I am using argparse's action to add various data to a class. I would like to use that action on the default value if that arg is not provided at the command line. Is this possible?
Thanks!
argparse does not use the action when applying the default. It just uses setattr. It may use the type if the default is a string. But you can invoke the action directly.
Here I use a custom action class borrowed from the documentation. In the first parse_args nothing happens. Then I create a new namespace, and invoke the action on the default. Then I pass that namespace to parse_args. To understand this, you many need to import it into an interactive shell, and examine the attributes of the namespace and action.
# sample custom action from docs
class FooAction(argparse.Action):
def __call__(self, parser, namespace, values, option_string=None):
print('Setting: %r %r %r' % (namespace, values, option_string))
setattr(namespace, self.dest, 'action:'+values)
p = argparse.ArgumentParser()
a1 = p.add_argument('--foo', action=FooAction, default='default')
print 'action:',a1
print p.parse_args([])
ns = argparse.Namespace()
a1(p, ns, a1.default, 'no string') # call action
print p.parse_args([],ns)
print p.parse_args(['--foo','1'],ns)
which produces:
action: FooAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default='default', type=None, choices=None, help=None, metavar=None)
Namespace(foo='default')
Setting: Namespace() 'default' 'no string'
Namespace(foo='action:default')
Setting: Namespace(foo='action:default') '1' '--foo'
Namespace(foo='action:1')
I tailored the output to highlight when the action is being used.
Here's a way of performing a special action on an argument that isn't given on the command line (or given with a value == to the default). It's a simplification of the class given in https://stackoverflow.com/a/24638908/901925.
class Parser1:
def __init__(self, desc):
self.parser = argparse.ArgumentParser(description=desc)
self.actions = []
def milestone(self, help_='milestone for latest release.', default=None):
action = self.parser.add_argument('-m', '--milestone', help=help_, default=default)
self.actions.append(action)
return self
def parse(self):
args = self.parser.parse_args()
for a in self.actions:
if getattr(args, a.dest) == a.default:
print 'Please specify', a.dest
values = raw_input('>')
setattr(args, a.dest, values)
return args
print Parser1('desc').milestone(default='PROMPT').parse()
The prompting is done after parse_args. I don't see any reason to call parse_args again.
I needed to prompt the user if an option was not specified - that's how I did it:
class _PromptUserAction(argparse.Action):
def __call__(self, parser, namespace, values, option_string=None):
if values == self.default:
print 'Please specify', self.dest
values = raw_input('>')
setattr(namespace, self.dest, values)
class Parser:
def __init__(self, desc, add_h=True):
self.parser = argparse.ArgumentParser(description=desc, add_help=add_h,
formatter_class=argparse.ArgumentDefaultsHelpFormatter)
#actions to be run on options if not specified (using default to check)
self.actions = []
#staticmethod
def new(description, add_help=True):
return Parser(description, add_help)
# ...
def milestone(self, help_='Specify the milestone for latest release.'):
action = self.parser.add_argument('-m', '--milestone',
dest='milestone',
action=_PromptUserAction,
default='PROMPT', # needed I think
type=str,
help=help_)
self.actions.append(action)
return self
def parse(self):
"""
Return an object which can be used to get the arguments as in:
parser_instance.parse().milestone
:return: ArgumentParser
"""
args = self.parser.parse_args()
# see: http://stackoverflow.com/a/21588198/281545
dic = vars(args)
ns = argparse.Namespace()
for a in self.actions:
if dic[a.dest] == a.default:
a(self.parser, ns, a.default) # call action
# duh - can I avoid it ?
import sys
return self.parser.parse_args(sys.argv[1:],ns)
I am interested if this can somehow be done without having to reparse the args (the import sys part). Maybe some constructor options for argparse.Action ?
I have to either store the command line argument in a variable or assign a default value to it.
What i am trying is the below
import sys
Var=sys.argv[1] or "somevalue"
I am getting the error out of index if i don't specify any argument. How to solve this?
Var=sys.argv[1] if len(sys.argv) > 1 else "somevalue"
The builtin argparse module is intended for exactly these sorts of tasks:
import argparse
# Set up argument parser
ap = argparse.ArgumentParser()
# Single positional argument, nargs makes it optional
ap.add_argument("thingy", nargs='?', default="blah")
# Do parsing
a = ap.parse_args()
# Use argument
print a.thingy
Or, if you are stuck with Python 2.6 or earlier, and don't wish to add a requirement on the backported argparse module, you can do similar things manually like so:
import optparse
opter = optparse.OptionParser()
# opter.add_option("-v", "--verbose") etc
opts, args = opter.parse_args()
if len(args) == 0:
var = "somevalue"
elif len(args) == 1:
var = args[0]
else:
opter.error("Only one argument expected, got %d" % len(args))
print var
Good question.
