16.4. argparse
— Parser for command-line options, arguments and sub-commands¶
Nuevo en la versión 3.2.
Source code: Lib/argparse.py
The argparse
module makes it easy to write user-friendly command-line
interfaces. The program defines what arguments it requires, and argparse
will figure out how to parse those out of sys.argv
. The argparse
module also automatically generates help and usage messages and issues errors
when users give the program invalid arguments.
16.4.1. Example¶
The following code is a Python program that takes a list of integers and produces either the sum or the max:
import argparse
parser = argparse.ArgumentParser(description='Process some integers.')
parser.add_argument('integers', metavar='N', type=int, nargs='+',
help='an integer for the accumulator')
parser.add_argument('--sum', dest='accumulate', action='store_const',
const=sum, default=max,
help='sum the integers (default: find the max)')
args = parser.parse_args()
print(args.accumulate(args.integers))
Assuming the Python code above is saved into a file called prog.py
, it can
be run at the command line and provides useful help messages:
$ python prog.py -h
usage: prog.py [-h] [--sum] N [N ...]
Process some integers.
positional arguments:
N an integer for the accumulator
optional arguments:
-h, --help show this help message and exit
--sum sum the integers (default: find the max)
When run with the appropriate arguments, it prints either the sum or the max of the command-line integers:
$ python prog.py 1 2 3 4
4
$ python prog.py 1 2 3 4 --sum
10
If invalid arguments are passed in, it will issue an error:
$ python prog.py a b c
usage: prog.py [-h] [--sum] N [N ...]
prog.py: error: argument N: invalid int value: 'a'
The following sections walk you through this example.
16.4.1.1. Creating a parser¶
The first step in using the argparse
is creating an
ArgumentParser
object:
>>> parser = argparse.ArgumentParser(description='Process some integers.')
The ArgumentParser
object will hold all the information necessary to
parse the command line into Python data types.
16.4.1.2. Adding arguments¶
Filling an ArgumentParser
with information about program arguments is
done by making calls to the add_argument()
method.
Generally, these calls tell the ArgumentParser
how to take the strings
on the command line and turn them into objects. This information is stored and
used when parse_args()
is called. For example:
>>> parser.add_argument('integers', metavar='N', type=int, nargs='+',
... help='an integer for the accumulator')
>>> parser.add_argument('--sum', dest='accumulate', action='store_const',
... const=sum, default=max,
... help='sum the integers (default: find the max)')
Later, calling parse_args()
will return an object with
two attributes, integers
and accumulate
. The integers
attribute
will be a list of one or more ints, and the accumulate
attribute will be
either the sum()
function, if --sum
was specified at the command line,
or the max()
function if it was not.
16.4.1.3. Parsing arguments¶
ArgumentParser
parses arguments through the
parse_args()
method. This will inspect the command line,
convert each argument to the appropriate type and then invoke the appropriate action.
In most cases, this means a simple Namespace
object will be built up from
attributes parsed out of the command line:
>>> parser.parse_args(['--sum', '7', '-1', '42'])
Namespace(accumulate=<built-in function sum>, integers=[7, -1, 42])
In a script, parse_args()
will typically be called with no
arguments, and the ArgumentParser
will automatically determine the
command-line arguments from sys.argv
.
16.4.2. ArgumentParser objects¶
-
class
argparse.
ArgumentParser
(prog=None, usage=None, description=None, epilog=None, parents=[], formatter_class=argparse.HelpFormatter, prefix_chars='-', fromfile_prefix_chars=None, argument_default=None, conflict_handler='error', add_help=True, allow_abbrev=True)¶ Create a new
ArgumentParser
object. All parameters should be passed as keyword arguments. Each parameter has its own more detailed description below, but in short they are:- prog - The name of the program (default:
sys.argv[0]
) - usage - The string describing the program usage (default: generated from arguments added to parser)
- description - Text to display before the argument help (default: none)
- epilog - Text to display after the argument help (default: none)
- parents - A list of
ArgumentParser
objects whose arguments should also be included - formatter_class - A class for customizing the help output
- prefix_chars - The set of characters that prefix optional arguments (default: “-“)
- fromfile_prefix_chars - The set of characters that prefix files from
which additional arguments should be read (default:
None
) - argument_default - The global default value for arguments
(default:
None
) - conflict_handler - The strategy for resolving conflicting optionals (usually unnecessary)
- add_help - Add a
-h/--help
option to the parser (default:True
) - allow_abbrev - Allows long options to be abbreviated if the
abbreviation is unambiguous. (default:
True
)
Distinto en la versión 3.5: allow_abbrev parameter was added.
