Develop Reference

How to pass an argument calling a function using ArgParse?

I'm trying to call function_3 with an argument but i'm receiving a error unrecognized arguments. I'm calling this way: python script.py --pass test
import argparse
import sys
def function_1():
print('Function 1')
def function_2():
print('Function 2')
def function_3(arg):
print(f'Function 3 {arg}')
if __name__ == "__main__":
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers()
parser_f1 = subparsers.add_parser('fc1', help='Function 1')
parser_f1.set_defaults(func=function_1)
parser_f2 = subparsers.add_parser('fc2', help='Function 2')
parser_f2.set_defaults(func=function_2)
parser_f3 = subparsers.add_parser('fc3', help='Function 3')
parser_f3.add_argument("pass", help="function 3 argument")
parser_f3.set_defaults(func=function_3)
if len(sys.argv) <= 1:
sys.argv.append('--help')
options = parser.parse_args()
options.func()
Error
usage: argtest.py [-h] {fc1,fc2,fc3} ...
argtest.py: error: unrecognized arguments: --arg

First, your pass option is only available to the fc3 subparser, so rather than python script.py --pass test, you would want:
python script.py fc3 --pass test
But you've defined a positional argument, not a command line option. You need to either call your script like this:
python script.py fc3 test
Or you need to fix your code:
parser_f3.add_argument('--pass', help='function 3 argument')
That would allow you run python script.py fc3 --pass test.
There are still additional problems with the code; your function3 function requires an argument, but you're calling options.func() with no arguments, which will result in a TypeError exception.

I don't suggest doing it this way, I'd personally suggest just using click instead, but this is one method for doing it I guess...
import sys
from argparse import ArgumentParser
def parse_args():
parser = ArgumentParser()
parser.add_argument(
"-f", "--function", type=str, help="name of the function to call", default="echo"
)
parser.add_argument("func_args", nargs="*")
return parser.parse_args(args=None if sys.argv[1:] else ["--help"])
def echo(*args):
print(' '.join(*args))
if __name__ == "__main__":
args = parse_args()
eval(f"{args.function}({args.func_args})")
This approach feels fragile and wrong to me, but I believe this is line with the behavior you're asking for.

Python ArgParse add help message when subparsers are omitted [duplicate]

With python's argparse, how do I make a subcommand a required argument? I want to do this because I want argparse to error out if a subcommand is not specified. I override the error method to print help instead. I have 3-deep nested subcommands, so it's not a matter of simply handling zero arguments at the top level.
In the following example, if this is called like so, I get:
$./simple.py
$
What I want it to do instead is for argparse to complain that the required subcommand was not specified:
import argparse
class MyArgumentParser(argparse.ArgumentParser):
def error(self, message):
self.print_help(sys.stderr)
self.exit(0, '%s: error: %s\n' % (self.prog, message))
def main():
parser = MyArgumentParser(description='Simple example')
subs = parser.add_subparsers()
sub_one = subs.add_parser('one', help='does something')
sub_two = subs.add_parser('two', help='does something else')
parser.parse_args()
if __name__ == '__main__':
main()

There was a change in 3.3 in the error message for required arguments, and subcommands got lost in the dust.
http://bugs.python.org/issue9253#msg186387
There I suggest this work around, setting the required attribute after the subparsers is defined.
parser = ArgumentParser(prog='test')
subparsers = parser.add_subparsers()
subparsers.required = True
subparsers.dest = 'command'
subparser = subparsers.add_parser("foo", help="run foo")
parser.parse_args()
update
A related pull-request: https://github.com/python/cpython/pull/3027

In addition to hpaulj's answer: you can also use the required keyword argument with ArgumentParser.add_subparsers() since Python 3.7. You also need to pass dest as argument. Otherwise you will get an error: TypeError: sequence item 0: expected str instance, NoneType found.
Example file example.py:
import argparse
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest='command', required=True)
foo_parser = subparsers.add_parser("foo", help="command foo")
args = parser.parse_args()
Output of the call without an argument:
$ python example.py
usage: example.py [-h] {foo} ...
example.py: error: the following arguments are required: command

How about using required=True? More info here.

