存钱

1. 存钱

存钱的关键代码：

def deposit(amount):
    balance = get_balance(account, ...)
    balance += amount 
    write_balance(account, balance)

考虑有两个操作，

• ATM存钱
• 在线转账

1.1 测试用例

import threading
import random
import sys

balance = 0

def deposit(list_amount):
    global balance
    for amount in list_amount:
        balance += amount

def main():
    # Generate random numbers 
    if len(sys.argv) != 2:
        print('The usage: python {} num'.format(sys.argv[0]))
        exit()

    times = int(sys.argv[1])

    # list_deposit_amount1 = [random.randint(1, 101) for _ in range(times)] 
    list_deposit_amount1 = [1] * times
    list_deposit_amount2 = [1] * times

    print('-----At the beginning-----')
    print('Initial balance: {}'.format(balance))
    print('The total amount of deposit1: {}'.format(sum(list_deposit_amount1)))
    print('The total amount of deposit2: {}'.format(sum(list_deposit_amount2)))
    expected_balance = balance + sum(list_deposit_amount1) + sum(list_deposit_amount2)
    print('The expected balance: {}'.format(expected_balance))

    # Multiple threads
    deposit_thread1 = threading.Thread(target=deposit, args=(list_deposit_amount1,))
    deposit_thread2 = threading.Thread(target=deposit, args=(list_deposit_amount2,))

    deposit_thread1.start()
    deposit_thread2.start()

    deposit_thread1.join() # Waits until the first thread is finished.
    deposit_thread2.join()

    print('\n-----In the end-----')
    print('The balance: {}'.format(balance))

if __name__ == '__main__':
    main()

1.2 测试结果

累计存钱次数少：

$ python .\deposit_money.py 1000
-----At the beginning-----
Initial balance: 0
The total amount of deposit1: 1000
The total amount of deposit2: 1000
The expected balance: 2000

-----In the end-----
The balance: 2000

加大存钱次数：

1.3 分析原因

balance -->  R

R         <--  R + amount

R         -->  balance

2. 修改后

2.1 加锁

2.2 测试用例

import threading
import random
import sys

balance = 0

mutex = threading.Lock()

def deposit(list_amount):
    global balance
    for amount in list_amount:
        mutex.acquire()
        balance += amount
        mutex.release()

def main():
    # Generate random numbers 
    if len(sys.argv) != 2:
        print('The usage: python {} num'.format(sys.argv[0]))
        exit()

    times = int(sys.argv[1])

    # list_deposit_amount1 = [random.randint(1, 101) for _ in range(times)] 
    list_deposit_amount1 = [1] * times
    list_deposit_amount2 = [1] * times

    print('-----At the beginning-----')
    print('Initial balance: {}'.format(balance))
    print('The total amount of deposit1: {}'.format(sum(list_deposit_amount1)))
    print('The total amount of deposit2: {}'.format(sum(list_deposit_amount2)))
    expected_balance = balance + sum(list_deposit_amount1) + sum(list_deposit_amount2)
    print('The expected balance: {}'.format(expected_balance))

    # Multiple threads
    deposit_thread1 = threading.Thread(target=deposit, args=(list_deposit_amount1,))
    deposit_thread2 = threading.Thread(target=deposit, args=(list_deposit_amount2,))

    deposit_thread1.start()
    deposit_thread2.start()

    deposit_thread1.join() # Waits until the first thread is finished.
    deposit_thread2.join()

    print('\n-----In the end-----')
    print('The balance: {}'.format(balance))

if __name__ == '__main__':
    main()


def withdraw(list_amount):
    global balance
    for amount in list_amount:
        if balance >= amount:
            balance -= amount

2.3 测试结果