Fix race condition in concurrent deposit

Concurrent bank account debugging (thread safety)

You are given a simple BankAccount object that is used concurrently from multiple threads. Two deposits run at the same time:

account = BankAccount(0)
with ThreadPoolExecutor(max_workers=2) as executor:
    futures = [
        executor.submit(account.deposit, 500),
        executor.submit(account.deposit, 700),
    ]

A simplified implementation of deposit() looks like this:

class BankAccount:
    def __init__(self, balance=0):
        self.balance = balance

    def deposit(self, amount):
        new_balance = self.balance + amount  # Read
        time.sleep(0.1)                      # Delay
        self.balance = new_balance           # Write

Tasks

1. Explain why this code can produce an incorrect final balance (e.g., 700 instead of the expected 1200 ).
2. Identify the underlying concurrency bug(s).
3. Propose and describe a correct fix that ensures thread safety for deposit() .
4. Mention any important caveats or alternatives (e.g., performance trade-offs, other synchronization approaches).