. @property goes on top, @abstractmethod below. This forces the subclass to fill in the property.
Example: @property\n@abstractmethod\ndef area(self):\n pass. The subclass fills area with @property. So shape.area works for every subclass; each provides its own area computation.
The subclass fills it in
The subclass fills the abstract property with @property. The pass body in the abstract class is replaced by a real computation. When the subclass instance is created, obj.area returns this computation.
Example: class Square(Shape):\n def __init__(self, side):\n self.side = side\n @property\n def area(self):\n return self.side ** 2. Square(3).area returns 9. If area is not filled, Square stays abstract and cannot be instantiated.
Exercise Shape abstract area, Square fills it
Start with from abc import ABC, abstractmethod. Define the abstract class Shape(ABC); inside, use @property, @abstractmethod and def area(self): with pass. Then define class Square(Shape):; write def __init__(self, side): and self.side = side. Add @property with def area(self): and return self.side ** 2. Finally create s = Square(3) and print with print(s.area).
Show example solution
# Write the Shape abstract class and the Square subclass
from abc import ABC, abstractmethod
class Shape(ABC):
@property
@abstractmethod
def area(self):
pass
class Square(Shape):
def __init__(self, side):
self.side = side
@property
def area(self):
return self.side ** 2
s = Square(3)
print(s.area)
Test yourself
-
What is the decorator order for an abstract property?
- First @abstractmethod, then @property
- Only @abstractmethod is enough
- Only @property is enough
- First @property, then @abstractmethod
Show answer
First @property, then @abstractmethod — @property on top, @abstractmethod below.
-
subclass @property line
class Square(Shape):
def __init__(self, side):
self.side = side
____
def area(self):
return self.side ** 2
Show answer
class Square(Shape):
def __init__(self, side):
self.side = side
@property
def area(self):
return self.side ** 2
-
Match the abstract property parts
- @property
- Top decorator
- @abstractmethod
- Bottom decorator
- def area(self)
- Abstract signature
- return self.side ** 2
- Implemented body
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