Private attributes
- Python course
- Intermediate
- Lesson 18/116
- 4 min
Learn name mangling with _ and __.
Private attribute convention
In Python, to mark an attribute as private, you prefix it with an underscore: _balance. This is not a rule but a convention; it does not block outside access, yet users are expected not to do it.
Two underscores __balance trigger name mangling: Python rewrites the attribute name as _ClassName__balance. This reduces clashes in subclasses. A single underscore is common for everyday encapsulation.
Outside access
Single-underscore attributes can be accessed from outside; Python does not block it. Still, this breaks the encapsulation agreement. Double-underscore attributes cannot be accessed directly; _ClassName__name is required.
Example: class Account:\n def __init__(self, balance):\n self._balance = balance. a._balance works but is discouraged. Encapsulation aims to make it hard for outside code to mutate internal data directly; we add control with @property.
Practice Add a private attribute to Account
Define a class named Account. Add the method def __init__(self, balance): and in the body write self._balance = balance. Then create an instance with a = Account(100) and print it with print(a._balance).
Starter code
# Define the Account classSample solution
# Define the Account class
class Account:
def __init__(self, balance):
self._balance = balance
a = Account(100)
print(a._balance)Expected output
100
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which prefix triggers name mangling?
- _value
- __value Correct answer
- value_
- Value
Explanation: Two underscores turn __value into _ClassName__value.
2 Complete the private attribute
class Account:
def __init__(self, balance):
self.____ = balanceclass Account:
def __init__(self, balance):
self._balance = balanceExplanation: _balance is a single-underscore private attribute.
3 Match the name conventions
| Term | Meaning |
|---|---|
_value |
Convention private |
__value |
Name mangling |
value |
Public |