PCAP Object-Oriented Programming — All Questions With Answers

Refer to the exhibit.

```python
class Base:
    def __init__(self):
        self._x = 10
    def get_x(self):
        return self._x

class Derived(Base):
    def __init__(self):
        self._x = 20

obj = Derived()
print(obj.get_x())
```

class A:
    def method(self):
        return 'A'

class B(A):
    def method(self):
        return 'B'

class C(A):
    def method(self):
        return 'C'

class D(B, C):
    pass

class Counter:
    count = 0
    def __init__(self):
        Counter.count += 1
    @classmethod
    def get_count(cls):
        return cls.count

c1 = Counter()
c2 = Counter()
print(Counter.get_count())
print(c1.get_count())
print(c2.get_count())

class MyClass:
    def __init__(self, x):
        self.__x = x
    def get_x(self):
        return self.__x

obj = MyClass(10)
print(obj.__x)

class MyClass:
    def __init__(self, value):
        self.value = value

obj = MyClass(10)
print(obj.value)

class A:
    def method(self):
        return 'A'
class B(A):
    def method(self):
        return 'B'
class C(A):
    def method(self):
        return 'C'
class D(B, C):
    pass

print(D.mro())

class Cache:
    def __init__(self, func):
        self.func = func
        self.cache = {}
    def __call__(self, *args):
        if args in self.cache:
            return self.cache[args]
        result = self.func(*args)
        self.cache[args] = result
        return result

@Cache
def add(a, b):
    return a + b

print(add(1, 2))
print(add(1, 2))

class MyClass:
    def __getattr__(self, name):
        return f"Default {name}"
    def __setattr__(self, name, value):
        self.__dict__[name] = value.upper()

obj = MyClass()
obj.x = "hello"
print(obj.x)

class A:
    def method(self):
        return "A"

class B(A):
    def method(self):
        return "B"

class C(A):
    def method(self):
        return "C"

class D(B, C):
    pass

print(D.__mro__)

class Circle:
    def __init__(self, radius):
        self._radius = radius

    @property
    def radius(self):
        return self._radius

    @radius.setter
    def radius(self, value):
        if value > 0:
            self._radius = value

c = Circle(5)
c.radius = -3
print(c.radius)

class A:
    def __init__(self):
        self.x = 1
class B(A):
    def __init__(self):
        super().__init__()
        self.x = 2
b = B()
print(b.x)

class MyClass:
    def __init__(self):
        self.name = "test"
obj = MyClass()
print(obj.age)

>>> help(MyClass)
Help on class MyClass in module __main__:
class MyClass(builtins.object)
 |  Methods defined here:
 |  
 |  __init__(self, value)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  display(self)
 |      Print the value.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  __dict__
 |      dictionary for instance variables (if defined)
 |  
 |  __weakref__
 |      list of weak references to the object (if defined)

class Dog:
    def __init__(self, name):
        self.name = name
    def bark(self):
        return "Woof!"

class MyClass:
    def __init__(self):
        self.__private = 10
obj = MyClass()
print(obj.__private)

class A:
    def method(self):
        print("A", end="")
class B(A):
    def method(self):
        print("B", end="")
        super().method()
class C(A):
    def method(self):
        print("C", end="")
        super().method()
class D(B, C):
    def method(self):
        print("D", end="")
        super().method()
obj = D()
obj.method()

class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.__balance = balance
    def deposit(self, amount):
        self.__balance += amount
    def withdraw(self, amount):
        if amount <= self.__balance:
            self.__balance -= amount
        else:
            print('Insufficient funds')
    def get_balance(self):
        return self.__balance

account = BankAccount('Alice', 100)
print(account.__balance)

class A:
    def __init__(self):
        self.x = 1
class B(A):
    def __init__(self):
        super().__init__()
        self.x = 2
class C(A):
    def __init__(self):
        self.x = 3
class D(B, C):
    pass

d = D()
print(d.x)

class MyClass:
    __slots__ = ('name', 'age')
    def __init__(self, name, age):
        self.name = name
        self.age = age

obj = MyClass('John', 30)
obj.city = 'New York'

Refer to the exhibit.

Traceback (most recent call last):
  File "test.py", line 10, in <module>
    print(obj.__private_attr)
AttributeError: 'MyClass' object has no attribute '__private_attr'

Given that MyClass defines __private_attr in __init__, why does this error occur?

class A:
    def __init__(self, val):
        self.val = val
    def __add__(self, other):
        return A(self.val + other.val + 1)
    def __str__(self):
        return str(self.val)

a = A(1)
b = A(2)
c = a + b
print(c)

class Base:
    def __init__(self, x):
        self.x = x
    def method(self):
        return self.x * 2

class Derived(Base):
    def __init__(self, x, y):
        Base.__init__(self, x)
        self.y = y
    def method(self):
        return super().method() + self.y

obj = Derived(3, 4)
print(obj.method())

class Counter:
    count = 0
    def __init__(self):
        Counter.count += 1
        self.id = Counter.count

c1 = Counter()
c2 = Counter()
print(c1.id, c2.id)

class MyClass:
    def __init__(self, value):
        self.__value = value
    def get_value(self):
        return self.__value

obj = MyClass(10)
print(obj.__value)

class Shape:
    def area(self):
        return 0

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius
    def area(self):
        return 3.14 * self.radius ** 2

shapes = [Shape(), Circle(2)]
for s in shapes:
    print(s.area(), end=' ')