Exceptions and arguments disappear when using argparse inside with statement #68258

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1. invisibleroads commented on Apr 28, 2015

   

   invisibleroadsmannequin

   on Apr 28, 2015

   MannequinAuthor

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   Exceptions and arguments disappear when using argparse inside a "with" statement. The behavior was confusing and frustrating because I could not pinpoint why certain arguments were missing or unrecognized.

   Unhandled exceptions inside the with statement typically trigger a traceback, unless __exit__ returns True, in which the exception is "swallowed" (https://www.python.org/dev/peps/pep-0343/).

   However, the NameError exception and tile_indices argument disappear because of a premature sys.exit (https://hg.python.org/cpython/file/default/Lib/argparse.py#l2385).

   """
   $ python exception-in-with.py
   EXPECTED
   Traceback (most recent call last):
     File "exception-in-with.py", line 51, in <module>
       abc  # !!!
   NameError: name 'abc' is not defined
   ACTUAL
   usage: exception-in-with.py [-h] --image_path PATH
   exception-in-with.py: error: argument --image_path is required
   
   $ python exception-in-with.py --image_path x
   EXPECTED == ACTUAL
   Traceback (most recent call last):
     File "exception-in-with.py", line 51, in <module>
       abc  # !!!
   NameError: name 'abc' is not defined
   
   $ python exception-in-with.py --image_path x --tile_indices 1
   EXPECTED
   Traceback (most recent call last):
     File "exception-in-with.py", line 51, in <module>
       abc  # !!!
   NameError: name 'abc' is not defined
   ACTUAL
   usage: exception-in-with.py [-h] --image_path PATH
   exception-in-with.py: error: unrecognized arguments: --tile_indices 1
   """
   from argparse import ArgumentParser
   
   
   class Starter(object):
   
       def __init__(self):
           self.argument_parser = ArgumentParser()
   
       def __enter__(self):
           return self
   
       def __exit__(self, exception_type, exception_value, exception_traceback):
           self.argument_parser.parse_args()
   
       def add_argument(self, *args, **kw):
           self.argument_parser.add_argument(*args, **kw)
   
   
   with Starter() as starter:
       starter.add_argument('--image_path', metavar='PATH', required=True)
       abc  # !!!
       starter.add_argument('--tile_indices', metavar='INTEGER')
   
2. invisibleroads

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 28, 2015
3. benjaminp commented on Apr 28, 2015

   

   benjaminp

   on Apr 28, 2015

   Contributor

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   I'm not sure why this is so surprising. parse_args in __exit__ raises a SystmExit, which the Python interpreter takes to mean the program is responsibly exiting. I don't think any other behavior would be reasonable. e.g. sys.exit() in an except clause shouldn't cause the exception to be printed.
4. benjaminp

   added

   invalid

   on Apr 28, 2015
5. benjaminp

   closed this as completedon Apr 28, 2015
6. invisibleroads commented on May 6, 2015

   

   invisibleroadsmannequin

   on May 6, 2015

   MannequinAuthor

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   The behavior may not be surprising from a technical perspective, but it is unintuitive.

   I think exceptions inside a with statement should trigger a traceback, unless you are saying that it is the responsibility of the author to catch and raise the exception inside __exit__, which feels to me like a workaround that is specific to parse_args.
7. bitdancer commented on May 6, 2015

   

   bitdancer

   on May 6, 2015

   Member

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   What is happening here is that the __exit__ method gets passed the exception, and then instead of returning and allowing the exception to propagate and be printed, it raises SystemExit (via parse_args), which causes Python to immediately shut down, *before* __exit__ returns and the exception is propagated.

   So yes, you'd have to do something specific for argparse (which raises a SystemExit exception if parsing fails) if you want to do something non-normal with SystemExit. That is, you'll need to catch SystemExit.

   This is really a duplicate of bpo-9938.
8. bitdancer

   removed

   invalid

   on May 6, 2015
9. ezio-melotti

   transferred this issue fromon Apr 10, 2022