argparse – Command line option and argument parsing.

Purpose:Command line option and argument parsing.
Available In:2.7 and later

The argparse module was added to Python 2.7 as a replacement for optparse. The implementation of argparse supports features that would not have been easy to add to optparse, and that would have required backwards-incompatible API changes, so a new module was brought into the library instead. optparse is still supported, but is not likely to receive new features.

Comparing with optparse

The API for argparse is similar to the one provided by optparse, and in many cases argparse can be used as a straightforward replacement by updating the names of the classes and methods used. There are a few places where direct compatibility could not be preserved as new features were added, however.

You will have to decide whether to upgrade existing programs on a case-by-case basis. If you have written extra code to work around limitations of optparse, you may want to upgrade to reduce the amount of code you need to maintain. New programs should probably use argparse, if it is available on all deployment platforms.

Setting up a Parser

The first step when using argparse is to create a parser object and tell it what arguments to expect. The parser can then be used to process the command line arguments when your program runs.

The parser class is ArgumentParser. The constructor takes several arguments to set up the description used in the help text for the program and other global behaviors or settings.

import argparse
parser = argparse.ArgumentParser(description='This is a PyMOTW sample program')

Defining Arguments

argparse is a complete argument processing library. Arguments can trigger different actions, specified by the action argument to add_argument(). Supported actions include storing the argument (singly, or as part of a list), storing a constant value when the argument is encountered (including special handling for true/false values for boolean switches), counting the number of times an argument is seen, and calling a callback.

The default action is to store the argument value. In this case, if a type is provided, the value is converted to that type before it is stored. If the dest argument is provided, the value is saved to an attribute of that name on the Namespace object returned when the command line arguments are parsed.

Parsing a Command Line

Once all of the arguments are defined, you can parse the command line by passing a sequence of argument strings to parse_args(). By default, the arguments are taken from sys.argv[1:], but you can also pass your own list. The options are processed using the GNU/POSIX syntax, so option and argument values can be mixed in the sequence.

The return value from parse_args() is a Namespace containing the arguments to the command. The object holds the argument values as attributes, so if your argument dest is "myoption", you access the value as args.myoption.

Simple Examples

Here is a simple example with 3 different options: a boolean option (-a), a simple string option (-b), and an integer option (-c).

import argparse

parser = argparse.ArgumentParser(description='Short sample app')

parser.add_argument('-a', action="store_true", default=False)
parser.add_argument('-b', action="store", dest="b")
parser.add_argument('-c', action="store", dest="c", type=int)

print parser.parse_args(['-a', '-bval', '-c', '3'])

There are a few ways to pass values to single character options. The example above uses two different forms, -bval and -c val.

$ python argparse_short.py

Namespace(a=True, b='val', c=3)

The type of the value associated with 'c' in the output is an integer, since the ArgumentParser was told to convert the argument before storing it.

“Long” option names, with more than a single character in their name, are handled in the same way.

import argparse

parser = argparse.ArgumentParser(description='Example with long option names')

parser.add_argument('--noarg', action="store_true", default=False)
parser.add_argument('--witharg', action="store", dest="witharg")
parser.add_argument('--witharg2', action="store", dest="witharg2", type=int)

print parser.parse_args([ '--noarg', '--witharg', 'val', '--witharg2=3' ])

And the results are similar:

$ python argparse_long.py

Namespace(noarg=True, witharg='val', witharg2=3)

One area in which argparse differs from optparse is the treatment of non-optional argument values. While optparse sticks to option parsing, argparse is a full command-line argument parser tool, and handles non-optional arguments as well.

import argparse

parser = argparse.ArgumentParser(description='Example with non-optional arguments')

parser.add_argument('count', action="store", type=int)
parser.add_argument('units', action="store")

print parser.parse_args()

In this example, the “count” argument is an integer and the “units” argument is saved as a string. If either is not provided on the command line, or the value given cannot be converted to the right type, an error is reported.

