如何使用 doctest 测试嵌套/重新引发的异常？答案

【问题标题】：How to test nested/re-raised exceptions with doctest?如何使用 doctest 测试嵌套/重新引发的异常？
【发布时间】：2018-11-11 23:42:35
【问题描述】：

下面的玩具脚本说明了这个问题：

#!/usr/bin/env python3

def bomb():
    """
    >>> bomb()
    Traceback (most recent call last):
      File "<string>", line 18, in bomb
    ZeroDivisionError: division by zero
    <BLANKLINE>
    During handling of the above exception, another exception occurred:
    <BLANKLINE>
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 20, in bomb
    Exception: re-raised
    """
    try:
        1/0
    except Exception as exception:
        raise Exception('re-raised')

if __name__ == '__main__' and '__file__' in globals():
    import sys
    if len(sys.argv) > 1 and sys.argv[1] == '-t':
        import doctest
        doctest.testmod()
    else:
        bomb()

如果我在 python 解释器中执行bomb()，我会得到文档字符串指定的输出：

% python3
Python 3.5.1 (default, May 24 2016, 20:04:39)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> exec(open('demo.py').read())
>>> bomb()
Traceback (most recent call last):
  File "<string>", line 18, in bomb
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 20, in bomb
Exception: re-raised
>>>

但是，doctest 错误地报告失败：

**********************************************************************
File "./demo.py", line 5, in __main__.bomb
Failed example:
    bomb()
Expected:
    Traceback (most recent call last):
      File "<string>", line 16, in bomb
    ZeroDivisionError: division by zero
    <BLANKLINE>
    During handling of the above exception, another exception occurred:
    <BLANKLINE>
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<string>", line 18, in bomb
    Exception: re-raised
Got:
    Traceback (most recent call last):
      File "./demo.py", line 18, in bomb
        1/0
    ZeroDivisionError: division by zero
    <BLANKLINE>
    During handling of the above exception, another exception occurred:
    <BLANKLINE>
    Traceback (most recent call last):
      File "/usr/lib/python3.5/doctest.py", line 1320, in __run
        compileflags, 1), test.globs)
      File "<doctest __main__.bomb[0]>", line 1, in <module>
        bomb()
      File "./demo.py", line 20, in bomb

【问题讨论】：

标签： python python-3.x exception doctest

【解决方案1】：

问题在于 doctest 在内部是如何工作的。基本上它在同一个 python 解释器中加载、编译和评估 python 的 sn-ps。

这与 python 加载和执行给定代码的方式不同。

这是发挥神奇作用的 doctest 的 sn-p：

# Don't blink!  This is where the user's code gets run.
exec(compile(example.source, filename, "single",
     compileflags, 1), test.globs)

当执行的代码引发和异常时，回溯包含 doctest 引擎的部分堆栈帧，使其与您的预期不同。

来自doctest documentation：

回溯标头后跟一个可选的回溯堆栈，其 doctest 忽略内容。回溯堆栈通常是从交互式会话中省略或逐字复制。

现在，最后一部分似乎仅适用于单个例外； “多重或嵌套异常”不是这样工作的，您可能无法检查它们的回溯。看到这个thread。

如果你仍然想检查这个，你可以使用另一个“doctest 引擎”，比如byexample。它有一个compatibility mode with doctest，所以你不需要重写所有东西。

免责声明：我是byexample 的作者。正如我在thread 中解释的那样，我是 doctests 的忠实粉丝，但它有其局限性。我希望byexample 可以填补空白并对其他人有用。

【讨论】：