[issue24070] Exceptions and arguments disappear when using argparse inside with statement

[R. David Murray]

R. David Murray added the comment:

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 issue 9938.

--
dependencies: +Documentation for argparse interactive use
nosy: +r.david.murray
resolution: not a bug -> duplicate
stage:  -> resolved
versions: +Python 3.4, Python 3.5 -Python 3.3

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[issue24070] Exceptions and arguments disappear when using argparse inside with statement

[Roy Hyunjin Han]

Roy Hyunjin Han added the comment:

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.

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[issue24070] Exceptions and arguments disappear when using argparse inside with statement

[Benjamin Peterson]

Benjamin Peterson added the comment:

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.

--
nosy: +benjamin.peterson
resolution:  -> not a bug
status: open -> closed

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[issue24070] Exceptions and arguments disappear when using argparse inside with statement

[Roy Hyunjin Han]

New submission from Roy Hyunjin Han:

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 
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 
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 
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')
```

--
components: Library (Lib)
files: argparse-with-exception.py
messages: 242192
nosy: invisibleroads
priority: normal
severity: normal
status: open
title: Exceptions and arguments disappear when using argparse inside with 
statement
type: behavior
versions: Python 2.7, Python 3.3
Added file: http://bugs.python.org/file39227/argparse-with-exception.py

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