$ python argparse_arguments.py 3 inches

Namespace(count=3, units='inches')

$ python argparse_arguments.py some inches

usage: argparse_arguments.py [-h] count units
argparse_arguments.py: error: argument count: invalid int value: 'some'

$ python argparse_arguments.py

usage: argparse_arguments.py [-h] count units
argparse_arguments.py: error: too few arguments

Argument Actions

There are six built-in actions that can be triggered when an argument is encountered:

store
Save the value, after optionally converting it to a different type. This is the default action taken if none is specified expliclity.
store_const
Save a value defined as part of the argument specification, rather than a value that comes from the arguments being parsed. This is typically used to implement command line flags that aren’t booleans.
store_true / store_false
Save the appropriate boolean value. These actions are used to implement boolean switches.
append
Save the value to a list. Multiple values are saved if the argument is repeated.
append_const
Save a value defined in the argument specification to a list.
version
Prints version details about the program and then exits.
import argparse

parser = argparse.ArgumentParser()

parser.add_argument('-s', action='store', dest='simple_value',
                    help='Store a simple value')

parser.add_argument('-c', action='store_const', dest='constant_value',
                    const='value-to-store',
                    help='Store a constant value')

parser.add_argument('-t', action='store_true', default=False,
                    dest='boolean_switch',
                    help='Set a switch to true')
parser.add_argument('-f', action='store_false', default=False,
                    dest='boolean_switch',
                    help='Set a switch to false')

parser.add_argument('-a', action='append', dest='collection',
                    default=[],
                    help='Add repeated values to a list',
                    )

parser.add_argument('-A', action='append_const', dest='const_collection',
                    const='value-1-to-append',
                    default=[],
                    help='Add different values to list')
parser.add_argument('-B', action='append_const', dest='const_collection',
                    const='value-2-to-append',
                    help='Add different values to list')

parser.add_argument('--version', action='version', version='%(prog)s 1.0')

results = parser.parse_args()
print 'simple_value     =', results.simple_value
print 'constant_value   =', results.constant_value
print 'boolean_switch   =', results.boolean_switch
print 'collection       =', results.collection
print 'const_collection =', results.const_collection
$ python argparse_action.py -h

usage: argparse_action.py [-h] [-s SIMPLE_VALUE] [-c] [-t] [-f]
                          [-a COLLECTION] [-A] [-B] [--version]

optional arguments:
  -h, --help       show this help message and exit
  -s SIMPLE_VALUE  Store a simple value
  -c               Store a constant value
  -t               Set a switch to true
  -f               Set a switch to false
  -a COLLECTION    Add repeated values to a list
  -A               Add different values to list
  -B               Add different values to list
  --version        show program's version number and exit

$ python argparse_action.py -s value

simple_value     = value
constant_value   = None
boolean_switch   = False
collection       = []
const_collection = []

$ python argparse_action.py -c

simple_value     = None
constant_value   = value-to-store
boolean_switch   = False
collection       = []
const_collection = []

$ python argparse_action.py -t

simple_value     = None
constant_value   = None
boolean_switch   = True
collection       = []
const_collection = []

$ python argparse_action.py -f

simple_value     = None
constant_value   = None
boolean_switch   = False
collection       = []
const_collection = []

$ python argparse_action.py -a one -a two -a three

simple_value     = None
constant_value   = None
boolean_switch   = False
collection       = ['one', 'two', 'three']
const_collection = []

$ python argparse_action.py -B -A

simple_value     = None
constant_value   = None
boolean_switch   = False
collection       = []
const_collection = ['value-2-to-append', 'value-1-to-append']

$ python argparse_action.py --version

argparse_action.py 1.0

Option Prefixes

The default syntax for options is based on the Unix convention of signifying command line switches using a prefix of “-”. argparse supports other prefixes, so you can make your program conform to the local platform default (i.e., use “/” on Windows) or follow a different convention.

