Python 接口：从协议到抽象基 类（Tombola子类的测试方法）

Tombola子类的测试方法

我编写的 Tombola 示例测试脚本用到两个类属性，用它们内省类的继
承关系。

__subclasses__()

这个方法返回类的直接子类列表，不含虚拟子类。

_abc_registry

只有抽象基类有这个数据属性，其值是一个 WeakSet 对象，即抽
象类注册的虚拟子类的弱引用。

为了测试 Tombola 的所有子类，我编写的脚本迭代
Tombola.subclasses() 和 Tombola._abc_registry 得到的列
表，然后把各个类赋值给在 doctest 中使用的 ConcreteTombola。
这个测试脚本成功运行时输出的结果如下：

$ python3 tombola_runner.py
BingoCage 24 tests, 0 failed - OK
LotteryBlower 24 tests, 0 failed - OK
TumblingDrum 24 tests, 0 failed - OK
TomboList 24 tests, 0 failed - OK

测试脚本的代码在示例 11-15 中，doctest 在示例 11-16 中。
示例 11-15　tombola_runner.py：Tombola 子类的测试运行程序

import doctest
from tombola import Tombola
# 要测试的模块
import bingo, lotto, tombolist, drum ➊
TEST_FILE = 'tombola_tests.rst'
TEST_MSG = '{0:16} {1.attempted:2} tests, {1.failed:2} failed - {2}'
def main(argv):
verbose = '-v' in argv
real_subclasses = Tombola.__subclasses__() ➋
virtual_subclasses = list(Tombola._abc_registry) ➌
for cls in real_subclasses + virtual_subclasses: ➍
  test(cls, verbose)
  def test(cls, verbose=False):
    res = doctest.testfile(
    TEST_FILE,
    globs={'ConcreteTombola': cls}, ➎
    verbose=verbose,
    optionflags=doctest.REPORT_ONLY_FIRST_FAILURE)
    tag = 'FAIL' if res.failed else 'OK'
    print(TEST_MSG.format(cls.__name__, res, tag)) ➏
if __name__ == '__main__':
import sys
main(sys.argv)

❶ 导入包含 Tombola 真实子类和虚拟子类的模块，用于测试。
❷ __subclasses__() 返回的列表是内存中存在的直接子代。即便源
码中用不到想测试的模块，也要将其导入，因为要把那些类载入内存。
❸ 把 _abc_registry（WeakSet 对象）转换成列表，这样方能与
__subclasses__() 的结果拼接起来。
❹ 迭代找到的各个子类，分别传给 test 函数。
❺ 把 cls 参数（要测试的类）绑定到全局命名空间里的
ConcreteTombola 名称上，供 doctest 使用。
❻ 输出测试结果，包含类的名称、尝试运行的测试数量、失败的测试
数量，以及 ‘OK’ 或 ‘FAIL’ 标记。

doctest 文件如示例 11-16 所示。
示例 11-16　tombola_tests.rst：Tombola 子类的 doctest

Tombola tests
==============
Every concrete subclass of Tombola should pass these tests.
Create and load instance from iterable::
>>> balls = list(range(3))
>>> globe = ConcreteTombola(balls)
>>> globe.loaded()
True
>>> globe.inspect()
(0, 1, 2)
Pick and collect balls::
>>> picks = []
>>> picks.append(globe.pick())
>>> picks.append(globe.pick())
>>> picks.append(globe.pick())
Check state and results::
>>> globe.loaded()
False
>>> sorted(picks) == balls
True
Reload::
>>> globe.load(balls)
>>> globe.loaded()
True
>>> picks = [globe.pick() for i in balls]
>>> globe.loaded()
False
Check that `LookupError` (or a subclass) is the exception
thrown when the device is empty::
>>> globe = ConcreteTombola([])
>>> try:
... globe.pick()
... except LookupError as exc:
... print('OK')
OK
Load and pick 100 balls to verify that they all come out::
>>> balls = list(range(100))
>>> globe = ConcreteTombola(balls)
>>> picks = []
>>> while globe.inspect():
... picks.append(globe.pick())
>>> len(picks) == len(balls)
True
>>> set(picks) == set(balls)
True
Check that the order has changed and is not simply reversed::
>>> picks != balls
True
>>> picks[::-1] != balls
True
Note: the previous 2 tests have a *very* small chance of failing
even if the implementation is OK. The probability of the 100
balls coming out, by chance, in the order they were inspect is
1/100!, or approximately 1.07e-158. It's much easier to win the
Lotto or to become a billionaire working as a programmer.
THE END

我们对 Tombola 抽象基类的分析到此结束。下一节说明 Python 如何使
用抽象基类的 register 函数。