Encapsulation, Properties and Abstract Classes

50 min · Intermediate

These questions test whether you understand Python's own rules instead of repeating other languages. Expect "does Python have private", "why properties" and "why no overloading".

Privacy by convention, not by lock

Name mangling is how Python renames an attribute written as __name inside a class body to _ClassName__name. It avoids accidental clashes in subclasses; it does not hide data.

A leading underscore says "internal, please do not touch". Two leading underscores trigger name mangling. Neither stops access.

Python

1class Account:2    def __init__(self):3        self._balance = 100      # internal by convention4        self.__pin = 1234        # name-mangled5
6a = Account()7print(vars(a))8print(a._Account__pin)

Example 1 · Mangled names are still reachable

Python

1class A:2    def __init__(self):3        self.__x = 14
5a = A()6try:7    print(a.__x)8except AttributeError:9    print("blocked by name")10print(a._A__x)11print([n for n in vars(a)])

Output

blocked by name
1
['_A__x']

What this code does

a.__x fails only because the real name is different.
Anyone who knows the pattern can still read it, so treat it as a clash guard.

Python renames __pin to _Account__pin so a subclass cannot clash with it by accident. It is a protection against name clashes, not a security feature.

Properties

Property is a built-in decorator that turns a method into an attribute. Reads call the getter, writes call the setter, and you can add validation or computed values.

A property is a method that reads like an attribute. You can start with a plain attribute and add checks later without changing a single caller.

Python

1class Account:2    def __init__(self, balance):3        self._balance = balance4
5    @property6    def balance(self):7        return self._balance8
9    @balance.setter10    def balance(self, value):11        if value < 0:12            raise ValueError("negative")13        self._balance = value

Example 2 · A property with validation and a computed value

Python

1class Rectangle:2    def __init__(self, w, h):3        self.w = w4        self._h = h5
6    @property7    def h(self):8        return self._h9
10    @h.setter11    def h(self, value):12        if value <= 0:13            raise ValueError("height must be positive")14        self._h = value15
16    @property17    def area(self):18        return self.w * self._h19
20r = Rectangle(3, 4)21print(r.area)22r.h = 523print(r.area)24try:25    r.h = 026except ValueError as e:27    print(e)

Output

12
15
height must be positive

What this code does

area has no setter, so it is computed each time and cannot be assigned.
Callers still write r.h = 5, exactly as for a plain attribute.

acct.balance calls the getter and acct.balance = 5 calls the setter. Without a setter the property is read-only.

Abstract classes

Abstract base class is a class that inherits from ABC and declares abstract methods. A subclass must implement all of them before it can be instantiated.

An abstract class defines a contract that subclasses must fill in. It cannot be created itself.

Python

1from abc import ABC, abstractmethod2
3class Shape(ABC):4    @abstractmethod5    def area(self):6        ...7
8class Square(Shape):9    def __init__(self, side):10        self.side = side11
12    def area(self):13        return self.side ** 2

Example 3 · An abstract class and its contract

Python

1from abc import ABC, abstractmethod2
3class Shape(ABC):4    @abstractmethod5    def area(self):6        ...7
8    def describe(self):9        return f"{type(self).__name__} with area {self.area()}"10
11class Square(Shape):12    def __init__(self, side):13        self.side = side14
15    def area(self):16        return self.side ** 217
18class Broken(Shape):19    pass20
21print(Square(3).describe())22for cls in (Shape, Broken):23    try:24        cls()25    except TypeError:26        print("cannot create", cls.__name__)

Output

Square with area 9
cannot create Shape
cannot create Broken

What this code does

describe is a normal method that uses the abstract area, so every subclass gets it.
A subclass that skips area stays abstract and cannot be created.

Example 4 · There is no overloading, so the last def wins

Python

1class Calc:2    def add(self, a):3        return a4
5    def add(self, a, b):6        return a + b7
8def area(w, h=None):9    return w * w if h is None else w * h10
11print(Calc().add(2, 3))12try:13    Calc().add(2)14except TypeError:15    print("first add is gone")16print(area(3), area(3, 4))

Output

5
first add is gone
9 12

What this code does

The second def add replaced the first, so only the two-argument version exists.
A default argument gives one function two call shapes. *args and functools.singledispatch cover other needs.

Shape() raises TypeError because area is abstract. Square is complete, so Square(3) works.

Next step

Check your understanding

12 quick questions

Privacy by convention, not by lock

Name mangling is how Python renames an attribute written as __name inside a class body to _ClassName__name. It avoids accidental clashes in subclasses; it does not hide data.

A leading underscore says "internal, please do not touch". Two leading underscores trigger name mangling. Neither stops access.

Properties

Property is a built-in decorator that turns a method into an attribute. Reads call the getter, writes call the setter, and you can add validation or computed values.

A property is a method that reads like an attribute. You can start with a plain attribute and add checks later without changing a single caller.

1class Account:2 def __init__(self, balance):3 self._balance = balance4 5 @property6 def balance(self):7 return self._balance8 9 @balance.setter10 def balance(self, value):11 if value < 0:12 raise ValueError("negative")13 self._balance = value

1class Rectangle:2 def __init__(self, w, h):3 self.w = w4 self._h = h5 6 @property7 def h(self):8 return self._h9 10 @h.setter11 def h(self, value):12 if value <= 0:13 raise ValueError("height must be positive")14 self._h = value15 16 @property17 def area(self):18 return self.w * self._h19 20r = Rectangle(3, 4)21print(r.area)22r.h = 523print(r.area)24try:25 r.h = 026except ValueError as e:27 print(e)

1from abc import ABC, abstractmethod2 3class Shape(ABC):4 @abstractmethod5 def area(self):6 ...7 8class Square(Shape):9 def __init__(self, side):10 self.side = side11 12 def area(self):13 return self.side ** 2

1from abc import ABC, abstractmethod2 3class Shape(ABC):4 @abstractmethod5 def area(self):6 ...7 8 def describe(self):9 return f"{type(self).__name__} with area {self.area()}"10 11class Square(Shape):12 def __init__(self, side):13 self.side = side14 15 def area(self):16 return self.side ** 217 18class Broken(Shape):19 pass20 21print(Square(3).describe())22for cls in (Shape, Broken):23 try:24 cls()25 except TypeError:26 print("cannot create", cls.__name__)

1class Calc:2 def add(self, a):3 return a4 5 def add(self, a, b):6 return a + b7 8def area(w, h=None):9 return w * w if h is None else w * h10 11print(Calc().add(2, 3))12try:13 Calc().add(2)14except TypeError:15 print("first add is gone")16print(area(3), area(3, 4))