import argparse

parser = argparse.ArgumentParser(description='Change the option prefix characters',
                                 prefix_chars='-+/',
                                 )

parser.add_argument('-a', action="store_false", default=None,
                    help='Turn A off',
                    )
parser.add_argument('+a', action="store_true", default=None,
                    help='Turn A on',
                    )
parser.add_argument('//noarg', '++noarg', action="store_true", default=False)

print parser.parse_args()

Set the prefix_chars parameter for the ArgumentParser to a string containing all of the characters that should be allowed to signify options. It is important to understand that although prefix_chars establishes the allowed switch characters, the individual argument definitions specify the syntax for a given switch. This gives you explicit control over whether options using different prefixes are aliases (such as might be the case for platform-independent command line syntax) or alternatives (e.g., using “+” to indicate turning a switch on and “-” to turn it off). In the example above, +a and -a are separate arguments, and //noarg can also be given as ++noarg, but not --noarg.

$ python argparse_prefix_chars.py -h

usage: argparse_prefix_chars.py [-h] [-a] [+a] [//noarg]

Change the option prefix characters

optional arguments:
  -h, --help        show this help message and exit
  -a                Turn A off
  +a                Turn A on
  //noarg, ++noarg

$ python argparse_prefix_chars.py +a

Namespace(a=True, noarg=False)

$ python argparse_prefix_chars.py -a

Namespace(a=False, noarg=False)

$ python argparse_prefix_chars.py //noarg

Namespace(a=None, noarg=True)

$ python argparse_prefix_chars.py ++noarg

Namespace(a=None, noarg=True)

$ python argparse_prefix_chars.py --noarg

usage: argparse_prefix_chars.py [-h] [-a] [+a] [//noarg]
argparse_prefix_chars.py: error: unrecognized arguments: --noarg

Sources of Arguments

In the examples so far, the list of arguments given to the parser have come from a list passed in explicitly, or were taken implicitly from sys.argv. Passing the list explicitly is useful when you are using argparse to process command line-like instructions that do not come from the command line (such as in a configuration file).

import argparse
from ConfigParser import ConfigParser
import shlex

parser = argparse.ArgumentParser(description='Short sample app')

parser.add_argument('-a', action="store_true", default=False)
parser.add_argument('-b', action="store", dest="b")
parser.add_argument('-c', action="store", dest="c", type=int)

config = ConfigParser()
config.read('argparse_witH_shlex.ini')
config_value = config.get('cli', 'options')
print 'Config  :', config_value

argument_list = shlex.split(config_value)
print 'Arg List:', argument_list

print 'Results :', parser.parse_args(argument_list)

shlex makes it easy to split the string stored in the configuration file.

$ python argparse_with_shlex.py

Config  : -a -b 2
Arg List: ['-a', '-b', '2']
Results : Namespace(a=True, b='2', c=None)

An alternative to processing the configuration file yourself is to tell argparse how to recognize an argument that specifies an input file containing a set of arguments to be processed using fromfile_prefix_chars.

import argparse
from ConfigParser import ConfigParser
import shlex

parser = argparse.ArgumentParser(description='Short sample app',
                                 fromfile_prefix_chars='@',
                                 )

parser.add_argument('-a', action="store_true", default=False)
parser.add_argument('-b', action="store", dest="b")
parser.add_argument('-c', action="store", dest="c", type=int)

print parser.parse_args(['@argparse_fromfile_prefix_chars.txt'])

This example stops when it finds an argument prefixed with @, then reads the named file to find more arguments. For example, an input file argparse_fromfile_prefix_chars.txt contains a series of arguments, one per line:

-a
-b
2

The output produced when processing the file is:

$ python argparse_fromfile_prefix_chars.py

Namespace(a=True, b='2', c=None)

Automatically Generated Options

argparse will automatically add options to generate help and show the version information for your application, if configured to do so.