I think the best solution would be to do
try:
var = sys.argv[1]
except IndexError:
var = "somevalue"
Try the following with a command-line-processing template:
def process_command_line(argv):
...
# add your option here
parser.add_option('--var',
default="somevalue",
help="your help text")
def main(argv=None):
settings, args = process_command_line(argv)
...
print settings, args # <- print your settings and args
Running ./your_script.py with the template below and your modifications above prints {'var': 'somevalue'} []
For an example of a command-line-processing template see an example in Code Like a Pythonista: Idiomatic Python (http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#command-line-processing):
#!/usr/bin/env python
"""
Module docstring.
"""
import sys
import optparse
def process_command_line(argv):
"""
Return a 2-tuple: (settings object, args list).
`argv` is a list of arguments, or `None` for ``sys.argv[1:]``.
"""
if argv is None:
argv = sys.argv[1:]
# initialize the parser object:
parser = optparse.OptionParser(
formatter=optparse.TitledHelpFormatter(width=78),
add_help_option=None)
# define options here:
parser.add_option( # customized description; put --help last
'-h', '--help', action='help',
help='Show this help message and exit.')
settings, args = parser.parse_args(argv)
# check number of arguments, verify values, etc.:
if args:
parser.error('program takes no command-line arguments; '
'"%s" ignored.' % (args,))
# further process settings & args if necessary
return settings, args
def main(argv=None):
settings, args = process_command_line(argv)
# application code here, like:
# run(settings, args)
return 0 # success
if __name__ == '__main__':
status = main()
sys.exit(status)
I would like to use argparse to parse boolean command-line arguments written as "--foo True" or "--foo False". For example:
my_program --my_boolean_flag False
However, the following test code does not do what I would like:
import argparse
parser = argparse.ArgumentParser(description="My parser")
parser.add_argument("--my_bool", type=bool)
cmd_line = ["--my_bool", "False"]
parsed_args = parser.parse(cmd_line)
Sadly, parsed_args.my_bool evaluates to True. This is the case even when I change cmd_line to be ["--my_bool", ""], which is surprising, since bool("") evalutates to False.
How can I get argparse to parse "False", "F", and their lower-case variants to be False?
I think a more canonical way to do this is via:
command --feature
and
command --no-feature
argparse supports this version nicely:
Python 3.9+:
parser.add_argument('--feature', action=argparse.BooleanOptionalAction)
Python < 3.9:
parser.add_argument('--feature', action='store_true')
parser.add_argument('--no-feature', dest='feature', action='store_false')
parser.set_defaults(feature=True)
Of course, if you really want the --arg <True|False> version, you could pass ast.literal_eval as the "type", or a user defined function ...
def t_or_f(arg):
ua = str(arg).upper()
if 'TRUE'.startswith(ua):
return True
elif 'FALSE'.startswith(ua):
return False
else:
pass #error condition maybe?
Yet another solution using the previous suggestions, but with the "correct" parse error from argparse:
def str2bool(v):
if isinstance(v, bool):
return v
if v.lower() in ('yes', 'true', 't', 'y', '1'):
return True
elif v.lower() in ('no', 'false', 'f', 'n', '0'):
return False
else:
raise argparse.ArgumentTypeError('Boolean value expected.')
This is very useful to make switches with default values; for instance
parser.add_argument("--nice", type=str2bool, nargs='?',
const=True, default=False,
help="Activate nice mode.")
allows me to use:
script --nice
script --nice <bool>
and still use a default value (specific to the user settings). One (indirectly related) downside with that approach is that the 'nargs' might catch a positional argument -- see this related question and this argparse bug report.
If you want to allow --feature and --no-feature at the same time (last one wins)
This allows users to make a shell alias with --feature, and overriding it with --no-feature.