You can use the dest argument, which is documented in the last example in the documentation for add_subparsers():
# required_subparser.py
import argparse
parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest='subparser_name')
one = subparsers.add_parser('one')
two = subparsers.add_parser('two')
args = parser.parse_args()
Running with Python 2.7:
$python required_subparser.py
usage: required_subparser.py [-h] {one,two} ...
required_subparser.py: error: too few arguments
$python required_subparser.py one
$# no error

argparse conflict when used with two connected python3 scripts

I am trying to run one python script (Main_Script) which is supposed to get argparse flag and this script at the same time calls another script(Sub_Script) which is also supposed to get the flag to input. And when I call Main_Script I get an error which says that I can't use the flag because it is not defined in the script but it is actually defined. The error notification makes me use the flag from subscript instead.
MAIN_SCRIPT
parser = argparse.ArgumentParser(add_help=True)
parser.add_argument('-p', '--print_positive_results', action='store_true')
args = parser.parse_args()
PRINT_POSITIVE = args.print_positive_results
#I then use rhi global variable PRINT_POSITIVE
SUB_SCRIPT
import argparse
parser = argparse.ArgumentParser(add_help=True)
parser.add_argument('-d', '--debug', action='store_true')
args = parser.parse_args()
And when I call python MAIN_SCRIPT.py -p I get this
usage: test_grammar.py [-h] [-d]
test_grammar.py: error: unrecognized arguments: -p
DEBUG = False
if (args.debug ):
DEBUG = True

Seems like the command line args from the main script are passed through to the sub script.
You could try to (and probably should) wrap the argparse stuff into:
if __name__ == '__main__':
<argparse stuff>
With this the code is only called when the script is called from the command line. The real code could be outsourced into a function. This way you can use the script from command line with argparse and only import the function from the script if you call it from another script:
Main script:
import argparse
import subscript
if __name__ == '__main__':
parser = argparse.ArgumentParser(add_help=True)
parser.add_argument('-p', '--print_positive_results', action='store_true')
args = parser.parse_args()
...
subscript.your_function(<whatever your args are>)
...
Sub script:
import argparse
def your_function(<your args>):
<your code>
if __name__ == '__main__':
parser = argparse.ArgumentParser(add_help=True)
parser.add_argument('-d', '--debug', action='store_true')
args = parser.parse_args()
your_function(<whatever your args are>)
...

How to handle optional and mandatory positional arguments with argparse(python)

I have been trying to execute my python script with some arguments passed. The way I want it to be is something like this:
python script.py filepath
but also to support this
python script.py filepath1 filepath2
My implementation has been:
parser = argparse.ArgumentParser()
mand = parser.add_argument_group("Mandatory arguments")
mand.add_argument("path1", help = "path1")
opt = parser.add_argument_group("Optional arguments")
opt.add_argument("path2", help = "path2")
args = parser.parse_args()
It seems to ask for both arguments. Does anyone have any suggestion as to what is the proper way to do this? One of my thoughts was to do two argument groups; one with the path1 and the other with both paths, but it still requested both.
Thanks guys and gals!
P.S. Python 2.7

Use nargs="?" (with argparse)
import argparse
parser = argparse.ArgumentParser()
mand = parser.add_argument_group("Mandatory arguments")
mand.add_argument("path1", help = "path1")
opt = parser.add_argument_group("Optional arguments")
opt.add_argument("path2", help = "path2", nargs="?")
args = parser.parse_args()
print args
See usage string:
$ python argscript.py
usage: argscript.py [-h] path1 [path2]
argscript.py: error: too few arguments
Or with -h:
$ python argscript.py -h
usage: argscript.py [-h] path1 [path2]
optional arguments:
-h, --help show this help message and exit
Mandatory arguments:
path1 path1
Optional arguments:
path2 path2
and try it with one positional argument only:
$ python argscript.py alfa
Namespace(path1='alfa', path2=None)
And with two:
$ python argscript.py alfa beta
Namespace(path1='alfa', path2='beta')

