Create argparse type "existing_dir[_empty]"

[pyi2ncommon] / src / argparse_helpers.py

# The software in this package is distributed under the GNU General
# Public License version 2 (with a special exception described below).
#
# A copy of GNU General Public License (GPL) is included in this distribution,
# in the file COPYING.GPL.
#
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# or inline functions from this file, or you compile this file and link it
# with other works to produce a work based on this file, this file
# does not by itself cause the resulting work to be covered
# by the GNU General Public License.
#
# However the source code for this file must still be made available
# in accordance with section (3) of the GNU General Public License.
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# This exception does not invalidate any other reasons why a work based
# on this file might be covered by the GNU General Public License.

"""
argparse_helpers: Some convenience helpers for argparse

Featuring
- NonExitingParser: a subclass of ArgumentParser that does not do sys.exit(2)
  on parse error but instead raises a ArgParserWantsExit exception.
- function existing_file which can be used as a type in add_argument() calls

.. codeauthor:: Intra2net
"""

from argparse import ArgumentParser, ArgumentTypeError
from os.path import isfile, isdir


class ArgParserWantsExit(Exception):
    """
    Exception raised from NonExitingParser instead of calling sys.exit(2).
    """

    def __init__(self, message):
        super(ArgParserWantsExit, self).__init__("Error parsing args: " +
                                                 message)


class NonExitingParser(ArgumentParser):
    """ArgumentParser that does not call sys.exit(2) on parse failure.

    Calling `sys.exit` also just raises a SystemExit exception. But that is not
    a subclass of Exception and not as explicit and specific as this one.

    Convenient e.g. for global try-except blocks e.g. in a daemon::

        def main():
            log = None
            try:
               log = create_log()
               parser = NonExitingParser()
               # ... add args ...
               args = parser.parse_args()
               # ... rest of your program ...
            except Exception as exc:
                log.error('uncaught exception: exc')  # will always show in log

    """
    def error(self, message):
        """Called when error occurred parsing args. Raise error, not exit."""
        raise ArgParserWantsExit(message)


def existing_file(filename):
    """
    Function that raises ArgumentTypeError if argument is not an existing file.

    Returns filename if it is valid.  Can be used as `type` argument in
    :py:meth:`argparse.ArgumentParser.add_argument`

    If you want to open the file, you can as well use the python built-in
    :py:class:`argparse.FileType` instead.
    """
    if isfile(filename):
        return filename
    raise ArgumentTypeError('{} is not an existing file'.format(filename))


def existing_file_or_empty(filename=''):
    """
    Like :py:func:`existing_file` but accepts empty filename (returns '' then).
    """
    if not filename.strip():
        return ''
    return existing_file(filename)


def existing_dir(path):
    """
    Function that raises ArgumentTypeError if argument is not an existing directory.

    .. seealso:: :py:func:`existing_file`, :py:func:`existing_dir_or_empty`
    """
    if isdir(path):
        return path
    raise ArgumentTypeError('{} is not an existing directory'.format(path))


def existing_dir_or_empty(path=''):
    """
    Like :py:func:`existing_dir` but accepts empty path (returns '' then).
    """
    if not path.strip():
        return ''
    return existing_dir(path)