Wanted to customize the python argparse module generated errors - python-2.7

Question

import argparse
parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group()
group.add_argument("-f", "--filepath", help="Input the file path")
group.add_argument("-d", "--dirpath", help="Input the directory path")
For Example if I use both options -f and -d,I'm getting the following error,which is generated by python:
error: argument -d/--dirpath: expected one argument
But I want to customise the error and it should be defined by me.

Answer 1

2140:~/mypy$ python3 stack50552334.py
Namespace(dirpath=None, filepath=None)
These are all produced when there's a mismatch between the Action nargs and available strings. While message format is the same, it is customized to the argument's name:
2140:~/mypy$ python3 stack50552334.py -f
usage: stack50552334.py [-h] [-f FILEPATH | -d DIRPATH]
stack50552334.py: error: argument -f/--filepath: expected one argument
2140:~/mypy$ python3 stack50552334.py -d
usage: stack50552334.py [-h] [-f FILEPATH | -d DIRPATH]
stack50552334.py: error: argument -d/--dirpath: expected one argument
2140:~/mypy$ python3 stack50552334.py -f -d
usage: stack50552334.py [-h] [-f FILEPATH | -d DIRPATH]
stack50552334.py: error: argument -f/--filepath: expected one argument
2141:~/mypy$ python3 stack50552334.py -f foo -d
usage: stack50552334.py [-h] [-f FILEPATH | -d DIRPATH]
stack50552334.py: error: argument -d/--dirpath: expected one argument
But when both flags are provided with the expected arguments, then the mutually_exclusive_group test comes into play, and raises its own error.
2141:~/mypy$ python3 stack50552334.py -f foo -d dir
usage: stack50552334.py [-h] [-f FILEPATH | -d DIRPATH]
stack50552334.py: error: argument -d/--dirpath: not allowed with argument -f/--filepath
The first class of message is produced in:
def _match_argument(self, action, arg_strings_pattern):
# match the pattern for this action to the arg strings
nargs_pattern = self._get_nargs_pattern(action)
match = _re.match(nargs_pattern, arg_strings_pattern)
# raise an exception if we weren't able to find a match
if match is None:
nargs_errors = {
None: _('expected one argument'),
OPTIONAL: _('expected at most one argument'),
ONE_OR_MORE: _('expected at least one argument'),
}
default = ngettext('expected %s argument',
'expected %s arguments',
action.nargs) % action.nargs
msg = nargs_errors.get(action.nargs, default)
raise ArgumentError(action, msg)
# return the number of arguments matched
return len(match.group(1))
The _ is alias for gettext.gettext. So there is a possibility of customizing the messages with this mechanism. https://docs.python.org/3/library/gettext.html
from gettext import gettext as _, ngettext
I don't have any experience with this multlingual localization.
The mutually_exclusive error is raised in:
def take_action(action, argument_strings, option_string=None):
...
# error if this argument is not allowed with other previously
# seen arguments, assuming that actions that use the default
# value don't really count as "present"
if argument_values is not action.default:
seen_non_default_actions.add(action)
for conflict_action in action_conflicts.get(action, []):
if conflict_action in seen_non_default_actions:
msg = _('not allowed with argument %s')
action_name = _get_action_name(conflict_action)
raise ArgumentError(action, msg % action_name)
Again, there is the possibility of gettext customization.
custom argparse.py
Of course there's nothing preventing you from modifying a copy of argparse.py file, and placing in your own directory so it is loaded instead of the default one.
post parsing testing
If the wording of the mutually_exclusive error is bothering you (as opposed to the expected one argument errors, you could omit that group from the parser, and do your own tests after parsing. If the user provides non-default values for both args.filepath and args.dirpath call parser.error('your own message'), or raise your own error.