Sorry for a bit off topic answer. I prefer using docopt so in case it would be of interest for you.
Using docopt
Install docopt first:
$ pip install docopt
and with the script:
"""script.py
Usage:
script.py <fname>...
"""
if __name__ == "__main__":
from docopt import docopt
args = docopt(__doc__)
print args
You can use it this way:
Asking for usage script:
$ python script.py
Usage:
script.py <fname>...
Calling with one file name:
$ python script.py alfa
{'<fname>': ['alfa']}
Using with two names:
$ python script.py alfa beta
{'<fname>': ['alfa', 'beta']}
Playing with alternative usage descriptions:
Following would allow one or two file names, but never three or more:
"""script.py
Usage:
script.py <input> [<output>]
"""
if __name__ == "__main__":
from docopt import docopt
args = docopt(__doc__)
print args

Simple argparse example wanted: 1 argument, 3 results

The documentation for the argparse python module, while excellent I'm sure, is too much for my tiny beginner brain to grasp right now. I don't need to do math on the command line or meddle with formatting lines on the screen or change option characters. All I want to do is "If arg is A, do this, if B do that, if none of the above show help and quit".

Here's the way I do it with argparse (with multiple args):
parser = argparse.ArgumentParser(description='Description of your program')
parser.add_argument('-f','--foo', help='Description for foo argument', required=True)
parser.add_argument('-b','--bar', help='Description for bar argument', required=True)
args = vars(parser.parse_args())
args will be a dictionary containing the arguments:
if args['foo'] == 'Hello':
# code here
if args['bar'] == 'World':
# code here
In your case simply add only one argument.

My understanding of the original question is two-fold. First, in terms of the simplest possible argparse example, I'm surprised that I haven't seen it here. Of course, to be dead-simple, it's also all overhead with little power, but it might get you started.
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("a")
args = parser.parse_args()
if args.a == 'magic.name':
print 'You nailed it!'
But this positional argument is now required. If you leave it out when invoking this program, you'll get an error about missing arguments. This leads me to the second part of the original question. Matt Wilkie seems to want a single optional argument without a named label (the --option labels). My suggestion would be to modify the code above as follows:
...
parser.add_argument("a", nargs='?', default="check_string_for_empty")
...
if args.a == 'check_string_for_empty':
print 'I can tell that no argument was given and I can deal with that here.'
elif args.a == 'magic.name':
print 'You nailed it!'
else:
print args.a
There may well be a more elegant solution, but this works and is minimalist.

The argparse documentation is reasonably good but leaves out a few useful details which might not be obvious. (#Diego Navarro already mentioned some of this but I'll try to expand on his answer slightly.) Basic usage is as follows:
parser = argparse.ArgumentParser()
parser.add_argument('-f', '--my-foo', default='foobar')
parser.add_argument('-b', '--bar-value', default=3.14)
args = parser.parse_args()
The object you get back from parse_args() is a 'Namespace' object: An object whose member variables are named after your command-line arguments. The Namespace object is how you access your arguments and the values associated with them:
args = parser.parse_args()
print (args.my_foo)
print (args.bar_value)
(Note that argparse replaces '-' in your argument names with underscores when naming the variables.)
In many situations you may wish to use arguments simply as flags which take no value. You can add those in argparse like this:
parser.add_argument('--foo', action='store_true')
parser.add_argument('--no-foo', action='store_false')
The above will create variables named 'foo' with value True, and 'no_foo' with value False, respectively:
if (args.foo):
print ("foo is true")
if (args.no_foo is False):
print ("nofoo is false")
Note also that you can use the "required" option when adding an argument:
parser.add_argument('-o', '--output', required=True)
That way if you omit this argument at the command line argparse will tell you it's missing and stop execution of your script.
Finally, note that it's possible to create a dict structure of your arguments using the vars function, if that makes life easier for you.
args = parser.parse_args()
argsdict = vars(args)
print (argsdict['my_foo'])
print (argsdict['bar_value'])
As you can see, vars returns a dict with your argument names as keys and their values as, er, values.
There are lots of other options and things you can do, but this should cover the most essential, common usage scenarios.

