[New-bugs-announce] [issue15832] argparse: typechecks default value too early

[Riccardo Murri]

New submission from Riccardo Murri:

The `argparse` module (tested with 2.7, but other versions might be
affected) checks the `default` value of an option too early: if the
default value raises an exception, then command-line parsing stops.

Consider for example the following code:

############### begin sample #####################
import os
import argparse

def existing_filename(path):
    if not os.access(path, os.F_OK | os.R_OK):
        raise argparse.ArgumentTypeError("File '%s' does not exist or cannot be read." % path)
    return path

parser = argparse.ArgumentParser(description='Process a file.')
parser.add_argument('file', type=existing_filename, default='/some/weird/default',
                    help='A file to process.')

args = parser.parse_args()
print ("Will process file '%s' ..." % args.file)
############### end sample #####################

The intention here is that the default value should be used *if and
only if* the script users do not provide their own file to process.
It may happen that the default is invalid, but that should be reported
as an error only if the default is used, i.e., users did not provide
any file name to the script.

What happens instead is that the argparse errors out in any case, even
when `--help` is requested or when a valid file name is passed, as the
following two examples show:

    rmurri at xenia:/tmp$ ./argparse-processes-defaults-too-early.py --help
    Traceback (most recent call last):
      File "./argparse-processes-defaults-too-early.py", line 18, in <module>
        args = parser.parse_args()
      File "/usr/lib/python2.7/argparse.py", line 1688, in parse_args
        args, argv = self.parse_known_args(args, namespace)
      File "/usr/lib/python2.7/argparse.py", line 1710, in parse_known_args
        default = self._get_value(action, default)
      File "/usr/lib/python2.7/argparse.py", line 2239, in _get_value
        raise ArgumentError(action, msg)
    argparse.ArgumentError: argument file: File '/some/weird/default' does not exist or cannot be read.

    rmurri at xenia:/tmp$ ./argparse-processes-defaults-too-early.py /tmp/xxx.py 
    Traceback (most recent call last):
      File "./argparse-processes-defaults-too-early.py", line 18, in <module>
        args = parser.parse_args()
      File "/usr/lib/python2.7/argparse.py", line 1688, in parse_args
        args, argv = self.parse_known_args(args, namespace)
      File "/usr/lib/python2.7/argparse.py", line 1710, in parse_known_args
        default = self._get_value(action, default)
      File "/usr/lib/python2.7/argparse.py", line 2239, in _get_value
        raise ArgumentError(action, msg)
    argparse.ArgumentError: argument file: File '/some/weird/default' does not exist or cannot be read.

Thanks!

----------
components: Library (Lib)
messages: 169548
nosy: riccardomurri
priority: normal
severity: normal
status: open
title: argparse: typechecks default value too early
type: behavior
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue15832>
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