This puzzle explores how Python handles “private” variables using name mangling, a mechanism that modifies variable names to avoid accidental access or override in subclasses.

Topics Covered

Name mangling
Private variables in classes
Attribute access and introspection

Puzzle Question

Why does direct access to __secret raise an error, and how can it still be accessed? What will the following code print?

Puzzle Code

class Spy:
    def __init__(self):
        self.__secret = "hidden"

    def reveal(self):
        return self.__secret

agent = Spy()

What will each of these lines do?

print(agent.reveal())
print(agent.__secret)
print(agent._Spy__secret)

Hints

Double underscores at the start of a variable name trigger name mangling.
Name mangling changes the variable name internally to avoid accidental access in subclasses.
It’s still possible to access the variable — if you know the mangled name.

Answer

hidden
AttributeError
hidden

Explanation

agent.reveal() works because it accesses __secret from within the class where it was defined.
agent.__secret raises an AttributeError because the name has been mangled to _Spy__secret.
agent._Spy__secret works because it manually accesses the mangled name.

Learnings

Name Mangling: Python uses name mangling to help protect class internals from accidental overwrites or clashes, especially in subclasses.
Not Truly Private: Python doesn’t have true private variables — name mangling is only a convention-based safeguard.
Debugging & Introspection: Understanding name mangling is important when debugging, using dir(), or reflecting on class attributes.

PyPuzzle 05 Name Mangling and Private Variables