Python 3.9 and above
parser.add_argument('--feature', default=True, action=argparse.BooleanOptionalAction)
Python 3.8 and below
I recommend mgilson's answer:
parser.add_argument('--feature', dest='feature', action='store_true')
parser.add_argument('--no-feature', dest='feature', action='store_false')
parser.set_defaults(feature=True)
If you DON'T want to allow --feature and --no-feature at the same time
You can use a mutually exclusive group:
feature_parser = parser.add_mutually_exclusive_group(required=False)
feature_parser.add_argument('--feature', dest='feature', action='store_true')
feature_parser.add_argument('--no-feature', dest='feature', action='store_false')
parser.set_defaults(feature=True)
You can use this helper if you are going to set many of them:
def add_bool_arg(parser, name, default=False):
group = parser.add_mutually_exclusive_group(required=False)
group.add_argument('--' + name, dest=name, action='store_true')
group.add_argument('--no-' + name, dest=name, action='store_false')
parser.set_defaults(**{name:default})
add_bool_arg(parser, 'useful-feature')
add_bool_arg(parser, 'even-more-useful-feature')
Here is another variation without extra row/s to set default values. The boolean value is always assigned, so that it can be used in logical statements without checking beforehand:
import argparse
parser = argparse.ArgumentParser(description="Parse bool")
parser.add_argument("--do-something", default=False, action="store_true",
help="Flag to do something")
args = parser.parse_args()
if args.do_something:
print("Do something")
else:
print("Don't do something")
print(f"Check that args.do_something={args.do_something} is always a bool.")
oneliner:
parser.add_argument('--is_debug', default=False, type=lambda x: (str(x).lower() == 'true'))
There seems to be some confusion as to what type=bool and type='bool' might mean. Should one (or both) mean 'run the function bool(), or 'return a boolean'? As it stands type='bool' means nothing. add_argument gives a 'bool' is not callable error, same as if you used type='foobar', or type='int'.
But argparse does have registry that lets you define keywords like this. It is mostly used for action, e.g. `action='store_true'. You can see the registered keywords with:
parser._registries
which displays a dictionary
{'action': {None: argparse._StoreAction,
'append': argparse._AppendAction,
'append_const': argparse._AppendConstAction,
...
'type': {None: <function argparse.identity>}}
There are lots of actions defined, but only one type, the default one, argparse.identity.
This code defines a 'bool' keyword:
def str2bool(v):
#susendberg's function
return v.lower() in ("yes", "true", "t", "1")
p = argparse.ArgumentParser()
p.register('type','bool',str2bool) # add type keyword to registries
p.add_argument('-b',type='bool') # do not use 'type=bool'
# p.add_argument('-b',type=str2bool) # works just as well
p.parse_args('-b false'.split())
Namespace(b=False)
parser.register() is not documented, but also not hidden. For the most part the programmer does not need to know about it because type and action take function and class values. There are lots of stackoverflow examples of defining custom values for both.
In case it isn't obvious from the previous discussion, bool() does not mean 'parse a string'. From the Python documentation:
bool(x): Convert a value to a Boolean, using the standard truth testing procedure.
Contrast this with
int(x): Convert a number or string x to an integer.
Simplest & most correct way is:
from distutils.util import strtobool
parser.add_argument('--feature', dest='feature',
type=lambda x: bool(strtobool(x)))
Do note that True values are y, yes, t, true, on and 1;
false values are n, no, f, false, off and 0. Raises ValueError if val is anything else.
A quite similar way is to use:
feature.add_argument('--feature',action='store_true')
and if you set the argument --feature in your command
command --feature
the argument will be True, if you do not set type --feature the arguments default is always False!
I was looking for the same issue, and imho the pretty solution is :
def str2bool(v):
return v.lower() in ("yes", "true", "t", "1")
and using that to parse the string to boolean as suggested above.
This works for everything I expect it to:
add_boolean_argument(parser, 'foo', default=True)
parser.parse_args([]) # Whatever the default was
parser.parse_args(['--foo']) # True
parser.parse_args(['--nofoo']) # False
parser.parse_args(['--foo=true']) # True
parser.parse_args(['--foo=false']) # False
parser.parse_args(['--foo', '--nofoo']) # Error
The code:
def _str_to_bool(s):
"""Convert string to bool (in argparse context)."""
if s.lower() not in ['true', 'false']:
raise ValueError('Need bool; got %r' % s)
return {'true': True, 'false': False}[s.lower()]
def add_boolean_argument(parser, name, default=False):
"""Add a boolean argument to an ArgumentParser instance."""
group = parser.add_mutually_exclusive_group()
group.add_argument(
'--' + name, nargs='?', default=default, const=True, type=_str_to_bool)
group.add_argument('--no' + name, dest=name, action='store_false')
Simplest. It's not flexible, but I prefer simplicity.
parser.add_argument('--boolean_flag',
help='This is a boolean flag.',
type=eval,
choices=[True, False],
default='True')
EDIT: If you don't trust the input, don't use eval.