- prog - The name of the program (default:
The following sections describe how each of these are used.
16.4.2.1. prog¶
By default, ArgumentParser
objects use sys.argv[0]
to determine
how to display the name of the program in help messages. This default is almost
always desirable because it will make the help messages match how the program was
invoked on the command line. For example, consider a file named
myprogram.py
with the following code:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', help='foo help')
args = parser.parse_args()
The help for this program will display myprogram.py
as the program name
(regardless of where the program was invoked from):
$ python myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]
optional arguments:
-h, --help show this help message and exit
--foo FOO foo help
$ cd ..
$ python subdir/myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]
optional arguments:
-h, --help show this help message and exit
--foo FOO foo help
To change this default behavior, another value can be supplied using the
prog=
argument to ArgumentParser
:
>>> parser = argparse.ArgumentParser(prog='myprogram')
>>> parser.print_help()
usage: myprogram [-h]
optional arguments:
-h, --help show this help message and exit
Note that the program name, whether determined from sys.argv[0]
or from the
prog=
argument, is available to help messages using the %(prog)s
format
specifier.
>>> parser = argparse.ArgumentParser(prog='myprogram')
>>> parser.add_argument('--foo', help='foo of the %(prog)s program')
>>> parser.print_help()
usage: myprogram [-h] [--foo FOO]
optional arguments:
-h, --help show this help message and exit
--foo FOO foo of the myprogram program
16.4.2.2. usage¶
By default, ArgumentParser
calculates the usage message from the
arguments it contains:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', nargs='?', help='foo help')
>>> parser.add_argument('bar', nargs='+', help='bar help')
>>> parser.print_help()
usage: PROG [-h] [--foo [FOO]] bar [bar ...]
positional arguments:
bar bar help
optional arguments:
-h, --help show this help message and exit
--foo [FOO] foo help
The default message can be overridden with the usage=
keyword argument:
>>> parser = argparse.ArgumentParser(prog='PROG', usage='%(prog)s [options]')
>>> parser.add_argument('--foo', nargs='?', help='foo help')
>>> parser.add_argument('bar', nargs='+', help='bar help')
>>> parser.print_help()
usage: PROG [options]
positional arguments:
bar bar help
optional arguments:
-h, --help show this help message and exit
--foo [FOO] foo help
The %(prog)s
format specifier is available to fill in the program name in
your usage messages.
16.4.2.3. description¶
Most calls to the ArgumentParser
constructor will use the
description=
keyword argument. This argument gives a brief description of
what the program does and how it works. In help messages, the description is
displayed between the command-line usage string and the help messages for the
various arguments:
>>> parser = argparse.ArgumentParser(description='A foo that bars')
>>> parser.print_help()
usage: argparse.py [-h]
A foo that bars
optional arguments:
-h, --help show this help message and exit
By default, the description will be line-wrapped so that it fits within the given space. To change this behavior, see the formatter_class argument.
16.4.2.4. epilog¶
Some programs like to display additional description of the program after the
description of the arguments. Such text can be specified using the epilog=
argument to ArgumentParser
:
>>> parser = argparse.ArgumentParser(
... description='A foo that bars',
... epilog="And that's how you'd foo a bar")
>>> parser.print_help()
usage: argparse.py [-h]
A foo that bars
optional arguments:
-h, --help show this help message and exit
And that's how you'd foo a bar
As with the description argument, the epilog=
text is by default
line-wrapped, but this behavior can be adjusted with the formatter_class
argument to ArgumentParser
.
16.4.2.5. parents¶
Sometimes, several parsers share a common set of arguments. Rather than
repeating the definitions of these arguments, a single parser with all the
shared arguments and passed to parents=
argument to ArgumentParser
can be used. The parents=
argument takes a list of ArgumentParser
objects, collects all the positional and optional actions from them, and adds
these actions to the ArgumentParser
object being constructed:
>>> parent_parser = argparse.ArgumentParser(add_help=False)
>>> parent_parser.add_argument('--parent', type=int)
>>> foo_parser = argparse.ArgumentParser(parents=[parent_parser])
>>> foo_parser.add_argument('foo')
>>> foo_parser.parse_args(['--parent', '2', 'XXX'])
Namespace(foo='XXX', parent=2)
>>> bar_parser = argparse.ArgumentParser(parents=[parent_parser])
>>> bar_parser.add_argument('--bar')
>>> bar_parser.parse_args(['--bar', 'YYY'])
Namespace(bar='YYY', parent=None)
Note that most parent parsers will specify add_help=False
. Otherwise, the
ArgumentParser
will see two -h/--help
options (one in the parent
and one in the child) and raise an error.
