Add bank account class

billing/accounts.py

@@ -0,0 +1,30 @@
+class BankAccount:
+    def __init__(self, owner, balance=0.0):
+        self.owner = owner
+        if balance < 0:
+            raise ValueError("Initial balance cannot be negative")
+        self.balance = balance
+
+    def deposit(self, amount):
+        if amount <= 0:
+            raise ValueError("Deposit amount must be positive")
+        self.balance += amount
+        return self.balance
+
+    def withdraw(self, amount):
+        if amount <= 0:
+            raise ValueError("Withdrawal amount must be positive")
+        if amount > self.balance:
+            raise ValueError("Insufficient funds")
+        self.balance -= amount
+        return self.balance
+
+    def transfer(self, other_account, amount):
+        if not isinstance(other_account, BankAccount):
+            raise ValueError("Recipient must be a BankAccount instance")
+        self.withdraw(amount)
+        other_account.deposit(amount)
+        return self.balance, other_account.balance
+
+    def get_balance(self):
+        return self.balance