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Python encapsulation and properties

Keep validation next to the state it protects.

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Before you start

Classes and exceptions.

Keep validation next to the state it protects.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

Encapsulation groups state with operations that maintain its rules. This account stores nonnegative integer units and rejects nonpositive deposits.

A read-only property exposes the balance through the public interface. A leading underscore is a convention for internal use, not enforced private access or a security boundary.

Code example

Python

class Account:
    def __init__(self):
        self._balance = 0
    @property
    def balance(self):
        return self._balance
    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("deposit must be positive")
        self._balance += amount

account = Account()
account.deposit(25)
print(account.balance)
try:
    account.deposit(0)
except ValueError:
    print("Rejected")
assert account.balance == 25

Output

25
Rejected

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1class Account:

2 def __init__(self):

3 self._balance = 0

4 @property

5 def balance(self):

6 return self._balance

7 def deposit(self, amount):

8 if amount <= 0:

9 raise ValueError("deposit must be positive")

10 self._balance += amount

12account = Account()

13account.deposit(25)

14print(account.balance)

15try:

16 account.deposit(0)

17except ValueError:

18 print("Rejected")

19assert account.balance == 25

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Initialize the balance.	`balance = 0`	—
Pass a positive amount.	`amount = 25`	—
Add the deposit.	`balance = 25`	—
Read through the property.	`balance = 25`	25
Try a zero deposit.	`amount = 0`	25
Reject it before changing state.	`balance = 25`	25
The caller handles the expected error.	`balance = 25`	25 Rejected

Behind the scenes

How Python processes this code

Python runs this source through its interpreter. It creates objects for values and resolves names as each statement runs.

Cost and common mistakes

O(1) work and space per bounded deposit.

Do not bypass validation by assigning internal state directly.
This teaching account omits persistence, concurrency, and currency rounding.

Try it yourself

Try a deposit of -5 and then check the balance.

Reveal answer and reasoning

It raises ValueError and the balance remains 25.