Nota
You must fully initialize the parsers before passing them via parents=
.
If you change the parent parsers after the child parser, those changes will
not be reflected in the child.
16.4.2.6. formatter_class¶
ArgumentParser
objects allow the help formatting to be customized by
specifying an alternate formatting class. Currently, there are four such
classes:
-
class
argparse.
RawDescriptionHelpFormatter
¶ -
class
argparse.
RawTextHelpFormatter
¶ -
class
argparse.
ArgumentDefaultsHelpFormatter
¶ -
class
argparse.
MetavarTypeHelpFormatter
¶
RawDescriptionHelpFormatter
and RawTextHelpFormatter
give
more control over how textual descriptions are displayed.
By default, ArgumentParser
objects line-wrap the description and
epilog texts in command-line help messages:
>>> parser = argparse.ArgumentParser(
... prog='PROG',
... description='''this description
... was indented weird
... but that is okay''',
... epilog='''
... likewise for this epilog whose whitespace will
... be cleaned up and whose words will be wrapped
... across a couple lines''')
>>> parser.print_help()
usage: PROG [-h]
this description was indented weird but that is okay
optional arguments:
-h, --help show this help message and exit
likewise for this epilog whose whitespace will be cleaned up and whose words
will be wrapped across a couple lines
Passing RawDescriptionHelpFormatter
as formatter_class=
indicates that description and epilog are already correctly formatted and
should not be line-wrapped:
>>> parser = argparse.ArgumentParser(
... prog='PROG',
... formatter_class=argparse.RawDescriptionHelpFormatter,
... description=textwrap.dedent('''\
... Please do not mess up this text!
... --------------------------------
... I have indented it
... exactly the way
... I want it
... '''))
>>> parser.print_help()
usage: PROG [-h]
Please do not mess up this text!
--------------------------------
I have indented it
exactly the way
I want it
optional arguments:
-h, --help show this help message and exit
RawTextHelpFormatter
maintains whitespace for all sorts of help text,
including argument descriptions.
ArgumentDefaultsHelpFormatter
automatically adds information about
default values to each of the argument help messages:
>>> parser = argparse.ArgumentParser(
... prog='PROG',
... formatter_class=argparse.ArgumentDefaultsHelpFormatter)
>>> parser.add_argument('--foo', type=int, default=42, help='FOO!')
>>> parser.add_argument('bar', nargs='*', default=[1, 2, 3], help='BAR!')
>>> parser.print_help()
usage: PROG [-h] [--foo FOO] [bar [bar ...]]
positional arguments:
bar BAR! (default: [1, 2, 3])
optional arguments:
-h, --help show this help message and exit
--foo FOO FOO! (default: 42)
MetavarTypeHelpFormatter
uses the name of the type argument for each
argument as the display name for its values (rather than using the dest
as the regular formatter does):
>>> parser = argparse.ArgumentParser(
... prog='PROG',
... formatter_class=argparse.MetavarTypeHelpFormatter)
>>> parser.add_argument('--foo', type=int)
>>> parser.add_argument('bar', type=float)
>>> parser.print_help()
usage: PROG [-h] [--foo int] float
positional arguments:
float
optional arguments:
-h, --help show this help message and exit
--foo int
16.4.2.7. prefix_chars¶
Most command-line options will use -
as the prefix, e.g. -f/--foo
.
Parsers that need to support different or additional prefix
characters, e.g. for options
like +f
or /foo
, may specify them using the prefix_chars=
argument
to the ArgumentParser constructor:
>>> parser = argparse.ArgumentParser(prog='PROG', prefix_chars='-+')
>>> parser.add_argument('+f')
>>> parser.add_argument('++bar')
>>> parser.parse_args('+f X ++bar Y'.split())
Namespace(bar='Y', f='X')
The prefix_chars=
argument defaults to '-'
. Supplying a set of
characters that does not include -
will cause -f/--foo
options to be
disallowed.
