Python Argparse

See the official docs for the full API.

Basic Parsing

import argparse

parser = argparse.ArgumentParser(description='Simple CLI')
parser.add_argument("name")

args = parser.parse_args() # Returns a Namespace
print(f"Hello {args.name}")

Produces:

$ python3 cli.py
usage: cli.py [-h] name
cli.py: error: the following arguments are required: name
$ python3 cli.py --help
usage: cli.py [-h] name

Simple CLI

positional arguments:
  name

optional arguments:
  -h, --help  show this help message and exit
$ python3 cli.py Hoani
Hello Hoani

Optional Arguments

Optional arguments are prefixed by --:

parser = argparse.ArgumentParser(description='Simple CLI')
parser.add_argument("name")
parser.add_argument("--age")

args = parser.parse_args()
print(f"Hello {args.name}", f"you are {args.age}" \
      if args.age else "")

Produces:

$ python3 cli.py Hoani --age 31
Hello Hoani you are 31
$ python3 cli.py Hoani
Hello Hoani

Argument options

• default - if the arg isn’t specified, this value is used
• type - specify the type
• help - help string
• nargs - the number of arguments
  • * => zero or more
  • + => one or more
  • 1..N => explicit number of args
    • all of the above values turns an argument into a List
• dest - change the name in the returned Namespace
• metavar - change the name used in help messages
• choices - limit what values can be used
  • e.g. choices=['tcp','bluetooth','uart']

Subparsers

Sometimes we want to write a cli which provides several services. For example, a simple remote get/set API.

python3 cli.py [ADDR] get [NAME] 
python3 cli.py [ADDR] set [NAME] [VALUE] 

import argparse

parser = argparse.ArgumentParser(description='Control')
parser.add_argument("addr")
subparsers = parser.add_subparsers(dest="command")
parser_get = subparsers.add_parser('get')
parser_get.add_argument("name")
parser_set = subparsers.add_parser('set')
parser_set.add_argument("name")
parser_set.add_argument("value")

args = parser.parse_args()
print(args)

Produces:

$ python3 cli.py localhost:1234         
Namespace(addr='localhost:1234', command=None)
$ python3 cli.py localhost:1234 get led
Namespace(addr='localhost:1234', command='get', name='led')
$ python3 cli.py localhost:1234 set led ON
Namespace(addr='localhost:1234', command='set', name='led', 
value='ON')

Unit testing a parser

When testing, we can pass a string of arguments to parse_args:

parser.parse_args(["hoani", "--age", "32"])