Matt is asking about positional parameters in argparse, and I agree that the Python documentation is lacking on this aspect. There's not a single, complete example in the ~20 odd pages that shows both parsing and using positional parameters.
None of the other answers here show a complete example of positional parameters, either, so here's a complete example:
# tested with python 2.7.1
import argparse
parser = argparse.ArgumentParser(description="An argparse example")
parser.add_argument('action', help='The action to take (e.g. install, remove, etc.)')
parser.add_argument('foo-bar', help='Hyphens are cumbersome in positional arguments')
args = parser.parse_args()
if args.action == "install":
print("You asked for installation")
else:
print("You asked for something other than installation")
# The following do not work:
# print(args.foo-bar)
# print(args.foo_bar)
# But this works:
print(getattr(args, 'foo-bar'))
The thing that threw me off is that argparse will convert the named argument "--foo-bar" into "foo_bar", but a positional parameter named "foo-bar" stays as "foo-bar", making it less obvious how to use it in your program.
Notice the two lines near the end of my example -- neither of those will work to get the value of the foo-bar positional param. The first one is obviously wrong (it's an arithmetic expression args.foo minus bar), but the second one doesn't work either:
AttributeError: 'Namespace' object has no attribute 'foo_bar'
If you want to use the foo-bar attribute, you must use getattr, as seen in the last line of my example. What's crazy is that if you tried to use dest=foo_bar to change the property name to something that's easier to access, you'd get a really bizarre error message:
ValueError: dest supplied twice for positional argument
Here's how the example above runs:
$ python test.py
usage: test.py [-h] action foo-bar
test.py: error: too few arguments
$ python test.py -h
usage: test.py [-h] action foo-bar
An argparse example
positional arguments:
action The action to take (e.g. install, remove, etc.)
foo-bar Hyphens are cumbersome in positional arguments
optional arguments:
-h, --help show this help message and exit
$ python test.py install foo
You asked for installation
foo

Yet another summary introduction, inspired by this post.
import argparse
# define functions, classes, etc.
# executes when your script is called from the command-line
if __name__ == "__main__":
parser = argparse.ArgumentParser()
#
# define each option with: parser.add_argument
#
args = parser.parse_args() # automatically looks at sys.argv
#
# access results with: args.argumentName
#
Arguments are defined with combinations of the following:
parser.add_argument( 'name', options... ) # positional argument
parser.add_argument( '-x', options... ) # single-char flag
parser.add_argument( '-x', '--long-name', options... ) # flag with long name
Common options are:
help: description for this arg when --help is used.
default: default value if the arg is omitted.
type: if you expect a float or int (otherwise is str).
dest: give a different name to a flag (e.g. '-x', '--long-name', dest='longName'). Note: by default --long-name is accessed with args.long_name
action: for special handling of certain arguments
store_true, store_false: for boolean args '--foo', action='store_true' => args.foo == True
store_const: to be used with option const '--foo', action='store_const', const=42 => args.foo == 42
count: for repeated options, as in ./myscript.py -vv '-v', action='count' => args.v == 2
append: for repeated options, as in ./myscript.py --foo 1 --foo 2 '--foo', action='append' => args.foo == ['1', '2']
required: if a flag is required, or a positional argument is not.
nargs: for a flag to capture N args ./myscript.py --foo a b => args.foo = ['a', 'b']
choices: to restrict possible inputs (specify as list of strings, or ints if type=int).

