Pytest：多个assert语句在同一测试函数中失败，如何知道是哪些assert语句导致失败？

Question

测试文件：

from RansomNote import Solution


def test_canConstruct():
    sol = Solution()
    assert sol.canConstruct('aa', 'aab') == True
    assert sol.canConstruct('xx','aaabc') == True
    assert sol.canConstruct('aab','ababa') == True

运行pytest -v后的输出：

==================================================================== short test summary info =====================================================================
FAILED Easy/test_ransomNote.py::test_canConstruct - AssertionError: assert False == True
======================================================================= 1 failed in 0.01s

问题： 首先，在同一个测试函数里有很多assert语句可以吗？还是不推荐这样做？ 输出结果只显示了“断言错误”，那怎么知道到底是哪个assert语句导致了失败呢？ 是不是每个assert语句都应该单独写一个新的测试函数比较好？

我尝试了pytest -v，想知道是哪个assert语句导致了失败。 输出结果： 失败了 Easy/test_ransomNote.py::test_canConstruct - AssertionError: assert False == True

Answer 1

Pytest会打印出紧跟在assert后面的字符串，所以你可以这样做：

from RansomNote import Solution


def test_canConstruct():
    sol = Solution()
    assert sol.canConstruct('aa', 'aab') == True, 'first one failed'
    assert sol.canConstruct('xx','aaabc') == True, 'second one failed'
    assert sol.canConstruct('aab','ababa') == True, 'third one failed'

Answer 2

你的输出结果中缺少了一些东西。

如果我用这段代码来重现你的问题：

def working_function():
    return True


def failing_function():
    return False


def test_some_function():
    assert working_function()
    assert failing_function()

运行 pytest -v 可以清楚地看到哪个断言语句失败了：

================================================================= FAILURES ==================================================================
____________________________________________________________ test_some_function _____________________________________________________________

    def test_some_function():
        assert working_function()
>       assert failing_function()
E       assert False
E        +  where False = failing_function()

test_assert.py:11: AssertionError
========================================================== short test summary info ==========================================================
FAILED test_assert.py::test_some_function - assert False
============================================================= 1 failed in 0.01s =============================================================

---

另外，这一点也要注意：

assert working_function()

这和下面的内容是完全一样的：

assert working_function() == True

也就是说，在你的 assert 语句中，没有必要加上 == True。

Answer 3

你可以使用参数

@pytest.mark.parametrize("x, y", (("aa", "aab"), ("xx", "aaabc"), ("aab", "ababa")))
def test_canConstruct(x, y):
    sol = Solution()
    assert sol.canConstruct(x, y) is True

然后单独获取结果

x.py::test_canConstruct[aa-aab] PASSED                 [ 33%]
x.py::test_canConstruct[xx-aaabc] PASSED               [ 66%]
x.py::test_canConstruct[aab-ababa] PASSED              [100%]