python中那个断言assert的优化-白红宇

python中那个断言assert的优化

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发布时间：2019-06-12

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Python Assert 为何不尽如人意

Python中的断言用起来非常简单，你可以在assert后面跟上任意判断条件，如果断言失败则会抛出异常。

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>>> assert 1 + 1 == 2 >>> assert isinstance('Hello', str) >>> assert isinstance('Hello', int) Traceback (most recent call last): File "", line 1, in 
    
      AssertionError

其实assert看上去不错，然而用起来并不爽。就比如有人告诉你程序错了，但是不告诉哪里错了。很多时候这样的assert还不如不写，写了我就想骂娘。直接抛一个异常来得更痛快一些。

改进方案 #1

一个稍微改进一丢丢的方案就是把必要的信息也放到assert语句后面，比如这样。

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>>> s = "nothin is impossible.">>> key = "nothing" >>> assert key in s, "Key: '{}' is not in Target: '{}'".format(key, s) Traceback (most recent call last): File "", line 1, in 
    
      AssertionError: Key: 'nothing' is not in Target: 'nothin is impossible.'

看上去还行吧，但是其实写的很蛋疼。假如你是一名测试汪，有成千上万的测试案例需要做断言做验证，相信你面对以上做法，心中一定有千万只那种马奔腾而过。

改进方案 #2

不管你是你是搞测试还是开发的，想必听过不少测试框架。你猜到我要说什么了吧？对，不用测试框架里的断言机制，你是不是洒。

py.test

是一个轻量级的测试框架，所以它压根就没写自己的断言系统，但是它对Python自带的断言做了强化处理，如果断言失败，那么框架本身会尽可能多地提供断言失败的原因。那么也就意味着，用py.test实现测试，你一行代码都不用改。

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import pytestdef test_case(): expected = "Hello" actual = "hello" assert expected == actual if __name__ == '__main__': pytest.main() """ ================================== FAILURES =================================== __________________________________ test_case __________________________________ def test_case(): expected = "Hello" actual = "hello" > assert expected == actual E assert 'Hello' == 'hello' E - Hello E ? ^ E + hello E ? ^ assertion_in_python.py:7: AssertionError ========================== 1 failed in 0.05 seconds =========================== """"

unittest

Python自带的单元测试框架就有了自己的断言方法self.assertXXX()，而且不推荐使用assert XXX语句。

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import unittestclass TestStringMethods(unittest.TestCase): def test_upper(self): self.assertEqual('foo'.upper(), 'FoO') if __name__ == '__main__': unittest.main() """ Failure Expected :'FOO' Actual :'FoO' Traceback (most recent call last): File "assertion_in_python.py", line 6, in test_upper self.assertEqual('foo'.upper(), 'FoO') AssertionError: 'FOO' != 'FoO' """

ptest

我非常喜欢，感谢Karl大神写了这么一个测试框架。ptest中的断言可读性很好，而且通过IDE的智能提示你能轻松完成各种断言语句。

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from ptest.decorator import *from ptest.assertion import * @TestClass() class TestCases: @Test() def test1(self): actual = 'foo' expected = 'bar' assert_that(expected).is_equal_to(actual) """ Start to run following 1 tests: ------------------------------ ... [demo.assertion_in_python.TestCases.test1@Test] Failed with following message: ... AssertionError: Unexpectedly that the str 
    
      is not equal to str 
     
      . """

改进方案 #3

不仅仅是你和我对Python中的断言表示不满足，所以大家都争相发明自己的assert包。在这里我强烈推荐这个包，它异常强大而且好评如潮。

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pip install assertpy

看例子:

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from assertpy import assert_thatdef test_something(): assert_that(1 + 2).is_equal_to(3) assert_that('foobar')\ .is_length(6)\ .starts_with('foo')\ .ends_with('bar') assert_that(['a', 'b', 'c'])\ .contains('a')\ .does_not_contain('x')

从它的文档上你会发现它支持了几乎你能想到的所有测试场景，包括但不限于以下列表。

Strings

Numbers

Lists

Tuples

Dicts

Sets

Booleans

Dates

Files

Objects

而且它的断言信息简洁明了，不多不少。

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Expected 
    
      to be of length <4>, but was <3>.Expected 
     
       to be empty string, but was not. Expected 
      
       , but was not. Expected 
       
         to contain only digits, but did not. Expected <123> to contain only alphabetic chars, but did not. Expected 
        
          to contain only uppercase chars, but did not. Expected 
         
           to contain only lowercase chars, but did not. Expected 
          
            to be equal to 
           
            , but was not. Expected 
            
              to be not equal to 
             
              , but was. Expected 
              
                to be case-insensitive equal to 
               
                , but was not.