Note the Argparse Tutorial in Python HOWTOs. It starts from most basic examples, like this one:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("square", type=int,
help="display a square of a given number")
args = parser.parse_args()
print(args.square**2)
and progresses to less basic ones.
There is an example with predefined choice for an option, like what is asked:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument("square", type=int,
help="display a square of a given number")
parser.add_argument("-v", "--verbosity", type=int, choices=[0, 1, 2],
help="increase output verbosity")
args = parser.parse_args()
answer = args.square**2
if args.verbosity == 2:
print("the square of {} equals {}".format(args.square, answer))
elif args.verbosity == 1:
print("{}^2 == {}".format(args.square, answer))
else:
print(answer)

Here's what I came up with in my learning project thanks mainly to #DMH...
Demo code:
import argparse
def main():
parser = argparse.ArgumentParser()
parser.add_argument('-f', '--flag', action='store_true', default=False) # can 'store_false' for no-xxx flags
parser.add_argument('-r', '--reqd', required=True)
parser.add_argument('-o', '--opt', default='fallback')
parser.add_argument('arg', nargs='*') # use '+' for 1 or more args (instead of 0 or more)
parsed = parser.parse_args()
# NOTE: args with '-' have it replaced with '_'
print('Result:', vars(parsed))
print('parsed.reqd:', parsed.reqd)
if __name__ == "__main__":
main()
This may have evolved and is available online: command-line.py
Script to give this code a workout: command-line-demo.sh

code file: argparseDemo.py
Simple: common case
name(abbr, full), with help
import argparse
argParser = argparse.ArgumentParser()
argParser.add_argument("-n", "--name", help="your name")
args = argParser.parse_args()
print("args=%s" % args)
print("args.name=%s" % args.name)
call
python argparseDemo.py -n Crifan
python argparseDemo.py --name Crifan
output: args=Namespace(name='Crifan') and args.name=Crifan
type
argParser.add_argument("-a", "--age", type=int, help="your current age")
print("type(args.age)=%s" % type(args.age))
call: python argparseDemo.py --age 30
output: type(args.age)=<class 'int'> and args.age=30
required
argParser.add_argument("-a", "--age", required=True, type=int, help="your current age")
call: python argparseDemo.py
output: error argparseDemo.py: error: the following arguments are required: -a/--age
default
argParser.add_argument("-a", "--age", type=int, default=20, help="your current age. Default is 20")
call: python argparseDemo.py
output: args.age=20
choices
argParser.add_argument("-f", "--love-fruit", choices=['apple', 'orange', 'banana'], help="your love fruits")
call: python argparseDemo.py -f apple
output: args=Namespace(love_fruit='apple') and args.love_fruit=apple
multi args
argParser.add_argument("-f", "--love-fruit", nargs=2, help="your love fruits")
call: python argparseDemo.py -f apple orange
output: args.love_fruit=['apple', 'orange']
Detail
most simple: -x
code:
import argparse
argParser = argparse.ArgumentParser()
argParser.add_argument("-a") # most simple -> got args.a, type is `str`
args = argParser.parse_args()
print("args.a=%s" % args.a)
usage = run in command line
python argparseDemo.py -a 30
or: ./argparseDemo.py -a 30
makesure argparseDemo.py is executable
if not, add it: chmod +x argparseDemo.py
output
args.a=30
Note
default type is str
argParser.add_argument("-a") == argParser.add_argument("-a", type=str)
print("type(args.a)=%s" % type(args.a)) -> type(args.a)=<class 'str'>
args type is Namespace
print("type(args)=%s" % type(args)) -> type(args)=<class 'argparse.Namespace'>
args value is Namespace(a='30')
print("args=%s" % args) -> args=Namespace(a='30')
so we can call/use args.a
parameter name
full parameter name: --xxx
code
argParser.add_argument("-a", "--age")
usage
python argparseDemo.py -a 30
or: python argparseDemo.py --age 30
get parsed value: args.age
Note: NOT args.a, and NOT exist args.a
full parameter name with multiple words: --xxx-yyy
code
argParser.add_argument("-a", "--current-age")
get parsed value: args.current_age
add help description: help
code
argParser.add_argument("-a", help="your age") # with help
output
use --help can see description
 python argparseDemo.py --help
usage: argparseDemo.py [-h] [-a A]
optional arguments:
-h, --help show this help message and exit
-a A your age
designate parameter type: type
code
argParser.add_argument("-a", type=int) # parsed arg is `int`, not default `str`
output
print("type(args.a)=%s" % type(args.a)) -> type(args.a)=<class 'int'>
print("args=%s" % args) -> args=Namespace(a=30)
add default value: default
code
argParser.add_argument("-a", type=int, default=20) # if not pass a, a use default value: 20
effect
usage: python argparseDemo.py
output: print("args.age=%s" % args.age) -> args=Namespace(a=20)