In addition to what #mgilson said, it should be noted that there's also a ArgumentParser.add_mutually_exclusive_group(required=False) method that would make it trivial to enforce that --flag and --no-flag aren't used at the same time.
This is actually outdated. For Python 3.7+, Argparse now supports boolean args (search BooleanOptionalAction).
The implementation looks like this:
import argparse
ap = argparse.ArgumentParser()
# List of args
ap.add_argument('--foo', default=True, type=bool, help='Some helpful text that is not bar. Default = True')
# Importable object
args = ap.parse_args()
One other thing to mention: this will block all entries other than True and False for the argument via argparse.ArgumentTypeError. You can create a custom error class for this if you want to try to change this for any reason.
A simpler way would be to use as below.
parser.add_argument('--feature', type=lambda s: s.lower() in ['true', 't', 'yes', '1'])
After previously following #akash-desarda 's excellence answer https://stackoverflow.com/a/59579733/315112 , to use strtobool via lambda, later, I decide to use strtobool directly instead.
import argparse
from distutils import util
parser.add_argument('--feature', type=util.strtobool)
Yes you're right, strtobool is returning an int, not a bool. But strtobool will not returning any other value except 0 and 1, and python will get them converted to a bool value seamlessy and consistently.
>>> 0 == False
True
>>> 0 == True
False
>>> 1 == False
False
>>> 1 == True
True
While on receiving a wrong input value like
python yours.py --feature wrong_value
An argparse.Action with strtobool compared to lambda will produce a slightly clearer/comprehensible error message:
yours.py: error: argument --feature: invalid strtobool value: 'wrong_value'
Compared to this code,
parser.add_argument('--feature', type=lambda x: bool(util.strtobool(x))
Which will produce a less clear error message:
yours.py: error: argument --feature: invalid <lambda> value: 'wrong_value'
Simplest way would be to use choices:
parser = argparse.ArgumentParser()
parser.add_argument('--my-flag',choices=('True','False'))
args = parser.parse_args()
flag = args.my_flag == 'True'
print(flag)
Not passing --my-flag evaluates to False. The required=True option could be added if you always want the user to explicitly specify a choice.
As an improvement to #Akash Desarda 's answer, you could do
import argparse
from distutils.util import strtobool
parser = argparse.ArgumentParser()
parser.add_argument("--foo",
type=lambda x:bool(strtobool(x)),
nargs='?', const=True, default=False)
args = parser.parse_args()
print(args.foo)
And it supports python test.py --foo
(base) [costa#costa-pc code]$ python test.py
False
(base) [costa#costa-pc code]$ python test.py --foo
True
(base) [costa#costa-pc code]$ python test.py --foo True
True
(base) [costa#costa-pc code]$ python test.py --foo False
False
Expanding on gerardw's answer
The reason parser.add_argument("--my_bool", type=bool) doesn't work is that bool("mystring") is True for any non-empty string so bool("False") is actually True.
What you want is
my_program.py
import argparse
parser = argparse.ArgumentParser(description="My parser")
parser.add_argument(
"--my_bool",
choices=["False", "True"],
)
parsed_args = parser.parse_args()
my_bool = parsed_args.my_bool == "True"
print(my_bool)
$ python my_program.py --my_bool False
False
$ python my_program.py --my_bool True
True
$ python my_program.py --my_bool true
usage: my_program.py [-h] [--my_bool {False,True}]
my_program.py: error: argument --my_bool: invalid choice: 'true' (choose from 'False', 'True')
I think the most canonical way will be:
parser.add_argument('--ensure', nargs='*', default=None)
ENSURE = config.ensure is None
I found good way to store default value of parameter as False and when it is present in commandline argument then its value should be true.
