diff --git a/other/bank_account.py b/other/bank_account.py
new file mode 100644
index 000000000000..97137d21dc4a
--- /dev/null
+++ b/other/bank_account.py
@@ -0,0 +1,98 @@
+"""
+Simple BankAccount class demonstrating core OOP concepts.
+
+Reference:
+Invariant-preserving bank account state model.
+
+This module implements a minimal transactional data model whose computational
+value is the enforcement of account invariants during constant-time state
+transitions: account identifiers must be present, balances may not start
+negative, deposits must be positive, and withdrawals may not overdraw the
+account.
+"""
+
+
+class BankAccount:
+    """
+    Stateful account abstraction with validated balance updates.
+
+    The class is intentionally small, but its purpose is not to serve as a
+    generic OOP tutorial. It provides a compact example of how to model a
+    mutable record while preserving correctness constraints across operations,
+    with each transaction executing in O(1) time.
+    >>> account = BankAccount("ACC-1001", initial_balance=100.0)
+    >>> account.balance
+    100.0
+    >>> account.deposit(50)
+    150.0
+    >>> account.withdraw(20)
+    130.0
+    >>> account.account_number
+    'ACC-1001'
+
+    >>> BankAccount("", initial_balance=10.0)
+    Traceback (most recent call last):
+    ...
+    ValueError: account_number must be provided
+
+    >>> BankAccount("ACC-1002", initial_balance=-1.0)
+    Traceback (most recent call last):
+    ...
+    ValueError: initial_balance cannot be negative
+
+    >>> account.deposit(0)
+    Traceback (most recent call last):
+    ...
+    ValueError: deposit amount must be positive
+
+    >>> account.withdraw(0)
+    Traceback (most recent call last):
+    ...
+    ValueError: withdraw amount must be positive
+
+    >>> account.withdraw(1000)
+    Traceback (most recent call last):
+    ...
+    ValueError: insufficient funds
+    """
+
+    def __init__(self, account_number: str, initial_balance: float = 0.0) -> None:
+        if not account_number:
+            raise ValueError("account_number must be provided")
+        if initial_balance < 0:
+            raise ValueError("initial_balance cannot be negative")
+
+        self.__account_number = account_number
+        self._balance = float(initial_balance)
+
+    @property
+    def account_number(self) -> str:
+        """Read-only public accessor for the encapsulated account number."""
+        return self.__account_number
+
+    @property
+    def balance(self) -> float:
+        """Current account balance."""
+        return self._balance
+
+    def deposit(self, amount: float) -> float:
+        """Deposit a positive amount and return updated balance."""
+        if amount <= 0:
+            raise ValueError("deposit amount must be positive")
+        self._balance += amount
+        return self._balance
+
+    def withdraw(self, amount: float) -> float:
+        """Withdraw a positive amount and return updated balance."""
+        if amount <= 0:
+            raise ValueError("withdraw amount must be positive")
+        if amount > self._balance:
+            raise ValueError("insufficient funds")
+        self._balance -= amount
+        return self._balance
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()