Classes

Abstract Base Classes

ABC and abstractmethod enforce that subclasses implement required methods.

ABC plus @abstractmethod declares the contract. Trying to construct the base class itself fails because at least one method has no implementation. A concrete describe() lives alongside the abstract area() so subclasses inherit shared behavior for free.

Source

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self) -> float:
        ...

    def describe(self) -> str:
        return f"shape with area {self.area()}"

try:
    Shape()
except TypeError as error:
    print(error)

Output

Can't instantiate abstract class Shape without an implementation for abstract method 'area'

A subclass that implements every abstract method is concrete and can be instantiated. It also inherits the non-abstract methods from the base class.

Source

class Square(Shape):
    def __init__(self, side):
        self.side = side

    def area(self):
        return self.side ** 2

print(Square(3).area())
print(Square(3).describe())

Output

9
shape with area 9

A subclass that forgets to implement an abstract method also cannot be instantiated — that is the value the ABC adds. The error fires at construction, not when something later tries to call the missing method.

Source

class Incomplete(Shape):
    pass

try:
    Incomplete()
except TypeError as error:
    print(error)

Output

Can't instantiate abstract class Incomplete without an implementation for abstract method 'area'

Contrast with Protocol. A HasArea protocol accepts any class with an area() method, no inheritance required. Triangle does not inherit from Shape, so it satisfies the protocol but fails isinstance(_, Shape). Square satisfies both because it explicitly inherited from the ABC.

Source

from typing import Protocol

class HasArea(Protocol):
    def area(self) -> float:
        ...

class Triangle:
    def __init__(self, base, height):
        self.base = base
        self.height = height

    def area(self):
        return 0.5 * self.base * self.height

def total_area(shapes: list[HasArea]) -> float:
    return sum(shape.area() for shape in shapes)

print(total_area([Square(3), Triangle(4, 3)]))
print(isinstance(Triangle(4, 3), Shape))
print(isinstance(Square(3), Shape))

Output

15.0
False
True

Notes

ABC plus @abstractmethod blocks instantiation until every abstract method has an implementation.
ABCs are nominal — subclasses opt in by inheriting; isinstance() reflects that opt-in.
Protocols are structural — any class with the right shape qualifies, regardless of inheritance.
Prefer an ABC when shared implementation or explicit opt-in matters; prefer a Protocol when only behavior at the API boundary matters.

Run the complete example

Example code

from abc import ABC, abstractmethod
from typing import Protocol

class Shape(ABC):
    @abstractmethod
    def area(self) -> float:
        ...

def describe(self) -> str:
        return f"shape with area {self.area()}"

try:
    Shape()
except TypeError as error:
    print(error)

class Square(Shape):
    def __init__(self, side):
        self.side = side

def area(self):
        return self.side ** 2

print(Square(3).area())
print(Square(3).describe())

class Incomplete(Shape):
    pass

try:
    Incomplete()
except TypeError as error:
    print(error)

class HasArea(Protocol):
    def area(self) -> float:
        ...

class Triangle:
    def __init__(self, base, height):
        self.base = base
        self.height = height

def area(self):
        return 0.5 * self.base * self.height

def total_area(shapes: list[HasArea]) -> float:
    return sum(shape.area() for shape in shapes)

print(total_area([Square(3), Triangle(4, 3)]))
print(isinstance(Triangle(4, 3), Shape))
print(isinstance(Square(3), Shape))

Expected output

Can't instantiate abstract class Shape without an implementation for abstract method 'area'
9
shape with area 9
Can't instantiate abstract class Incomplete without an implementation for abstract method 'area'
15.0
False
True

Execution time appears here after you run the example.

Abstract Base Classes

See also

Run the complete example

Example code

Expected output