这篇文章主要给大家介绍了关于pytest中文文档之编写断言的相关资料，文中通过示例代码介绍的非常详细，对大家学习或者使用pytest具有一定的参考学习价值，需要的朋友们下面来一起学习学习吧
编写断言
使用assert编写断言
pytest允许你使用python标准的assert表达式写断言；
例如，你可以这样做：
# test_sample.py def func(x): return x + 1 def test_sample(): assert func(3) == 5
如果这个断言失败，你会看到func(3)实际的返回值：
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_sample.py ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:Personal FilesPythonpytest-chinese-docsrc, inifile: pytest.ini collected 1 item test_sample.py F [100%] ====================================================== FAILURES ======================================================= _____________________________________________________ test_sample _____________________________________________________ def test_sample(): > assert func(3) == 5 E assert 4 == 5 E + where 4 = func(3) test_sample.py:28: AssertionError ================================================== 1 Failed in 0.05s ==================================================
pytest支持显示常见的python子表达式的值，包括：调用、属性、比较、二进制和一元运算符等（参考pytest支持的python失败时报告的演示）;
这允许你在没有模版代码参考的情况下，可以使用的python的数据结构，而无须担心丢失自省的问题；
同时，你也可以为断言指定了一条说明信息，用于失败时的情况说明：
assert a % 2 == 0, "value was odd, should be even"
编写触发期望异常的断言
你可以使用pytest.raises()作为上下文管理器，来编写一个触发期望异常的断言：import pytest def myfunc(): raise ValueError("Exception 123 raised") def test_match(): with pytest.raises(ValueError): myfunc()当用例没有返回ValueError或者没有异常返回时，断言判断失败；如果你希望同时访问异常的属性，可以这样：import pytest def myfunc(): raise ValueError("Exception 123 raised") def test_match(): with pytest.raises(ValueError) as excinfo: myfunc() assert '123' in str(excinfo.value)其中，excinfo是ExceptionInfo的一个实例，它封装了异常的信息；常用的属性包括：.type、.value和.traceback；注意：在上下文管理器的作用域中，raises代码必须是最后一行，否则，其后面的代码将不会执行；所以，如果上述例子改成：def test_match(): with pytest.raises(ValueError) as excinfo: myfunc() assert '456' in str(excinfo.value)则测试将永远成功，因为assert '456' in str(excinfo.value)并不会执行；你也可以给pytest.raises()传递一个关键字参数match，来测试异常的字符串表示str(excinfo.value)是否符合给定的正则表达式（和unittest中的TestCase.assertRaisesRegexp方法类似）：import pytest def myfunc(): raise ValueError("Exception 123 raised") def test_match(): with pytest.raises((ValueError, RuntimeError), match=r'.* 123 .*'): myfunc()pytest实际调用的是re.search()方法来做上述检查；并且，pytest.raises()也支持检查多个期望异常（以元组的形式传递参数），我们只需要触发其中任意一个；pytest.raises还有另外的一种使用形式：首先，我们来看一下它在源码中的定义：# _pytest/python_api.py def raises( # noqa: F811 expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]], *args: Any, match: Optional[Union[str, "Pattern"]] = None, **kwargs: Any ) -> Union["RaisesContext[_E]", Optional[_pytest._code.ExceptionInfo[_E]]]:它接收一个位置参数expected_exception，一组可变参数args，一个关键字参数match和一组关键字参数kwargs；接着往下看：# _pytest/python_api.py if not args: if kwargs: msg = "Unexpected keyword arguments passed to pytest.raises: " msg += ", ".join(sorted(kwargs)) msg += "nUse context-manager form instead?" raise TypeError(msg) return RaisesContext(expected_exception, message, match) else: func = args[0] if not callable(func): raise TypeError( "{!r} object (type: {}) must be callable".format(func, type(func)) ) try: func(*args[1:], **kwargs) except expected_exception as e: # We just caught the exception - there is a traceback. assert e.__traceback__ is not None return _pytest._code.ExceptionInfo.from_exc_info( (type(e), e, e.__traceback__) ) fail(message)其中，args如果存在，那么它的第一个参数必须是一个可调用的对象，否则会报TypeError异常；同时，它会把剩余的args参数和所有kwargs参数传递给这个可调用对象，然后检查这个对象执行之后是否触发指定异常；所以我们有了一种新的写法：pytest.raises(ZeroDivisionError, lambda x: 1/x, 0) # 或者 pytest.raises(ZeroDivisionError, lambda x: 1/x, x=0)这个时候如果你再传递match参数，是不生效的，因为它只有在if not args:的时候生效；另外，pytest.mark.xfail()也可以接收一个raises参数，来判断用例是否因为一个具体的异常而导致失败：@pytest.mark.xfail(raises=IndexError) def test_f(): f()如果f()触发一个IndexError异常，则用例标记为xFailed；如果没有，则正常执行f()；注意：如果f()测试成功，用例的结果是xpassed，而不是passed；pytest.raises适用于检查由代码故意引发的异常；而@pytest.mark.xfail()更适合用于记录一些未修复的Bug；特殊数据结构比较时的优化# test_special_compare.py def test_set_comparison(): set1 = set('1308') set2 = set('8035') assert set1 == set2 def test_long_str_comparison(): str1 = 'show me codes' str2 = 'show me money' assert str1 == str2 def test_dict_comparison(): dict1 = { 'x': 1, 'y': 2, } dict2 = { 'x': 1, 'y': 1, } assert dict1 == dict2上面，我们检查了三种数据结构的比较：集合、字符串和字典；/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_special_compare.py ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:Personal FilesPythonpytest-chinese-docsrc, inifile: pytest.ini collected 3 items test_special_compare.py FFF [100%] ====================================================== FAILURES ======================================================= _________________________________________________ test_set_comparison _________________________________________________ def test_set_comparison(): set1 = set('1308') set2 = set('8035') > assert set1 == set2 E AssertionError: assert {'0', '1', '3', '8'} == {'0', '3', '5', '8'} E