abc

Abstract Base Classes (abc)

Define interfaces that subclasses must implement — fail at instantiation if methods are missing.

Files

File	Description
example.py	Shape ABC, incomplete subclass trap, storage interface

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Descriptive Example

Scenario

Define a Shape interface — any concrete shape must implement area().

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

class BrokenShape(Shape):
    pass    # missing area()

rect = Rectangle(4, 5)
print(rect.area())          # 20

BrokenShape()               # TypeError: Can't instantiate abstract class

Swappable implementations

class Storage(ABC):
    @abstractmethod
    def save(self, data): ...
    @abstractmethod
    def load(self, key): ...

class MemoryStorage(Storage):
    def __init__(self):
        self._data = {}
    def save(self, data):
        key = str(len(self._data))
        self._data[key] = data
        return key
    def load(self, key):
        return self._data[key]

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Interview Q&A

Q1: What is an ABC?
A: Abstract Base Class — cannot be instantiated directly. Subclasses must implement all @abstractmethods or instantiation raises TypeError.

Q2: ABC vs duck typing?
A: Duck typing: "if it quacks, use it" — no formal contract. ABC: explicit interface enforced at instantiation. Both are valid Python styles.

Q3: What is collections.abc.Iterable?
A: ABC for objects with __iter__. Lets you check isinstance(obj, Iterable) without importing concrete types.

Q4: ABC vs Protocol (typing)?
A: ABC: runtime check via inheritance. Protocol: static structural typing — checked by mypy/pyright, no inheritance required.

Q5: Can you instantiate an ABC with all methods implemented?
A: Yes — once all abstract methods are overridden in a concrete subclass, it can be instantiated normally.

Q6: What is @abstractclassmethod / @abstractstaticmethod?
A: Variants for abstract class methods and static methods. Same rule — must be implemented in concrete subclass.

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Run

python3 example.py