The add_help argument to ArgumentParser controls the help-related options.

import argparse

parser = argparse.ArgumentParser(add_help=True)

parser.add_argument('-a', action="store_true", default=False)
parser.add_argument('-b', action="store", dest="b")
parser.add_argument('-c', action="store", dest="c", type=int)

print parser.parse_args()

The help options (-h and --help) are added by default, but can be disabled by setting add_help to false.

import argparse

parser = argparse.ArgumentParser(add_help=False)

parser.add_argument('-a', action="store_true", default=False)
parser.add_argument('-b', action="store", dest="b")
parser.add_argument('-c', action="store", dest="c", type=int)

print parser.parse_args()

Although -h and --help are defacto standard option names for requesting help, some applications or uses of argparse either don’t need to provide help or need to use those option names for other purposes.

$ python argparse_with_help.py -h

usage: argparse_with_help.py [-h] [-a] [-b B] [-c C]

optional arguments:
  -h, --help  show this help message and exit
  -a
  -b B
  -c C

$ python argparse_without_help.py -h

usage: argparse_without_help.py [-a] [-b B] [-c C]
argparse_without_help.py: error: unrecognized arguments: -h

The version options (-v and --version) are added when version is set in the ArgumentParser constructor.

import argparse

parser = argparse.ArgumentParser(version='1.0')

parser.add_argument('-a', action="store_true", default=False)
parser.add_argument('-b', action="store", dest="b")
parser.add_argument('-c', action="store", dest="c", type=int)

print parser.parse_args()

print 'This is not printed'

Both forms of the option print the program’s version string, then cause it to exit immediately.

$ python argparse_with_version.py -h

usage: argparse_with_version.py [-h] [-v] [-a] [-b B] [-c C]

optional arguments:
  -h, --help     show this help message and exit
  -v, --version  show program's version number and exit
  -a
  -b B
  -c C

$ python argparse_with_version.py -v

1.0

$ python argparse_with_version.py --version

1.0

Parser Organization

argparse includes several features for organizing your argument parsers, to make implementation easier or to improve the usability of the help output.

Sharing Parser Rules

It is common to need to implement a suite of command line programs that all take a set of arguments, and then specialize in some way. For example, if the programs all need to authenticate the user before taking any real action, they would all need to support --user and --password options. Rather than add the options explicitly to every ArgumentParser, you can define a “parent” parser with the shared options, and then have the parsers for the individual programs inherit from its options.

The first step is to set up the parser with the shared argument definitions. Since each subsequent user of the parent parser is going to try to add the same help options, causing an exception, we turn off automatic help generation in the base parser.

import argparse

parser = argparse.ArgumentParser(add_help=False)

parser.add_argument('--user', action="store")
parser.add_argument('--password', action="store")

Next, create another parser with parents set:

import argparse
import argparse_parent_base

parser = argparse.ArgumentParser(parents=[argparse_parent_base.parser])

parser.add_argument('--local-arg', action="store_true", default=False)

print parser.parse_args()

And the resulting program takes all three options:

$ python argparse_uses_parent.py -h

usage: argparse_uses_parent.py [-h] [--user USER] [--password PASSWORD]
                               [--local-arg]

optional arguments:
  -h, --help           show this help message and exit
  --user USER
  --password PASSWORD
  --local-arg

Conflicting Options

The previous example pointed out that adding two argument handlers to a parser using the same argument name causes an exception. Change the conflict resolution behavior by passing a conflict_handler. The two built-in handlers are error (the default), and resolve, which picks a handler based on the order they are added.

import argparse

parser = argparse.ArgumentParser(conflict_handler='resolve')

parser.add_argument('-a', action="store")
parser.add_argument('-b', action="store", help='Short alone')
parser.add_argument('--long-b', '-b', action="store", help='Long and short together')

print parser.parse_args(['-h'])

Since the last handler with a given argument name is used, in this example the stand-alone option -b is masked by the alias for --long-b.

