3、pytest中文文档--编写断言

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,excinfoExceptionInfo的一个实例,它封装了异常的信息;常用的属性包括:.type.value.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__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__()__repr__()的区别,可以参考StackFlow上的这个问题中的回答:http://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;

posted @ 2019-09-11 15:44 luizyao 阅读(...) 评论(...) 编辑 收藏