argparse

Wed, 04 Jan 2023 00:00:00 +0000

Basic usage

import argparse

def get_args_parser():
 # set up a parser
 parser = argparse.ArgumentParser(description=f"{program_description}")

 # define parsing rules and operations for different arguments 
 # parser.add_argument()
 # ...

 return parser

if __name__ == '__main__':
 parser = get_parser()

 # parse command line 
 args = parser.parse_args()

 # further operations based on parsed arguments

`add_argument()`

Parameters:

name or flags - Either a name or a list of option strings, e.g. foo or -f, --foo.
action - The basic type of action to be taken when this argument is encountered at the command line.
nargs - The number of command-line arguments that should be consumed.
const - A constant value required by some action and nargs selections.
default - The value produced if the argument is absent from the command line.
type - The type to which the command-line argument should be converted.
choices - A container of the allowable values for the argument.
required - Whether or not the command-line option may be omitted (optionals only).
help - A brief description of what the argument does.
metavar - A name for the argument in usage messages.
dest - The name of the attribute to be added to the object returned by parse_args().

name or flags

The add_argument() method must know whether an optional argument, or a positional argument is expected. Therefore, the first arguments passed to add_argument() must be either a series of flags, or a simple argument name.

When parse_args() is called, optional arguments will be identified by the - prefix, and the remaining arguments will be assumed to be positional.

For example, an optional argument could be created like:

parser.add_argument('-f', '--foo')

A positional argument could be created like:

parser.add_argument('bar')

action

The action keyword argument specifies how the command-line arguments should be
handled. The supplied actions are:
- 'store'
  
  This just stores the argument’s value.
- 'store_const'
  
  This stores the value specified by the const keyword argument. The 'store_const' action is most commonly used with optional arguments that specify some sort of flag
- 'store_true' and 'store_false'
  
  These are special cases of 'store_const' used for storing the values True and False respectively. In addition, they create default values of False and True respectively.
- 'append'
  
  This stores a list, and appends each argument value to the list. This is useful to allow an option to be specified multiple times.
- 'append_const'
  
  This stores a list, and appends the value specified by the const keyword argument to the list.
- 'count'
  
  This counts the number of times a keyword argument occurs.
- 'help'
  
  This prints a complete help message for all the options in the current parser and then exits. By default a help action is automatically added to the parser.
- 'version'
  
  This expects a version= keyword argument in the add_argument() call, and prints version information and exits when invoked
- 'extend'
  
  This stores a list, and extends each argument value to the list.

nargs

The nargs specifies the number of command-line arguments that should be consumed.

Supported value	Meaning
`N`	The absolute number of arguments (e.g., `3`).
`?`	0 or 1 argument
`*`	0 or all arguments
`+`	All, and at least one, argument

const

The const argument of add_argument() is used to hold constant values that are not read from the command line but are required for the various ArgumentParser actions.

default

All optional arguments and some positional arguments may be omitted at the command line. The default keyword argument of add_argument() specifies what value should be used if the command-line argument is not present.

type

By default, ArgumentParser objects read command-line arguments in as simple strings. The type keyword argument of add_argument() allows any necessary type-checking and type conversions to be performed.

choices

Some command-line arguments should be selected from a restricted set of values.
These can be handled by passing a container object as the choices keyword argument to add_argument().
When the command line is parsed, argument values will be checked, and an error message will be displayed if the argument was not one of the acceptable values

required

An argument is made required with the required option. If the required argument is not given, an error will be raised.

help

The help value is a string containing a brief description of the argument.
When a user requests help (usually by using -h or --help at the command line), these help descriptions will be displayed with each argument:

metavar

When ArgumentParser generates help messages, it needs some way to refer to each expected argument.
By default
- ArgumentParser objects use the dest value as the “name” of each object.
- for positional argument actions, the dest value is used directly
  - E.g. a single positional argument with dest='bar' will be referred to as bar
- for optional argument actions, the dest value is uppercased
  - E.g. A single optional argument --foo that should be followed by a single command-line argument will be referred to as FOO.

The metavar option gives the argument a name to the expected value displayed in error and help outputs.

dest

Most ArgumentParser actions add some value as an attribute of the object returned by parse_args(). The name of this attribute is determined by the dest keyword argument of add_argument().
- For positional argument actions, dest is normally supplied as the first argument to add_argument()
  
  E.g.
```
parser.add_argument('bar')
```
  The value of dest is bar
- For optional argument actions, the value of dest is normally inferred from the option strings.
  - ArgumentParser generates the value of dest by taking the first long option string and stripping away the initial -- string.
  - If no long option strings were supplied, dest will be derived from the first short option string by stripping the initial - character.
  - Any internal - characters will be converted to _ characters to make sure the string is a valid attribute name.
  E.g.
```
parser.add_argument("-n", "--name", type=str, required=True, help="Name of the person")
```
  The value of dest is name. After calling
```
args = parser.parse_args()
```
  we can use the argument with args.name

Example

Assume we have a python file person-info.py:

import argparse


def get_args_parser():
 parser = argparse.ArgumentParser(description="Person info program")
 parser.add_argument("-n", "--name", type=str, required=True, help="Name of the person")
 parser.add_argument("-a", "--age", type=int, required=True, help="Age of the person")
 parser.add_argument(
 "-g",
 metavar="gender",
 dest="gender",
 default="male",
 choices=["male, female"],
 help="Gender of the person. (male, female)",
 )
 parser.add_argument("-s", "--single", dest="is_single", action="store_true", help="Is the person single?")
 parser.add_argument("-i", "--interests", metavar="interests", action="append", help="Interests of the person")
 parser.add_argument(
 "-e", metavar="education", dest="education", nargs="+", help="Educational experience of the person"
 )

 return parser


if __name__ == "__main__":
 parser = get_args_parser()
 args = parser.parse_args()

 print(f"Name: {args.name}")
 print(f"Age: {args.age}")
 print(f"Gender: {args.gender}")
 print(f"Is single?: {args.is_single}")
 print(f"Interests: {args.interests}")
 print(f"Education: {args.education}")

Print the help information:

$ python person-info.py -h

usage: person-info.py [-h] -n NAME -a AGE [-g gender] [-s] [-i interests]
 [-e education [education ...]]

Person info program

optional arguments:
 -h, --help show this help message and exit
 -n NAME, --name NAME Name of the person
 -a AGE, --age AGE Age of the person
 -g gender Gender of the person. (male, female)
 -s, --single Is the person single?
 -i interests, --interests interests
 Interests of the person
 -e education [education ...]
 Educational experience of the person

Run the script by given necesary arguments:

$ python person-info.py -n Ecko -a 26 -s -i coding -e KIT KIT -i basketball -i fitness

Name: Ecko
Age: 26
Gender: male
Is single?: True
Interests: ['coding', 'basketball', 'fitness']
Education: ['KIT', 'KIT']

Violation by not given age arguement -a :

python person-info.py -n Ecko -s -i coding -e KIT KIT -i basketball -i fitness

usage: person-info.py [-h] -n NAME -a AGE [-g gender] [-s] [-i interests]
 [-e education [education ...]]
person-info.py: error: the following arguments are required: -a/--age

Reference

Python documentation: argparse — Parser for command-line options, arguments and sub-commands¶
Python argparse tutorial
argparse简要用法总结
argparse – Command line option and argument parsing

Argparse | Haobin Tan