$ python argparse_conflict_handler_resolve.py

usage: argparse_conflict_handler_resolve.py [-h] [-a A] [--long-b LONG_B]

optional arguments:
  -h, --help            show this help message and exit
  -a A
  --long-b LONG_B, -b LONG_B
                        Long and short together

Switching the order of the calls to add_argument() unmasks the stand-alone option:

import argparse

parser = argparse.ArgumentParser(conflict_handler='resolve')

parser.add_argument('-a', action="store")
parser.add_argument('--long-b', '-b', action="store", help='Long and short together')
parser.add_argument('-b', action="store", help='Short alone')

print parser.parse_args(['-h'])

Now both options can be used together.

$ python argparse_conflict_handler_resolve2.py

usage: argparse_conflict_handler_resolve2.py [-h] [-a A] [--long-b LONG_B]
                                             [-b B]

optional arguments:
  -h, --help       show this help message and exit
  -a A
  --long-b LONG_B  Long and short together
  -b B             Short alone

Argument Groups

argparse combines the argument definitions into “groups.” By default, it uses two groups, with one for options and another for required position-based arguments.

import argparse

parser = argparse.ArgumentParser(description='Short sample app')

parser.add_argument('--optional', action="store_true", default=False)
parser.add_argument('positional', action="store")

print parser.parse_args()

The grouping is reflected in the separate “positional arguments” and “optional arguments” section of the help output:

$ python argparse_default_grouping.py -h

usage: argparse_default_grouping.py [-h] [--optional] positional

Short sample app

positional arguments:
  positional

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

You can adjust the grouping to make it more logical in the help, so that related options or values are documented together. The shared-option example from earlier could be written using custom grouping so that the authentication options are shown together in the help.

Create the “authentication” group with add_argument_group() and then add each of the authentication-related options to the group, instead of the base parser.

import argparse

parser = argparse.ArgumentParser(add_help=False)

group = parser.add_argument_group('authentication')

group.add_argument('--user', action="store")
group.add_argument('--password', action="store")

The program using the group-based parent lists it in the parents value, just as before.

import argparse
import argparse_parent_with_group

parser = argparse.ArgumentParser(parents=[argparse_parent_with_group.parser])

parser.add_argument('--local-arg', action="store_true", default=False)

print parser.parse_args()

The help output now shows the authentication options together.

$ python argparse_uses_parent_with_group.py -h

usage: argparse_uses_parent_with_group.py [-h] [--user USER]
                                          [--password PASSWORD] [--local-arg]

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

authentication:
  --user USER
  --password PASSWORD

Mutually Exclusive Options

Defining mutually exclusive options is a special case of the option grouping feature, and uses add_mutually_exclusive_group() instead of add_argument_group().

import argparse

parser = argparse.ArgumentParser()

group = parser.add_mutually_exclusive_group()
group.add_argument('-a', action='store_true')
group.add_argument('-b', action='store_true')

print parser.parse_args()

argparse enforces the mutal exclusivity for you, so that only one of the options from the group can be given.

$ python argparse_mutually_exclusive.py -h

usage: argparse_mutually_exclusive.py [-h] [-a | -b]

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

$ python argparse_mutually_exclusive.py -a

Namespace(a=True, b=False)

$ python argparse_mutually_exclusive.py -b

Namespace(a=False, b=True)

$ python argparse_mutually_exclusive.py -a -b

usage: argparse_mutually_exclusive.py [-h] [-a | -b]
argparse_mutually_exclusive.py: error: argument -b: not allowed with argument -a

Nesting Parsers

The parent parser approach described above is one way to share options between related commands. An alternate approach is to combine the commands into a single program, and use subparsers to handle each portion of the command line. The result works in the way svn, hg, and other programs with multiple command line actions, or sub-commands, does.