cmd command
when you want argument to be true:
python main.py --csv
when you want your argument should be false:
python main.py
import argparse
from ast import parse
import sys
parser = argparse.ArgumentParser(description='')
parser.add_argument('--csv', action='store_true', default = False
,help='read from csv')
args = parser.parse_args()
if args.csv:
print('reading from csv')
Quick and easy, but only for arguments 0 or 1:
parser.add_argument("mybool", default=True,type=lambda x: bool(int(x)))
myargs=parser.parse_args()
print(myargs.mybool)
The output will be "False" after calling from terminal:
python myscript.py 0
class FlagAction(argparse.Action):
# From http://bugs.python.org/issue8538
def __init__(self, option_strings, dest, default=None,
required=False, help=None, metavar=None,
positive_prefixes=['--'], negative_prefixes=['--no-']):
self.positive_strings = set()
self.negative_strings = set()
for string in option_strings:
assert re.match(r'--[A-z]+', string)
suffix = string[2:]
for positive_prefix in positive_prefixes:
self.positive_strings.add(positive_prefix + suffix)
for negative_prefix in negative_prefixes:
self.negative_strings.add(negative_prefix + suffix)
strings = list(self.positive_strings | self.negative_strings)
super(FlagAction, self).__init__(option_strings=strings, dest=dest,
nargs=0, const=None, default=default, type=bool, choices=None,
required=required, help=help, metavar=metavar)
def __call__(self, parser, namespace, values, option_string=None):
if option_string in self.positive_strings:
setattr(namespace, self.dest, True)
else:
setattr(namespace, self.dest, False)
Similar to #Akash but here is another approach that I've used. It uses str than lambda because python lambda always gives me an alien-feelings.
import argparse
from distutils.util import strtobool
parser = argparse.ArgumentParser()
parser.add_argument("--my_bool", type=str, default="False")
args = parser.parse_args()
if bool(strtobool(args.my_bool)) is True:
print("OK")
just do the following , you can make --test = True by using
python filename --test
parser.add_argument("--test" , default=False ,help="test ?", dest='test', action='store_true')
Convert the value:
def __arg_to_bool__(arg):
"""__arg_to_bool__
Convert string / int arg to bool
:param arg: argument to be converted
:type arg: str or int
:return: converted arg
:rtype: bool
"""
str_true_values = (
'1',
'ENABLED',
'ON',
'TRUE',
'YES',
)
str_false_values = (
'0',
'DISABLED',
'OFF',
'FALSE',
'NO',
)
if isinstance(arg, str):
arg = arg.upper()
if arg in str_true_values:
return True
elif arg in str_false_values:
return False
if isinstance(arg, int):
if arg == 1:
return True
elif arg == 0:
return False
if isinstance(arg, bool):
return arg
# if any other value not covered above, consider argument as False
# or you could just raise and error
return False
[...]
args = ap.parse_args()
my_arg = options.my_arg
my_arg = __arg_to_bool__(my_arg)
You can create a BoolAction and then use it
class BoolAction(Action):
def __init__(
self,
option_strings,
dest,
nargs=None,
default: bool = False,
**kwargs,
):
if nargs is not None:
raise ValueError('nargs not allowed')
super().__init__(option_strings, dest, default=default, **kwargs)
def __call__(self, parser, namespace, values, option_string=None):
input_value = values.lower()
b = input_value in ['true', 'yes', '1']
if not b and input_value not in ['false', 'no', '0']:
raise ValueError('Invalid boolean value "%s".)
setattr(namespace, self.dest, b)
and then set action=BoolAction in parser.add_argument()
I have a script where I ask the user for a list of pre-defined actions to perform. I also want the ability to assume a particular list of actions when the user doesn't define anything. however, it seems like trying to do both of these together is impossible.
when the user gives no arguments, they receive an error that the default choice is invalid
acts = ['clear','copy','dump','lock']
p = argparse.ArgumentParser()
p.add_argument('action', nargs='*', action='append', choices=acts, default=[['dump', 'clear']])
args = p.parse_args([])
>>> usage: [-h] [{clear,copy,dump,lock} [{clear,copy,dump,lock} ...]]
