Python unittest 成功断言 None 为 False [英] Python unittest successfully asserts None is False

问题描述

为什么 assertFalse 在 None 上成功?

导入单元测试类 TestNoneIsFalse(unittest.TestCase):def test_none_is_false(self):self.assertFalse(无)

结果:

<代码>>python -m unittest temp.----------------------------------------------------------------------在 0.001 秒内运行 1 次测试好的

似乎这种行为会导致函数不总是返回值的错误.例如:

def is_lower_than_5(x):如果 x <5:返回真elif x >5:返回错误....def test_5_is_not_lower_than_5(self):self.assertFalse(is_lower_than_5(5))

即使上面的测试应该失败，它也会通过.它在代码中遗漏了一个应该被捕获的错误.

我们应该如何断言该值是字面上的 False 而不仅仅是布尔上下文中的 false?例如

self.assertEquals(False, None) # 断言失败.好的！

解决方案

None 是假的，还有 0, "",[], ...

assertFalse 不会通过身份检查给定值是否为 False.此行为与 if 语句一致:

 如果不是 None:打印('假值！')

类似地，assertTrue 不会检查一个值是否为 True，而是像 1、"abc"<这样的值/code>, [1, 2, 3] 测试通过.有关详细信息，请参阅真值测试.

此行为也明确记录:

<块引用>

assertTrue(expr, msg=None)assertFalse(expr, msg=None)

测试 expr 为真(或假).

注意这等价于 bool(expr) is True 而不是 expr is True

如果您真的想确定某个值是 True 还是 False，请使用 assertIs.

Why does assertFalse succeed on None?

import unittest

class TestNoneIsFalse(unittest.TestCase):
    def test_none_is_false(self):
        self.assertFalse(None)

Results:

> python -m unittest temp
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK

It seems as if this behaviour invites errors where a function does not always return a value. For example:

def is_lower_than_5(x):
    if x < 5:
        return True
    elif x > 5:
        return False

....

def test_5_is_not_lower_than_5(self):
   self.assertFalse(is_lower_than_5(5))

The above test would pass even though it should fail. It is missing an error in the code that should be caught.

How should we assert that the value is literally False and not merely false in a boolean context? e.g.

self.assertEquals(False, None)  # assert fails. good!

解决方案

None is falsy, as well as 0, "", [], ...

assertFalse does not check whether the given value is False by identity. This behavior is consistent with the if statement:

if not None:
    print('falsy value!')

Similarly, assertTrue does not check whether a value is True, and as such values like 1, "abc", [1, 2, 3] pass the test. See Truth Value Testing for more information.

This behavior is also explicitly documented:

assertTrue(expr, msg=None)
assertFalse(expr, msg=None)

Test that expr is true (or false).

Note that this is equivalent to bool(expr) is True and not to expr is True

If you really want to be sure that a value is True or False, use assertIs.

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