You could also use plac (a wrapper around argparse).
As a bonus it generates neat help instructions - see below.
Example script:
#!/usr/bin/env python3
def main(
arg: ('Argument with two possible values', 'positional', None, None, ['A', 'B'])
):
"""General help for application"""
if arg == 'A':
print("Argument has value A")
elif arg == 'B':
print("Argument has value B")
if __name__ == '__main__':
import plac
plac.call(main)
Example output:
No arguments supplied - example.py:
usage: example.py [-h] {A,B}
example.py: error: the following arguments are required: arg
Unexpected argument supplied - example.py C:
usage: example.py [-h] {A,B}
example.py: error: argument arg: invalid choice: 'C' (choose from 'A', 'B')
Correct argument supplied - example.py A :
Argument has value A
Full help menu (generated automatically) - example.py -h:
usage: example.py [-h] {A,B}
General help for application
positional arguments:
{A,B} Argument with two possible values
optional arguments:
-h, --help show this help message and exit
Short explanation:
The name of the argument usually equals the parameter name (arg).
The tuple annotation after arg parameter has the following meaning:
Description (Argument with two possible values)
Type of argument - one of 'flag', 'option' or 'positional' (positional)
Abbreviation (None)
Type of argument value - eg. float, string (None)
Restricted set of choices (['A', 'B'])
Documentation:
To learn more about using plac check out its great documentation:
Plac: Parsing the Command Line the Easy Way

To add to what others have stated:
I usually like to use the 'dest' parameter to specify a variable name and then use 'globals().update()' to put those variables in the global namespace.
Usage:
$ python script.py -i "Hello, World!"
Code:
...
parser.add_argument('-i', '--input', ..., dest='inputted_variable',...)
globals().update(vars(parser.parse_args()))
...
print(inputted_variable) # Prints "Hello, World!"

New to this, but combining Python with Powershell and using this template, being inspired by an in-depth and great Python Command Line Arguments – Real Python
There is a lot you can do within the init_argparse() and I am covering just the most simple scenario here.
import argparse
use if __name__ == "__main__": main() pattern to execute from terminal
parse arguments within the main() function that has no parameters as all
define a init_argparse() function
create a parser object by calling argparse.ArgumentParser()
declare one or more argumnent with parser.add_argument("--<long_param_name>")
return parser
parse args by creating an args object by calling parser.parse_args()
define a function proper with param1, param2, ...
call function_proper with params being assigned as attributes of an args object
e.g. function_proper(param1=args.param1, param2=args.param2)
within a shell call the module with named arguments:
e.g. python foobar.py --param1="foo" --param2=="bar"
#file: foobar.py
import argparse
def function_proper(param1, param2):
#CODE...
def init_argparse() -> argparse.ArgumentParser:
parser = argparse.ArgumentParser()
parser.add_argument("--param1")
parser.add_argument("--param2")
return parser
def main() -> None:
parser = init_argparse()
args = parser.parse_args()
function_proper(param1=args.param1, param2=args.param2)
if __name__ == "__main__":
main()
>>> python .\foobar.py --param1="foo" --param2=="bar"

