Pytest之断言
在上一篇Pytest系列文章：Pytest之收集用例及命令行参数，主要介绍Pytest用例收集规则及常用的命令行参数。
在自动化测试过程中，需要判断预期结果和实际结果是否一致，这时需要使用到断言。
什么是断言呢？简单来说就是实际结果和期望结果去对比。
一 断言用法
在pytest中，使用assert进行断言，格式为：assert 表达式。
如果表达式返回结果为True，则断言成功，否则断言失败。
二 常用断言
unittest的三种断言:
assertIn（expect，result）断言包含（被包含的写前面）;
assertEqual（expect，result）断言相等;
assertTure（条件）断言是否为真。返回Ture或False;
Pytest里的断言实际上就是Python中的assert断言方法,常用断言方法如下：
- assert xx ：判断 xx 为真;
- assert not xx ：判断 xx 不为真;
- assert a in b ：判断 b 包含 a;
- assert a == b ：判断 a 等于 b;
- assert a != b ：判断 a 不等于 b;
import pytest
def test_demo1():
a = 1
assert a
def test_demo2():
a = 0
assert not a
def test_demo3():
s = 'hello'
assert 'h' in s
def test_demo4():
a = 3
assert a == 3
def test_demo5():
a = 4
assert a != 3
if __name__ == '__main__':
pytest.main()
运行结果如下：
Testing started at 18:22 ...
C:\Users\96984\Desktop\code\pytest\venv\Scripts\python.exe "C:\ruanjian\pycharm2019.3\PyCharm 2019.3.1\plugins\python\helpers\pycharm\_jb_pytest_runner.py" --path C:/Users/96984/Desktop/code/pytest/demo/demo_pytest.py
Launching pytest with arguments C:/Users/96984/Desktop/code/learn_pytest/demo/demo_pytest.py in C:\Users\96984\Desktop\code\learn_pytest\demo
============================= test session starts =============================
platform win32 -- Python 3.6.8, pytest-5.4.3, py-1.9.0, pluggy-0.13.1 -- C:\Users\96984\Desktop\code\learn_pytest\venv\Scripts\python.exe
cachedir: .pytest_cache
metadata: {'Python': '3.6.8', 'Platform': 'Windows-10-10.0.18362-SP0', 'Packages': {'pytest': '5.4.3', 'py': '1.9.0', 'pluggy': '0.13.1'}, 'Plugins': {'html': '2.1.1', 'metadata': '1.10.0'}, 'JAVA_HOME': 'C:\\Program Files\\Java\\jdk1.8.0_77'}
rootdir: C:\Users\96984\Desktop\code\learn_pytest\demo
plugins: html-2.1.1, metadata-1.10.0
collecting ... collected 5 items
demo_pytest.py::test_demo1 PASSED [ 20%]
demo_pytest.py::test_demo2 PASSED [ 40%]
demo_pytest.py::test_demo3 PASSED [ 60%]
demo_pytest.py::test_demo4 PASSED [ 80%]
demo_pytest.py::test_demo5 PASSED [100%]
============================== 5 passed in 0.06s ==============================
Process finished with exit code 0
三 异常断言
在测试过程中，有时需要对特定异常进行断言，可以使用 pytest.raises 作为上下文管理器，当抛出异常时可以获取到对应的异常实例。
import pytest
def test_zero_division():
1 / 0
if __name__ == '__main__':
pytest.main()
运行结果：
================================== FAILURES ===================================
_____________________________ test_zero_division ______________________________
def test_zero_division():
> 1 / 0
E ZeroDivisionError: division by zero
所以我们需要捕获并断言异常。
断言场景：断言抛出的异常是否符合预期。
预期结果：ZeroDivisionError: division by zero，其中ZeroDivisionError为错误类型，division by zero为具体错误值。
断言方式: 断言异常的type和value值。
断言代码如下：
import pytest
def test_zero_division_long():
with pytest.raises(ZeroDivisionError) as excinfo:
1 / 0
# 断言异常类型 type
assert excinfo.type == ZeroDivisionError
# 断言异常 value 值
assert "division by zero" in str(excinfo.value)
if __name__ == '__main__':
pytest.main()
代码详细说明：
① pytest.raises 捕获异常，源码如下：
def raises( # noqa: F811
expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
*args: Any,
**kwargs: Any
) -> Union["RaisesContext[_E]", _pytest._code.ExceptionInfo[_E]]:
__tracebackhide__ = True
for exc in filterfalse(
inspect.isclass, always_iterable(expected_exception, BASE_TYPE) # type: ignore[arg-type] # noqa: F821
):
msg = "exceptions must be derived from BaseException, not %s"
raise TypeError(msg % type(exc))
message = "DID NOT RAISE {}".format(expected_exception)
if not args:
match = kwargs.pop("match", None)
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)
raises.Exception = fail.Exception # type: ignore
② excinfo作为异常信息实例，拥有type 、value等属性，源码如下：
@property
def type(self) -> "Type[_E]":
"""the exception class"""
assert (
self._excinfo is not None
), ".type can only be used after the context manager exits"
return self._excinfo[0]
@property
def value(self) -> _E:
"""the exception value"""
assert (
self._excinfo is not None
), ".value can only be used after the context manager exits"
return self._excinfo[1]
③ excinfo.value的值是元组，所以要转成字符串。
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