A program to work with directories on the filesystem might define commands for creating, deleting, and listing the contents of a directory like this:

import argparse

parser = argparse.ArgumentParser()

subparsers = parser.add_subparsers(help='commands')

# A list command
list_parser = subparsers.add_parser('list', help='List contents')
list_parser.add_argument('dirname', action='store', help='Directory to list')

# A create command
create_parser = subparsers.add_parser('create', help='Create a directory')
create_parser.add_argument('dirname', action='store', help='New directory to create')
create_parser.add_argument('--read-only', default=False, action='store_true',
                           help='Set permissions to prevent writing to the directory',
                           )

# A delete command
delete_parser = subparsers.add_parser('delete', help='Remove a directory')
delete_parser.add_argument('dirname', action='store', help='The directory to remove')
delete_parser.add_argument('--recursive', '-r', default=False, action='store_true',
                           help='Remove the contents of the directory, too',
                           )

print parser.parse_args()

The help output shows the named subparsers as “commands” that can be specified on the command line as positional arguments.

$ python argparse_subparsers.py -h

usage: argparse_subparsers.py [-h] {list,create,delete} ...

positional arguments:
  {list,create,delete}  commands
    list                List contents
    create              Create a directory
    delete              Remove a directory

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

Each subparser also has its own help, describing the arguments and options for that command.

$ python argparse_subparsers.py create -h

usage: argparse_subparsers.py create [-h] [--read-only] dirname

positional arguments:
  dirname      New directory to create

optional arguments:
  -h, --help   show this help message and exit
  --read-only  Set permissions to prevent writing to the directory

And when the arguments are parsed, the Namespace object returned by parse_args() includes only the values related to the command specified.

$ python argparse_subparsers.py delete -r foo

Namespace(dirname='foo', recursive=True)

Advanced Argument Processing

The examples so far have shown simple boolean flags, options with string or numerical arguments, and positional arguments. argparse supports sophisticated argument specification for variable-length argument list, enumerations, and constant values as well.

Variable Argument Lists

You can configure a single argument defintion to consume multiple arguments on the command line being parsed. Set nargs to one of these flag values, based on the number of required or expected arguments:

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

parser = argparse.ArgumentParser()

parser.add_argument('--three', nargs=3)
parser.add_argument('--optional', nargs='?')
parser.add_argument('--all', nargs='*', dest='all')
parser.add_argument('--one-or-more', nargs='+')

print parser.parse_args()

The parser enforces the argument count instructions, and generates an accurate syntax diagram as part of the command help text.

$ python argparse_nargs.py -h

usage: argparse_nargs.py [-h] [--three THREE THREE THREE]
                         [--optional [OPTIONAL]] [--all [ALL [ALL ...]]]
                         [--one-or-more ONE_OR_MORE [ONE_OR_MORE ...]]

optional arguments:
  -h, --help            show this help message and exit
  --three THREE THREE THREE
  --optional [OPTIONAL]
  --all [ALL [ALL ...]]
  --one-or-more ONE_OR_MORE [ONE_OR_MORE ...]

$ python argparse_nargs.py

Namespace(all=None, one_or_more=None, optional=None, three=None)

$ python argparse_nargs.py --three

usage: argparse_nargs.py [-h] [--three THREE THREE THREE]
                         [--optional [OPTIONAL]] [--all [ALL [ALL ...]]]
                         [--one-or-more ONE_OR_MORE [ONE_OR_MORE ...]]
argparse_nargs.py: error: argument --three: expected 3 argument(s)

$ python argparse_nargs.py --three a b c

Namespace(all=None, one_or_more=None, optional=None, three=['a', 'b', 'c'])

$ python argparse_nargs.py --optional

Namespace(all=None, one_or_more=None, optional=None, three=None)

$ python argparse_nargs.py --optional with_value

Namespace(all=None, one_or_more=None, optional='with_value', three=None)

$ python argparse_nargs.py --all with multiple values

Namespace(all=['with', 'multiple', 'values'], one_or_more=None, optional=None, three=None)

