如何使用 doctest 测试嵌套/重新引发的异常？

Question

The following toy script illustrates the problem:

#!/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()

If I execute bomb() in the python interpreter, I get the output specified by the docstring:

% 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
>>>

doctest , however, incorrectly reports a failure:

**********************************************************************
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

Answer 1

The problem is how the doctest internally works. Basically it loads, compiles and evals the snippets of python inside the same python interpreter.

This is different in how python loads and executes a given piece of code.

This is the snippet of doctest that does the magic:

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

When the executed code raise and exception, the traceback contains part of the stack frame of the doctest engine making it different from what you expect.

From the doctest documentation :

The traceback header is followed by an optional traceback stack, whose contents are ignored by doctest. The traceback stack is typically omitted, or copied verbatim from an interactive session.

Now, the last part of that seems that applies only for single exceptions ; "multi or nested exceptions" don't work like that way and you may not be able to check their tracebacks. See this thread .

If you still want to check this, you could use another "doctest engine" like byexample . It has a compatibility mode with doctest so you should not need to rewrite everything.

Disclaimer: I'm the author of byexample . An as I explained in this thread I'm a really big fan of doctests but it has its limitations. I hope that byexample could fill the gap and be useful for other people.