Python中不盡如人意的斷言Assertion

時間 2019-12-05

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Python Assert 爲什麼不盡如人意

Python中的斷言用起來很是簡單，你能夠在assert後面跟上任意判斷條件，若是斷言失敗則會拋出異常。html

>>> assert 1 + 1 == 2
>>> assert isinstance('Hello', str)
>>> assert isinstance('Hello', int)

Traceback (most recent call last):
  File "<input>", line 1, in <module>
AssertionError

其實assert看上去不錯，然而用起來並不爽。就好比有人告訴你程序錯了，可是不告訴哪裏錯了。不少時候這樣的assert還不如不寫，寫了我就想罵娘。直接拋一個異常來得更痛快一些。python

改進方案 #1

一個稍微改進一丟丟的方案就是把必要的信息也放到assert語句後面，好比這樣。git

>>> 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.'

看上去還行吧，可是其實寫的很蛋疼。假如你是一名測試汪，有成千上萬的測試案例須要作斷言作驗證，相信你面對以上作法，心中必定有千萬只那種馬奔騰而過。github

改進方案 #2

無論你是你是搞測試仍是開發的，想必聽過很多測試框架。你猜到我要說什麼了吧？對，不用測試框架裏的斷言機制，你是否是灑。shell

py.test

py.test 是一個輕量級的測試框架，因此它壓根就沒寫本身的斷言系統，可是它對Python自帶的斷言作了強化處理，若是斷言失敗，那麼框架自己會盡量多地提供斷言失敗的緣由。那麼也就意味着，用py.test實現測試，你一行代碼都不用改。框架

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 ===========================
""""

unittest

Python自帶的unittest單元測試框架就有了本身的斷言方法self.assertXXX()，並且不推薦使用assert XXX語句。單元測試

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'
"""

ptest

我很是喜歡ptest，感謝Karl大神寫了這麼一個測試框架。ptest中的斷言可讀性很好，並且智能提示也很方便你經過IDE輕鬆完成各類斷言語句。測試

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>.
"""

改進方案 #3

不只僅是你和我對Python中的斷言表示不知足，因此你們都爭相發明本身的assert包。在這裏我強烈推薦assertpy 這個包，它異常強大並且好評如潮。spa

pip install assertpy

看例子:調試

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')

從它的github 主頁文檔上你會發現它支持了幾乎你能想到的全部測試場景，包括但不限於如下列表。

Strings
Numbers
Lists
Tuples
Dicts
Sets
Booleans
Dates
Files
Objects

並且它的斷言信息簡潔明瞭，很少很多。

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.

在發現assertpy以前我也想寫一個相似的包，儘量通用一些。可是如今，我爲毛要從新去造輪子？徹底不必！