Python中不尽如人意的断言Assertion

brwi0519 5年前
   <h3><strong>Python Assert 为何不尽如人意</strong></h3>    <p>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。</p>    <pre>  <code class="language-python">>>> assert 1 + 1 == 2  >>> assert isinstance('Hello', str)  >>> assert isinstance('Hello', int)    Traceback (most recent call last):    File "<input>", line 1, in <module>  AssertionError</code></pre>    <p>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。</p>    <h3><strong>改进方案 #1</strong></h3>    <p>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。</p>    <pre>  <code class="language-python">>>> s = "nothin is impossible."  >>> key = "nothing"  >>> assert key in s, "Key: '{}' is not in Target: '{}'".format(key, s)    Traceback (most recent call last):    File "<input>", line 1, in <module>  AssertionError: Key: 'nothing' is not in Target: 'nothin is impossible.'</code></pre>    <p>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。</p>    <h3><strong>改进方案 #2</strong></h3>    <p>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。</p>    <p><strong>py.test</strong></p>    <p>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 <strong>py.test</strong> 实现测试,你一行代码都不用改。</p>    <pre>  <code class="language-python">import pytest    def 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 ===========================  """"</code></pre>    <p><strong>unittest</strong></p>    <p>Python自带的 unittest 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。</p>    <pre>  <code class="language-python">import unittest    class 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'  """</code></pre>    <p><strong>ptest</strong></p>    <p>我非常喜欢 ptest ,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。</p>    <pre>  <code class="language-python">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 <bar> is not equal to str <foo>.  """</code></pre>    <h3><strong>改进方案 #3</strong></h3>    <p>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 assertpy 这个包,它异常强大而且好评如潮。</p>    <pre>  <code class="language-python">pip install assertpy</code></pre>    <p>看例子:</p>    <pre>  <code class="language-python">from assertpy import assert_that    def 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')</code></pre>    <p>从它的 github 主页 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。</p>    <ul>     <li>Strings</li>     <li>Numbers</li>     <li>Lists</li>     <li>Tuples</li>     <li>Dicts</li>     <li>Sets</li>     <li>Booleans</li>     <li>Dates</li>     <li>Files</li>     <li>Objects</li>    </ul>    <p>而且它的断言信息简洁明了,不多不少。</p>    <pre>  <code class="language-python">Expected <foo> to be of length <4>, but was <3>.  Expected <foo> to be empty string, but was not.  Expected <False>, but was not.  Expected <foo> to contain only digits, but did not.  Expected <123> to contain only alphabetic chars, but did not.  Expected <foo> to contain only uppercase chars, but did not.  Expected <FOO> to contain only lowercase chars, but did not.  Expected <foo> to be equal to <bar>, but was not.  Expected <foo> to be not equal to <foo>, but was.  Expected <foo> to be case-insensitive equal to <BAR>, but was not.</code></pre>    <p>在发现assertpy之前我也想写一个类似的包,尽可能通用一些。但是现在,我为毛要重新去造轮子?完全没必要!</p>    <h3><strong>总结</strong></h3>    <p>断言在软件系统中有非常重要的作用,写的好可以让你的系统更稳定,也可以让你有更多真正面对对象的时间,而不是在调试代码。</p>    <p>Python中默认的断言语句其实还有一个作用,如果你写了一个类型相关的断言,IDE会把这个对象当成这种类型,这时候智能提示就有如神助。</p>    <p>要不要把内置的断言语句换成可读性更好功能更强大的第三方断言,完全取决于实际情况。比如你真的需要验证某个东西并且很关心验证结果,那么必须不能用简单的assert;如果你只是担心某个点可能有坑或者让IDE认识某个对象,用内置的assert既简单又方便。</p>    <p> </p>    <p> </p>    <p>来自:http://www.cnblogs.com/cicaday/p/python-assert.html</p>    <p> </p>