3、pytest中文文档--编写断言
目录
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);
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
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:https://*.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;