$ python argparse_nargs.py --one-or-more with_value

Namespace(all=None, one_or_more=['with_value'], optional=None, three=None)

$ python argparse_nargs.py --one-or-more with multiple values

Namespace(all=None, one_or_more=['with', 'multiple', 'values'], optional=None, three=None)

$ python argparse_nargs.py --one-or-more

usage: argparse_nargs.py [-h] [--three THREE THREE THREE]
                         [--optional [OPTIONAL]] [--all [ALL [ALL ...]]]
                         [--one-or-more ONE_OR_MORE [ONE_OR_MORE ...]]
argparse_nargs.py: error: argument --one-or-more: expected at least one argument

Argument Types

argparse treats all argument values as strings, unless you tell it to convert the string to another type. The type parameter to add_argument() expects a converter function used by the ArgumentParser to transform the argument value from a string to some other type.

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('-i', type=int)
parser.add_argument('-f', type=float)
parser.add_argument('--file', type=file)

try:
    print parser.parse_args()
except IOError, msg:
    parser.error(str(msg))

Any callable that takes a single string argument can be passed as type, including built-in types like int(), float(), and file().

$ python argparse_type.py -i 1

Namespace(f=None, file=None, i=1)

$ python argparse_type.py -f 3.14

Namespace(f=3.14, file=None, i=None)

$ python argparse_type.py --file argparse_type.py

Namespace(f=None, file=<open file 'argparse_type.py', mode 'r' at 0x1004de270>, i=None)

If the type conversion fails, argparse raises an exception. TypeError and ValueError exceptions are trapped automatically and converted to a simple error message for the user. Other exceptions, such as the IOError in the example below where the input file does not exist, must be handled by the caller.

$ python argparse_type.py -i a

usage: argparse_type.py [-h] [-i I] [-f F] [--file FILE]
argparse_type.py: error: argument -i: invalid int value: 'a'

$ python argparse_type.py -f 3.14.15

usage: argparse_type.py [-h] [-i I] [-f F] [--file FILE]
argparse_type.py: error: argument -f: invalid float value: '3.14.15'

$ python argparse_type.py --file does_not_exist.txt

usage: argparse_type.py [-h] [-i I] [-f F] [--file FILE]
argparse_type.py: error: [Errno 2] No such file or directory: 'does_not_exist.txt'

To limit an input argument to a value within a pre-defined set, use the choices parameter.

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('--mode', choices=('read-only', 'read-write'))

print parser.parse_args()

If the argument to --mode is not one of the allowed values, an error is generated and processing stops.

$ python argparse_choices.py -h

usage: argparse_choices.py [-h] [--mode {read-only,read-write}]

optional arguments:
  -h, --help            show this help message and exit
  --mode {read-only,read-write}

$ python argparse_choices.py --mode read-only

Namespace(mode='read-only')

$ python argparse_choices.py --mode invalid

usage: argparse_choices.py [-h] [--mode {read-only,read-write}]
argparse_choices.py: error: argument --mode: invalid choice: 'invalid'
(choose from 'read-only', 'read-write')

File Arguments

Although file objects can instantiated with a single string argument, that does not allow you to specify the access mode. FileType gives you a more flexible way of specifying that an argument should be a file, including the mode and buffer size.

import argparse

parser = argparse.ArgumentParser()

parser.add_argument('-i', metavar='in-file', type=argparse.FileType('rt'))
parser.add_argument('-o', metavar='out-file', type=argparse.FileType('wt'))

try:
    results = parser.parse_args()
    print 'Input file:', results.i
    print 'Output file:', results.o
except IOError, msg:
    parser.error(str(msg))
    

The value associated with the argument name is the open file handle. You are responsible for closing the file yourself when you are done with it.

