Using argparse for mandatory argument without prefix

Question

I'm trying to use argparse module within my python application. My application should be run with single mandatory argument without any prefixes. I could not come up with a way to do it.

Answer 1

Here is a simple example using argparse to require exactly one integer argument:

import argparse

parser = argparse.ArgumentParser(description='process an integer')
parser.add_argument('integer', metavar='N', type=int, nargs=1,
               help='an integer')
args = parser.parse_args()
print(args.integer)

Saving this code in argparse1.py and running it gives:

$ python argparse1.py 5
[5]  

$ python argparse1.py
usage: argparse1.py [-h] N
argpars1.py: error: the following arguments are required: N

$ python argparse1.py 5 7
usage: argparse1.py [-h] N
argparse1.py: error: unrecognized arguments: 7

$ python argparse1.py test
usage: argparse1.py N
argparse1.py: error: argument N: invalid int value: 'test'

$ python argparse1.py -h
usage: argparse1.py [-h] N

process an integer

positional arguments:
  N           an integer

optional arguments:
  -h, --help  show this help message and exit

To remove the optional arguments define ArgumentParser with add_help=False as follows:

import argparse

parser = argparse.ArgumentParser(add_help=False)
parser.add_argument('integer', metavar='N', type=int, nargs=1)
args = parser.parse_args()
print(args.integer)

Putting this code in argparse2.py and testing it gives:

$ python argparse2.py
usage: argparse2.py N
argparse2.py: error: the following arguments are required: N

$ python argparse2.py 5
[5]

$ python argparse2.py 5 7
usage: argparse2.py N
argparse2.py: error: unrecognized arguments: 7

$ python argparse2.py hello
usage: argparse2.py N
argparse2.py: error: argument N: invalid int value: 'hello'

$ python argparse2.py -h
usage: argparse2.py N
argparse2.py: error: the following arguments are required: N

All usage messages can be suppressed by importing argparse.SUPPRESS and configuring ArgumentParser with usage=SUPPRESS as follows:

from argparse import ArgumentParser,SUPPRESS

parser = ArgumentParser(add_help=False, usage=SUPPRESS)
parser.add_argument('integer', metavar='N', type=int, nargs=1)
args = parser.parse_args()
print(args.integer)

Saving this code in argparse3.py and running it gives:

$ python argparse3.py 5
[5]

$ python argparse3.py 5 7 9
argparse3.py: error: unrecognized arguments: 7 9

$ python argparse3.py -h
argparse3.py: error: the following arguments are required: N

The "unrecognized arguments" error is hardcoded in ArgumentParser.parse_args() (https://hg.python.org/cpython/file/3.4/Lib/argparse.py#l1727) and the "following arguments are required" error is separately hardcoded (https://hg.python.org/cpython/file/3.4/Lib/argparse.py#l1994) both without recourse to modification from the API. The latter appears to be in _parse_known_args(self, arg_strings, namespace) that I suppose could be overridden but would involve copying about 250 lines of code in order to modify 1 line to change that error message.

However, it is possible to avoid these error messages by adding just one argument with nargs='*' and no type, which effectively allows all command lines, and then adding custom error handling of arguments including your own error and usage messages. For example:

import os
import sys
from argparse import ArgumentParser,SUPPRESS

script =  os.path.basename(sys.argv[0])
usage = 'usage: ' + script + ' n (int)'

parser = ArgumentParser(add_help=False, usage=SUPPRESS)
parser.add_argument('integer', nargs='*')
args = parser.parse_args()

if (len(args.integer) == 0):
    print('error: no argument provided\n', usage, file=sys.stderr, sep = '')
    sys.exit(1)


if (len(args.integer) > 1):
    print('error: too many arguments\n', usage, file=sys.stderr, sep = '')
    sys.exit(1)

v = args.integer[0]

try:
    v = int(v)
except ValueError:
    print("error: '", v, "'", ' is not an integer\n', usage, file=sys.stderr, sep='')
    sys.exit(1)

print(v + 2)

Putting this code in argparse4.py and testing it gives:

$ python argparse4.py 5
7

$ python argparse4.py
error: no argument provided
usage: argparse4.py n (int)

$ python argparse4.py 5 9
error: too many arguments
usage: argparse4.py n (int)

$ python argparse4.py hello
error: 'hello' is not an integer
usage: argparse4.py n (int)