Pytest series - detailed use of assert assertion

foreword

• Unlike unittest, pytest uses the assert keyword that comes with python to assert
• The assert keyword can be followed by an expression, as long as the final result of the expression is True, then the assertion passes and the use case executes successfully, otherwise the use case execution fails

assert little chestnut

I want to output some prompt information after an exception is thrown, and it is convenient to check the reason after execution

# 异常信息
def f():
    return 3
def test_function():
    a = f()
    assert a % 2 == 0, "判断 a 为偶数，当前 a 的值为：%s" % a

Results of the

common assertion

The assertion in pytest is actually the assert assertion method in python. The commonly used methods are as follows

• assert xx: judge that xx is true
• assert not xx: judge that xx is not true
• assert a in b : judge that b contains a
• assert a == b : judge that a is equal to b
• assert a != b : judge that a is not equal to b

exception assertion

You can use pytest.raises as a context manager, and you can get the corresponding exception instance when an exception is thrown

# 断言异常
def test_zero_division():
    with pytest.raises(ZeroDivisionError):
        1 / 0

Assertion scene: assert whether the exception it throws is expected

Code execution: 1/0

Expected result: The exception thrown is ZeroDivisionError: division by zero

How to assert: usually the type and value of the assertion exception

Specific method: Here, the exception type of 1/0 is ZeroDivisionError, and the value of the exception is divisionby zero

# 详细断言异常
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)

excinfo : is an instance of exception information

Main attributes:  .type , .value , .traceback 

Note: When asserting the type, the exception type does not need to be quoted, and when asserting the value value, it needs to be converted to str

 

Expansion 1: match

The match keyword argument can be passed to the context manager to test whether a regular expression matches the string representation of the exception

Note: This method can only assert value, not type

# 自定义消息
def test_zero_division_long():
    with pytest.raises(ZeroDivisionError, match=".*zero.*") as excinfo:
        1 / 0

 The regexp parameter of the match method matches the re.search function, so match='zero' in the example above would also work

 Extension 2: Check the assertion decorator

# 断言装饰器
@pytest.mark.xfail(raises=ZeroDivisionError)
def test_f():
    1 / 0

 Results of the

knowledge points

• The code throws an exception, but it matches the exception class specified by raises, so there is no assertion failure
• It is equivalent to a check exception decorator , function: check whether there is an exception, not sure whether there is an exception
• With pytest.raise(ZeroDivisionError) it may be better to use with pytest.raise(ZeroDivisionError) for cases where exception code is intentionally tested
• 而@pytest.mark.xfail(raises=ZeroDivisionError) For checking for unfixed bugs (i.e., exceptions may occur), it may be better to use a checked assertion