Fix type annotations and bugs in type_checking_with_mypy.py

- Fix open_account() parameter: change 'balance' to 'balances' (NameError bug)
- Add missing type annotations to all function parameters and return types
- Run through mypy to validate all type hints are correct
- Verify code runs without errors

Author: Iswanna

prep/type_checking_with_mypy.py

@@ -0,0 +1,30 @@
+def open_account(balances: dict[str, int], name: str, amount: int) -> None:
+    balances[name] = amount
+
+def sum_balances(accounts: dict[str, int]) -> int:
+    total = 0
+    for name, pence in accounts.items():
+        print(f"{name} had balance {pence}")
+        total += pence
+    return total
+
+def format_pence_as_string(total_pence: int) -> str:
+    if total_pence < 100:
+        return f"{total_pence}p"
+    pounds = int(total_pence / 100)
+    pence = total_pence % 100
+    return f"£{pounds}.{pence:02d}"
+
+balances: dict[str, int] = {
+    "Sima": 700,
+    "Linn": 545,
+    "Georg": 831,
+}
+
+open_account(balances, "Tobi", 913)
+open_account(balances, "Olya", 713)
+
+total_pence: int = sum_balances(balances)
+total_string: str = format_pence_as_string(total_pence)
+
+print(f"The bank accounts total {total_string}")