$ python argparse_FileType.py -h

usage: argparse_FileType.py [-h] [-i in-file] [-o out-file]

optional arguments:
  -h, --help   show this help message and exit
  -i in-file
  -o out-file

$ python argparse_FileType.py -i argparse_FileType.py -o temporary_file.\
txt

Input file: <open file 'argparse_FileType.py', mode 'rt' at 0x1004de270>
Output file: <open file 'temporary_file.txt', mode 'wt' at 0x1004de300>

$ python argparse_FileType.py -i no_such_file.txt

usage: argparse_FileType.py [-h] [-i in-file] [-o out-file]
argparse_FileType.py: error: argument -i: can't open 'no_such_file.txt': [Errno 2] No such file or directory: 'no_such_file.txt'

Custom Actions

In addition to the built-in actions described earlier, you can define custom actions by providing an object that implements the Action API. The object passed to add_argument() as action should take parameters describing the argument being defined (all of the same arguments given to add_argument()) and return a callable object that takes as parameters the parser processing the arguments, the namespace holding the parse results, the value of the argument being acted on, and the option_string that triggered the action.

A class Action is provided as a convenient starting point for defining new actions. The constructor handles the argument definitions, so you only need to override __call__() in the subclass.

import argparse

class CustomAction(argparse.Action):
    def __init__(self,
                 option_strings,
                 dest,
                 nargs=None,
                 const=None,
                 default=None,
                 type=None,
                 choices=None,
                 required=False,
                 help=None,
                 metavar=None):
        argparse.Action.__init__(self,
                                 option_strings=option_strings,
                                 dest=dest,
                                 nargs=nargs,
                                 const=const,
                                 default=default,
                                 type=type,
                                 choices=choices,
                                 required=required,
                                 help=help,
                                 metavar=metavar,
                                 )
        print
        print 'Initializing CustomAction'
        for name,value in sorted(locals().items()):
            if name == 'self' or value is None:
                continue
            print '  %s = %r' % (name, value)
        return

    def __call__(self, parser, namespace, values, option_string=None):
        print
        print 'Processing CustomAction for "%s"' % self.dest
        print '  parser = %s' % id(parser)
        print '  values = %r' % values
        print '  option_string = %r' % option_string
        
        # Do some arbitrary processing of the input values
        if isinstance(values, list):
            values = [ v.upper() for v in values ]
        else:
            values = values.upper()
        # Save the results in the namespace using the destination
        # variable given to our constructor.
        setattr(namespace, self.dest, values)

parser = argparse.ArgumentParser()

parser.add_argument('-a', action=CustomAction)
parser.add_argument('-m', nargs='*', action=CustomAction)
parser.add_argument('positional', action=CustomAction)

results = parser.parse_args(['-a', 'value', '-m' 'multi-value', 'positional-value'])
print
print results

The type of values depends on the value of nargs. If the argument allows multiple values, values will be a list even if it only contains one item.

The value of option_string also depends on the original argument specifiation. For positional, required, arguments, option_string is always None.

$ python argparse_custom_action.py


Initializing CustomAction
  dest = 'a'
  option_strings = ['-a']
  required = False

Initializing CustomAction
  dest = 'm'
  nargs = '*'
  option_strings = ['-m']
  required = False

Initializing CustomAction
  dest = 'positional'
  option_strings = []
  required = True

Processing CustomAction for "a"
  parser = 4299616464
  values = 'value'
  option_string = '-a'

Processing CustomAction for "m"
  parser = 4299616464
  values = ['multi-value']
  option_string = '-m'

Processing CustomAction for "positional"
  parser = 4299616464
  values = 'positional-value'
  option_string = None

Namespace(a='VALUE', m=['MULTI-VALUE'], positional='POSITIONAL-VALUE')

See also

argparse
The standard library documentation for this module.
original argparse
The PyPI page for the version of argparse from outside of the standard libary. This version is compatible with older versions of Python, and can be installed separately.
ConfigParser
Read and write configuration files.
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