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3、pytest中文文档--编写断言

dazhizhi 2019-09-11 18:09 67 查看 https://www.cnblogs.com/luizya

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\Pyth
aec
on\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
ExceptionInfo
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         Ex
aec
tra 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
ae4
-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: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)
1062
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;

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