doctest mishandles exceptions raised within generators #42523

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1. twegener commented on Oct 26, 2005

   

   twegenermannequin

   on Oct 26, 2005

   MannequinAuthor

   More actions

   If a generator raises an exception while iterating over it, doctest
   will only register the exception output, and will miss output that
   occurred during previous loop iterations.

   The following code clarifies and reproduces the problem:
   (also included as an attachment)

   """Reproduce bug with exceptions in a generator in doctest tests.

   This bug has been seen to occur in:

   Linux (RH9):
   Python 2.4.1
   Python 2.3.5
   Python 2.2.2 (using from __future__ import generators)

   Windows XP:
   Python 2.4.2
   Python 2.3.5

   """

   def error_generator():
       """Yield 0 to 2 and then try and raise an exception.

       >>> for j in error_generator():
       ...    print j
       0
       1
       2
       Traceback (most recent call last):
       Exception: Contrived exception for sake of example

   """
   # Note: This is obviously a contrived example of generator use.
   for i in range(3):
       yield i
   

       if 1:
           raise Exception("Contrived exception for sake of example")
   
       raise StopIteration
   
   
   if __name__ == '__main__':
       # Run the doctest tests.
       import sys
       import doctest
       doctest.testmod(sys.modules['__main__'])

   print
   print 'What should have happened...'
   for j in error_generator():
       print j
   
2. twegener

   closed this as completedon Oct 26, 2005
3. twegener

   closed this as completedon Oct 26, 2005
4. twegener

   added

   docsDocumentation in the Doc dirDocumentation in the Doc dir

   on Oct 26, 2005
5. tim-one commented on Apr 24, 2006

   

   tim-one

   on Apr 24, 2006

   Member

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   user_id=31435

   doctest doesn't support mixing "expected normal output" with
   an exception, regardless of how such a thing may arise. For
   example, this one-liner can't work either as a doctest:

   >>> print 'hi', 1/0
   hi
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ZeroDivisionError: integer division or modulo by zero

   So there's nothing specific to generators in this, rather
   it's a doctest limitation. The docs do say, wrt testing
   exceptions:

   """
   No problem, provided that the traceback is the only output
   produced by the example: just paste in the traceback.
   """

   In any case, it wasn't intended that doctest support this,
   and the docs do try to communicate that. I added text
   spelling out the other half (if there's expected output in
   addition to the traceback, problem ;-)), in rev 45677 on the
   trunk and rev 45678 on the 2.4 branch, and am closing this
   report as a doc bug.

   Edward Loper tried supporting this, as a new feature, before
   2.4 was released, but it so grossly complicated the docs and
   the implementation that we agreed to drop it. So you could
   re-open this as a feature request, but it's unlikely to change.
6. ezio-melotti

   transferred this issue fromon Apr 10, 2022