代码通过手动测试，但在单元测试中失败，可能吗？ [关闭]

Question

我已经为自学在线 Python 课程编程了 3 周。目标是扩展一个基本的 BankAccount 类，您可以通过子类 MinimumBalanceAccount 尽可能多地取款，其中过多的取款被拒绝。这是我目前所拥有的：

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

    def deposit(self, amount):
        self.balance += amount
        return self.balance

    def withdraw(self, amount):
        if(amount > self.balance):
            print "invalid transaction."
        else:
            self.balance -= amount
            return self.balance   

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

    def withdraw(self, amount):
        if(self.balance - amount < self.minimum_balance):
            print "Minimum balance exceeded."
        else:
            self.balance -= amount
            return self.balance

由于在线课程具有在线单元测试功能，我从练习定义中编写了以下单元测试：

import unittest

class AccountBalanceTestCases(unittest.TestCase):
    def setUp(self):
       self.my_account = BankAccount(90)
    # omitting tests that pass ok...
    def test_invalid_operation(self):
       self.assertEqual(self.my_account.withdraw(1000), "invalid transaction", msg='Invalid transaction')

unittest.main(verbosity=2)

测试有望通过，但是：

$ python bank2.py
test_invalid_operation (__main__.AccountBalanceTestCases) ... invalid transaction.
FAIL

======================================================================
FAIL: test_invalid_operation (__main__.AccountBalanceTestCases)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "bank2.py", line 36, in test_invalid_operation
    self.assertEqual(self.my_account.withdraw(1000), "invalid transaction", msg='Invalid transaction')
AssertionError: Invalid transaction

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)
$ 

我做错了什么？

Answer 1

单元测试返回：

..invalid transaction.
F..
======================================================================
FAIL: test_invalid_operation (__main__.AccountBalanceTestCases)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "bank.py", line 48, in test_invalid_operation
    self.assertEqual(self.my_account.withdraw(1000), "invalid transaction", msg='Invalid transaction')
AssertionError: Invalid transaction

----------------------------------------------------------------------
Ran 5 tests in 0.001s

FAILED (failures=1)
$ 

第一句话是马上进入单元测试的拥抱。但是，unittest.Testcase 不应该 print 任何东西，因为这会将垃圾推入 unittest 输出中。

在这种情况下，我们从 self.my_account.withdraw(1000) 不返回 "invalid transaction" 的异常中读取。确实：

def withdraw(self, amount):
    if(self.balance - amount < self.minimum_balance):
        print "Minimum balance exceeded."
    else:
        self.balance -= amount
        return self.balance

withdraw 在特殊情况下不返回任何内容。所以你的测试代码没问题，它已经很好地为你服务了:)

要通过测试，withdraw 方法必须返回字符串 "invalid transaction"。这里的继承没有错，但是当你覆盖一个方法时，你并没有从那个方法的超类实现继承（除非你显式地调用它）。

def withdraw(self, amount):
    if(self.balance - amount < self.minimum_balance):
        print "Minimum balance exceeded."
        return "invalid transaction"
    else:
        self.balance -= amount
        return self.balance

关于提问的一般评论：当代码正确缩进时以及当我们立即遇到异常时，这将有很大帮助;)

更新更深入地探讨继承问题：当您希望 MinimumBalanceAccount 成为真正的 BankAccount 时，您不仅要关心 minimum_balance，还要关心开头balance 因为这是超类需要的部分：

class MinimumBalanceAccount(BankAccount):
    def __init__(self, balance, minimum_balance):
        BankAccount.__init__(self, balance)
        self.minimum_balance = minimum_balance
    def withdraw(self, amount):
        ...

当然，我做了这个测试驱动：

class MBATest(unittest.TestCase):
  def setUp(self):
    self.my_mba = MinimumBalanceAccount(90, 10)
  def test_mba_withdraw(self):
    # put in one test to be sure about the oder of execution ;)
    self.assertEqual(self.my_mba.withdraw(10), 80)
    self.assertEqual(self.my_mba.withdraw(100), "invalid transaction")

这个会通过的。因为我喜欢冗长，所以我使用unittest.main(verbosity=2) 并最终得到：

$ python bank.py
test_balance (__main__.AccountBalanceTestCases) ... ok
test_deposit (__main__.AccountBalanceTestCases) ... ok
test_invalid_operation (__main__.AccountBalanceTestCases) ... ok
test_sub_class (__main__.AccountBalanceTestCases) ... ok
test_withdraw (__main__.AccountBalanceTestCases) ... ok
test_mba_withdraw (__main__.MBATest) ... ok
----------------------------------------------------------------------
Ran 6 tests in 0.000s
OK
$

也许你应该通过this question 工作，如果你喜欢它，也可以通过this one...