Python Object-Oriented Programming Basics 5

Question 1

What is the output of the following?
```python
class A:
    x = 5
a = A()
a.x = 10
print(A.x)
```

Accepted Answer

5

Answer

Assigning `a.x = 10` creates an instance attribute on `a` without modifying the class attribute `A.x`, which remains 5.

Question 2

Which dunder method enables the `len(obj)` call on a custom class?

Accepted Answer

__len__

Answer

Python calls `__len__` on the object when `len()` is invoked, so defining it lets your class support the built-in.

Question 3

What does `issubclass(Dog, Animal)` return if `Dog` inherits from `Animal`?

Accepted Answer

True

Answer

`issubclass()` returns `True` when the first argument is a subclass of the second, including indirect inheritance.

Question 4

How do you make a class attribute read-only for instances?

Accepted Answer

Use `@property` with only a getter and no setter

Answer

A `@property` with only a getter raises `AttributeError` on assignment, effectively making the attribute read-only from outside.

Question 5

What is the purpose of `__new__` in Python classes?

Accepted Answer

To allocate and return a new instance before `__init__` is called

Answer

`__new__` is a static method that creates and returns the new object; `__init__` then initializes it.

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