目录 编写断言 使用assert编写断言 编写触发期望异常的断言 特殊数据结构比较时的优化 为失败断言添加自定义的说明 关于断言自省的细节 复写缓存文件 去使能断言自省 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 Files\Python\pytest-chinese-doc\src, 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 Files\Python\pytest-chinese-doc\src, 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 Files\Python\pytest-chinese-doc\src, 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 <src.test_foo_compare.Foo object at 0x0000020E90C4E978> == <src.test_foo_compare.Foo object at 0x0000020E90C4E630> 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 Files\Python\pytest-chinese-doc\src, 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://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 Files\Python\pytest-chinese-doc\src, 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 Files\Python\pytest-chinese-doc\src, 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;
转载自://www.cnblogs.com/luizyao/p/11506664.html