: error: argument action: invalid choice: [['dump', 'clear']] (choose from 'clear', 'copy', 'dump', 'lock')
and when they do define a set of actions, the resultant namespace has the user's actions appended to the default, rather than replacing the default
acts = ['clear','copy','dump','lock']
p = argparse.ArgumentParser()
p.add_argument('action', nargs='*', action='append', choices=acts, default=[['dump', 'clear']])
args = p.parse_args(['lock'])
args
>>> Namespace(action=[['dump', 'clear'], ['dump']])
What you need can be done using a customized argparse.Action as in the following example:
import argparse
parser = argparse.ArgumentParser()
class DefaultListAction(argparse.Action):
CHOICES = ['clear','copy','dump','lock']
def __call__(self, parser, namespace, values, option_string=None):
if values:
for value in values:
if value not in self.CHOICES:
message = ("invalid choice: {0!r} (choose from {1})"
.format(value,
', '.join([repr(action)
for action in self.CHOICES])))
raise argparse.ArgumentError(self, message)
setattr(namespace, self.dest, values)
parser.add_argument('actions', nargs='*', action=DefaultListAction,
default = ['dump', 'clear'],
metavar='ACTION')
print parser.parse_args([])
print parser.parse_args(['lock'])
The output of the script is:
$ python test.py
Namespace(actions=['dump', 'clear'])
Namespace(actions=['lock'])
In the documentation (http://docs.python.org/dev/library/argparse.html#default), it is said :
For positional arguments with nargs equal to ? or *, the default value is used when no command-line argument was present.
Then, if we do :
acts = ['clear','copy','dump','lock']
p = argparse.ArgumentParser()
p.add_argument('action', nargs='*', choices=acts, default='clear')
print p.parse_args([])
We get what we expect
Namespace(action='clear')
The problem is when you put a list as a default.
But I've seen it in the doc,
parser.add_argument('bar', nargs='*', default=[1, 2, 3], help='BAR!')
So, I don't know :-(
Anyhow, here is a workaround that does the job you want :
import sys, argparse
acts = ['clear','copy','dump','lock']
p = argparse.ArgumentParser()
p.add_argument('action', nargs='*', choices=acts)
args = ['dump', 'clear'] # I set the default here ...
if sys.argv[1:]:
args = p.parse_args()
print args
I ended up doing the following:
no append
add the empty list to the possible choices or else the empty input breaks
without default
check for an empty list afterwards and set the actual default in that case
Example:
parser = argparse.ArgumentParser()
parser.add_argument(
'is',
type=int,
choices=[[], 1, 2, 3],
nargs='*',
)
args = parser.parse_args(['1', '3'])
assert args.a == [1, 3]
args = parser.parse_args([])
assert args.a == []
if args.a == []:
args.a = [1, 2]
args = parser.parse_args(['1', '4'])
# Error: '4' is not valid.
You could test whether the user is supplying actions (in which case parse it as a required, position argument), or is supplying no actions (in which case parse it as an optional argument with default):
import argparse
import sys
acts = ['clear', 'copy', 'dump', 'lock']
p = argparse.ArgumentParser()
if sys.argv[1:]:
p.add_argument('action', nargs = '*', choices = acts)
else:
p.add_argument('--action', default = ['dump', 'clear'])
args = p.parse_args()
print(args)
when run, yields these results:
% test.py
Namespace(action=['dump', 'clear'])
% test.py lock
Namespace(action=['lock'])
% test.py lock dump
Namespace(action=['lock', 'dump'])
You probably have other options to parse as well. In that case, you could use parse_known_args to parse the other options, and then handle the unknown arguments in a second pass:
import argparse
acts = ['clear', 'copy', 'dump', 'lock']
p = argparse.ArgumentParser()
p.add_argument('--foo')
args, unknown = p.parse_known_args()
if unknown:
p.add_argument('action', nargs = '*', choices = acts)
else:
p.add_argument('--action', default = ['dump', 'clear'])
p.parse_args(unknown, namespace = args)
print(args)
when run, yields these results:
% test.py
Namespace(action=['dump', 'clear'], foo=None)
% test.py --foo bar
Namespace(action=['dump', 'clear'], foo='bar')
% test.py lock dump
Namespace(action=['lock', 'dump'], foo=None)
% test.py lock dump --foo bar
Namespace(action=['lock', 'dump'], foo='bar')
The action was being appended because of the "action='append'" parameter you passed to argparse.
After removing this parameter, the arguments passed by a user would be displayed on their own, but the program would throw an error when no arguments were passed.
Adding a '--' prefix to the first parameter resolves this in the laziest way.
acts = ['clear','copy','dump','lock']
p = argparse.ArgumentParser()
p.add_argument('--action', nargs='*', choices=acts, default=[['dump', 'clear']])
args = p.parse_args()
The downside to this approach is that the options passed by the user must now be preceded by '--action', like:
app.py --action clear dump copy