I went through all the examples and answers and in a way or another they didn't address my need. So I will list her a scenario that I need more help and I hope this can explain the idea more.
Initial Problem
I need to develop a tool which is getting a file to process it and it needs some optional configuration file to be used to configure the tool.
so what I need is something like the following
mytool.py file.text -config config-file.json
The solution
Here is the solution code
import argparse
def main():
parser = argparse.ArgumentParser(description='This example for a tool to process a file and configure the tool using a config file.')
parser.add_argument('filename', help="Input file either text, image or video")
# parser.add_argument('config_file', help="a JSON file to load the initial configuration ")
# parser.add_argument('-c', '--config_file', help="a JSON file to load the initial configuration ", default='configFile.json', required=False)
parser.add_argument('-c', '--config', default='configFile.json', dest='config_file', help="a JSON file to load the initial configuration " )
parser.add_argument('-d', '--debug', action="store_true", help="Enable the debug mode for logging debug statements." )
args = parser.parse_args()
filename = args.filename
configfile = args.config_file
print("The file to be processed is", filename)
print("The config file is", configfile)
if args.debug:
print("Debug mode enabled")
else:
print("Debug mode disabled")
print("and all arguments are: ", args)
if __name__ == '__main__':
main()
I will show the solution in multiple enhancements to show the idea
First Round: List the arguments
List all input as mandatory inputs so second argument will be
parser.add_argument('config_file', help="a JSON file to load the initial configuration ")
When we get the help command for this tool we find the following outcome
(base) > python .\argparser_example.py -h
usage: argparser_example.py [-h] filename config_file
This example for a tool to process a file and configure the tool using a config file.
positional arguments:
filename Input file either text, image or video
config_file a JSON file to load the initial configuration
optional arguments:
-h, --help show this help message and exit
and when I execute it as the following
(base) > python .\argparser_example.py filename.txt configfile.json
the outcome will be
The file to be processed is filename.txt
The config file is configfile.json
and all arguments are: Namespace(config_file='configfile.json', filename='filename.txt')
But the config file should be optional, I removed it from the arguments
(base) > python .\argparser_example.py filename.txt
The outcome will be is:
usage: argparser_example.py [-h] filename config_file
argparser_example.py: error: the following arguments are required: c
Which means we have a problem in the tool
Second Round : Make it optimal
So to make it optional I modified the program as follows
parser.add_argument('-c', '--config', help="a JSON file to load the initial configuration ", default='configFile.json', required=False)
The help outcome should be
usage: argparser_example.py [-h] [-c CONFIG] filename
This example for a tool to process a file and configure the tool using a config file.
positional arguments:
filename Input file either text, image or video
optional arguments:
-h, --help show this help message and exit
-c CONFIG, --config CONFIG
a JSON file to load the initial configuration
so when I execute the program
(base) > python .\argparser_example.py filename.txt
the outcome will be
The file to be processed is filename.txt
The config file is configFile.json
and all arguments are: Namespace(config_file='configFile.json', filename='filename.txt')
with arguments like
(base) > python .\argparser_example.py filename.txt --config_file anotherConfig.json
The outcome will be
The file to be processed is filename.txt
The config file is anotherConfig.json
and all arguments are: Namespace(config_file='anotherConfig.json', filename='filename.txt')
Round 3: Enhancements
to change the flag name from --config_file to --config while we keep the variable name as is we modify the code to include dest='config_file' as the following:
parser.add_argument('-c', '--config', help="a JSON file to load the initial configuration ", default='configFile.json', dest='config_file')
and the command will be
(base) > python .\argparser_example.py filename.txt --config anotherConfig.json
To add the support for having a debug mode flag, we need to add a flag in the arguments to support a boolean debug flag. To implement it i added the following:
parser.add_argument('-d', '--debug', action="store_true", help="Enable the debug mode for logging debug statements." )
the tool command will be:
(carnd-term1-38) > python .\argparser_example.py image.jpg -c imageConfig,json --debug
the outcome will be
The file to be processed is image.jpg
The config file is imageConfig,json
Debug mode enabled
and all arguments are: Namespace(config_file='imageConfig,json', debug=True, filename='image.jpg')

A really simple way to use argparse and amend the '-h'/ '--help' switches to display your own personal code help instructions is to set the default help to False, you can also add as many additional .add_arguments as you like:
import argparse
parser = argparse.ArgumentParser(add_help=False)
parser.add_argument('-h', '--help', action='help',
help='To run this script please provide two arguments')
parser.parse_args()
Run: python test.py -h
Output:
usage: test.py [-h]
optional arguments:
-h, --help To run this script please provide two arguments

