Pytest之断言

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。

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。

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

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]