Extra items in the left set: E '1' E Extra items in the right set: E '5' E Use -v to get the full diff test_special_compare.py:26: AssertionError ______________________________________________ test_long_str_comparison _______________________________________________ def test_long_str_comparison(): str1 = 'show me codes' str2 = 'show me money' > assert str1 == str2 E AssertionError: assert 'show me codes' == 'show me money' E - show me codes E ? ^ ^ ^ E + show me money E ? ^ ^ ^ test_special_compare.py:32: AssertionError ________________________________________________ test_dict_comparison _________________________________________________ def test_dict_comparison(): dict1 = { 'x': 1, 'y': 2, } dict2 = { 'x': 1, 'y': 1, } > assert dict1 == dict2 E AssertionError: assert {'x': 1, 'y': 2} == {'x': 1, 'y': 1} E Omitting 1 identical items, use -vv to show E Differing items: E {'y': 2} != {'y': 1} E Use -v to get the full diff test_special_compare.py:44: AssertionError ================================================== 3 Failed in 0.09s ==================================================针对一些特殊的数据结构间的比较，pytest对结果的显示做了一些优化：集合、列表等：标记出第一个不同的元素；字符串：标记出不同的部分；字典：标记出不同的条目；更多例子参考pytest支持的python失败时报告的演示为失败断言添加自定义的说明# test_foo_compare.py class Foo: def __init__(self, val): self.val = val def __eq__(self, other): return self.val == other.val def test_foo_compare(): f1 = Foo(1) f2 = Foo(2) assert f1 == f2我们定义了一个Foo对象，也复写了它的__eq__()方法，但当我们执行这个用例时：/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_foo_compare.py ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:Personal FilesPythonpytest-chinese-docsrc, inifile: pytest.ini collected 1 item test_foo_compare.py F [100%] ====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________ def test_foo_compare(): f1 = Foo(1) f2 = Foo(2) > assert f1 == f2 E assert == test_foo_compare.py:37: AssertionError ================================================== 1 Failed in 0.04s ==================================================并不能直观的看出来失败的原因；在这种情况下，我们有两种方法来解决：复写Foo的__repr__()方法：def __repr__(self): return str(self.val)我们再执行用例：luyao@NJ-LUYAO-T460 /d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_foo_compare.py ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:Personal FilesPythonpytest-chinese-docsrc, inifile: pytest.ini collected 1 item test_foo_compare.py F [100%] ====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________ def test_foo_compare(): f1 = Foo(1) f2 = Foo(2) > assert f1 == f2 E assert 1 == 2 test_foo_compare.py:37: AssertionError ================================================== 1 Failed in 0.06s ==================================================这时，我们能看到失败的原因是因为1 == 2不成立；至于__str__()和__repr__()的区别，可以参考StackFlow上的这个问题中的回答：https://stackoverflow.com/questions/1436703/difference-between-str-and-repr使用pytest_assertrepr_compare这个钩子方法添加自定义的失败说明# conftest.py from .test_foo_compare import Foo def pytest_assertrepr_compare(op, left, right): if isinstance(left, Foo) and isinstance(right, Foo) and op == "==": return [ "比较两个Foo实例:", # 顶头写概要 " 值: {} != {}".format(left.val, right.val), # 除了第一个行，其余都可以缩进 ]再次执行：/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_foo_compare.py ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:Personal FilesPythonpytest-chinese-docsrc, inifile: pytest.ini collected 1 item test_foo_compare.py F [100%] ====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________ def test_foo_compare(): f1 = Foo(1) f2 = Foo(2) > assert f1 == f2 E assert 比较两个Foo实例: E 值: 1 != 2 test_foo_compare.py:37: AssertionError ================================================== 1 Failed in 0.05s ==================================================我们会看到一个更友好的失败说明；关于断言自省的细节当断言失败时，pytest为我们提供了非常人性化的失败说明，中间往往夹杂着相应变量的自省信息，这个我们称为断言的自省；那么，pytest是如何做到这样的：pytest发现测试模块，并引入他们，与此同时，pytest会复写断言语句，添加自省信息；但是，不是测试模块的断言语句并不会被复写；复写缓存文件pytest会把被复写的模块存储到本地作为缓存使用，你可以通过在测试用例的根文件夹中的conftest.py里添加如下配置：import sys sys.dont_write_bytecode = True来禁止这种行为；但是，它并不会妨碍你享受断言自省的好处，只是不会在本地存储.pyc文件了。去使能断言自省你可以通过一下两种方法：
在需要去使能模块的docstring中添加PYTEST_DONT_REWRITE字符串；
执行pytest时，添加--assert=plain选项；
我们来看一下去使能后的效果：
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_foo_compare.py --assert=plain ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:Personal FilesPythonpytest-chinese-docsrc, inifile: pytest.ini collected 1 item test_foo_compare.py F [100%] ====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________ def test_foo_compare(): f1 = Foo(1) f2 = Foo(2) > assert f1 == f2 E AssertionError test_foo_compare.py:37: AssertionError ================================================== 1 Failed in 0.05s ==================================================
断言失败时的信息就非常的不完整了，我们几乎看不出任何有用的Debug信息；
总结
以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，谢谢大家对编程之家的支持。
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