As an addition to existing answers, if you are lazy enough, it is possible to use code generation tool called protoargs. It generates arguments parser from the configuration. For python it uses argparse.
Configuration with optional A and B:
syntax = "proto2";
message protoargs
{
optional string A = 1; // A param description
optional string B = 2; // B param description
}//protoargs
Configuration with required A and B:
syntax = "proto2";
message protoargs
{
required string A = 1; // A param description
required string B = 2; // B param description
}//protoargs
Configuration with positional A and B:
syntax = "proto2";
message protoargs
{
required string A = 1; // A param description
required string B = 2; // B param description
}//protoargs
message protoargs_links
{
}//protoargs_links
Now all you should run is:
python ./protoargs.py -i test.proto -o . --py
And use it (it is possible to take other examples here):
import sys
import test_pa
class ArgsParser:
program = "test"
description = "Simple A and B parser test."
def parse(self, argv):
self.config = test_pa.parse(self.program, self.description, argv)
def usage(self):
return test_pa.usage(self.program, self.description)
if __name__ == "__main__":
parser = ArgsParser()
if len(sys.argv) == 1:
print(parser.usage())
else:
parser.parse(sys.argv[1:])
if parser.config.A:
print(parser.config.A)
if parser.config.B:
print(parser.config.B)
If you want more - change configuration, regenerate parser, use an updated parser.config.
UPD: As mentioned in rules, I must specify that this is my own project

code:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument('-A', default=False, action='store_true')
parser.add_argument('-B', default=False, action='store_true')
args=parser.parse_args()
if args.A:
print('do this')
elif args.B:
print('do that')
else:
print('help')
running result:
$ python3 test.py
help
$ python3 test.py -A
do this
$ python3 test.py -B
do that
$ python3 test.py -C
usage: test.py [-h] [-A] [-B]
test.py: error: unrecognized arguments: -C
As for the original request (if A ....), I would use argv to solve it, not using argparse at all:
import sys
if len(sys.argv)==2:
if sys.argv[1] == 'A':
print('do this')
elif sys.argv[1] == 'B':
print('do that')
else:
print('help')
else:
print('help')

Since you have not clarified wheather the arguments 'A' and 'B' are positional or optional, I'll make a mix of both.
Positional arguments are required by default. If not giving one will throw 'Few arguments given' which is not the case for the optional arguments going by their name. This program will take a number and return its square by default, if the cube option is used it shall return its cube.
import argparse
parser = argparse.ArgumentParser('number-game')
parser.add_argument(
"number",
type=int,
help="enter a number"
)
parser.add_argument(
"-c", "--choice",
choices=['square','cube'],
help="choose what you need to do with the number"
)
# all the results will be parsed by the parser and stored in args
args = parser.parse_args()
# if square is selected return the square, same for cube
if args.c == 'square':
print("{} is the result".format(args.number**2))
elif args.c == 'cube':
print("{} is the result".format(args.number**3))
else:
print("{} is not changed".format(args.number))
usage
$python3 script.py 4 -c square
16
Here the optional arguments are taking value, if you just wanted to use it like a flag you can too. So by using -s for square and -c for cube we change the behaviour, by adding action = "store_true". It is changed to true only when used.
parser.add_argument(
"-s", "--square",
help="returns the square of number",
action="store_true"
)
parser.add_argument(
"-c", "--cube",
help="returns the cube of number",
action="store_true"
)
so the conditional block can be changed to,
if args.s:
print("{} is the result".format(args.number**2))
elif args.c:
print("{} is the result".format(args.number**3))
else:
print("{} is not changed".format(args.number))
usage
$python3 script.py 4 -c
64

The simplest answer!
P.S. the one who wrote the document of argparse is foolish
python code:
import argparse
parser = argparse.ArgumentParser(description='')
parser.add_argument('--o_dct_fname',type=str)
parser.add_argument('--tp',type=str)
parser.add_argument('--new_res_set',type=int)
args = parser.parse_args()
o_dct_fname = args.o_dct_fname
tp = args.tp
new_res_set = args.new_res_set
running code
python produce_result.py --o_dct_fname o_dct --tp father_child --new_res_set 1