Yclept Nemo added the comment: Well that won't work. Example: import argparse class TestAction(argparse.Action): def __call__(self, parser, namespace, values, option_string=None): print("default: {}({})\tdest: {}({})".format(self.default, type(self.default), getattr(namespace, self.dest), type(getattr(namespace, self.dest)))) if getattr(namespace, self.dest) is self.default: print("Replacing with: ", values) setattr(namespace, self.dest, values) # extra logical code not necessary for testcase parser = argparse.ArgumentParser() parser.add_argument\ ( "-o", "--output" , type=int , action=TestAction , default=42 ) args = parser.parse_args() $ ./argparse_test -o 42 -o 100 default: 42(<class 'int'>) dest: 42(<class 'int'>) Replacing with: 42 default: 42(<class 'int'>) dest: 42(<class 'int'>) Replacing with: 100 100 Use this approach: class ExtendAction(argparse.Action): def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) if not isinstance(self.default, collections.abc.Iterable): self.default = [self.default] self.reset_dest = False def __call__(self, parser, namespace, values, option_string=None): if not self.reset_dest: setattr(namespace, self.dest, []) self.reset_dest = True getattr(namespace, self.dest).extend(values) Anyway, this should be properly documented... ---------- nosy: +Yclept.Nemo _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue16399> _______________________________________