16.4.2.8. fromfile_prefix_chars¶
Sometimes, for example when dealing with a particularly long argument lists, it
may make sense to keep the list of arguments in a file rather than typing it out
at the command line. If the fromfile_prefix_chars=
argument is given to the
ArgumentParser
constructor, then arguments that start with any of the
specified characters will be treated as files, and will be replaced by the
arguments they contain. For example:
>>> with open('args.txt', 'w') as fp:
... fp.write('-f\nbar')
>>> parser = argparse.ArgumentParser(fromfile_prefix_chars='@')
>>> parser.add_argument('-f')
>>> parser.parse_args(['-f', 'foo', '@args.txt'])
Namespace(f='bar')
Arguments read from a file must by default be one per line (but see also
convert_arg_line_to_args()
) and are treated as if they
were in the same place as the original file referencing argument on the command
line. So in the example above, the expression ['-f', 'foo', '@args.txt']
is considered equivalent to the expression ['-f', 'foo', '-f', 'bar']
.
The fromfile_prefix_chars=
argument defaults to None
, meaning that
arguments will never be treated as file references.
16.4.2.9. argument_default¶
Generally, argument defaults are specified either by passing a default to
add_argument()
or by calling the
set_defaults()
methods with a specific set of name-value
pairs. Sometimes however, it may be useful to specify a single parser-wide
default for arguments. This can be accomplished by passing the
argument_default=
keyword argument to ArgumentParser
. For example,
to globally suppress attribute creation on parse_args()
calls, we supply argument_default=SUPPRESS
:
>>> parser = argparse.ArgumentParser(argument_default=argparse.SUPPRESS)
>>> parser.add_argument('--foo')
>>> parser.add_argument('bar', nargs='?')
>>> parser.parse_args(['--foo', '1', 'BAR'])
Namespace(bar='BAR', foo='1')
>>> parser.parse_args([])
Namespace()
16.4.2.10. allow_abbrev¶
Normally, when you pass an argument list to the
parse_args()
method of an ArgumentParser
,
it recognizes abbreviations of long options.
This feature can be disabled by setting allow_abbrev
to False
:
>>> parser = argparse.ArgumentParser(prog='PROG', allow_abbrev=False)
>>> parser.add_argument('--foobar', action='store_true')
>>> parser.add_argument('--foonley', action='store_false')
>>> parser.parse_args(['--foon'])
usage: PROG [-h] [--foobar] [--foonley]
PROG: error: unrecognized arguments: --foon
Nuevo en la versión 3.5.
16.4.2.11. conflict_handler¶
ArgumentParser
objects do not allow two actions with the same option
string. By default, ArgumentParser
objects raise an exception if an
attempt is made to create an argument with an option string that is already in
use:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-f', '--foo', help='old foo help')
>>> parser.add_argument('--foo', help='new foo help')
Traceback (most recent call last):
..
ArgumentError: argument --foo: conflicting option string(s): --foo
Sometimes (e.g. when using parents) it may be useful to simply override any
older arguments with the same option string. To get this behavior, the value
'resolve'
can be supplied to the conflict_handler=
argument of
ArgumentParser
:
>>> parser = argparse.ArgumentParser(prog='PROG', conflict_handler='resolve')
>>> parser.add_argument('-f', '--foo', help='old foo help')
>>> parser.add_argument('--foo', help='new foo help')
>>> parser.print_help()
usage: PROG [-h] [-f FOO] [--foo FOO]
optional arguments:
-h, --help show this help message and exit
-f FOO old foo help
--foo FOO new foo help
Note that ArgumentParser
objects only remove an action if all of its
option strings are overridden. So, in the example above, the old -f/--foo
action is retained as the -f
action, because only the --foo
option
string was overridden.
16.4.2.12. add_help¶
By default, ArgumentParser objects add an option which simply displays
the parser’s help message. For example, consider a file named
myprogram.py
containing the following code:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', help='foo help')
args = parser.parse_args()
If -h
or --help
is supplied at the command line, the ArgumentParser
help will be printed:
$ python myprogram.py --help
usage: myprogram.py [-h] [--foo FOO]
optional arguments:
-h, --help show this help message and exit
--foo FOO foo help
Occasionally, it may be useful to disable the addition of this help option.
This can be achieved by passing False
as the add_help=
argument to
ArgumentParser
:
>>> parser = argparse.ArgumentParser(prog='PROG', add_help=False)
>>> parser.add_argument('--foo', help='foo help')
>>> parser.print_help()
usage: PROG [--foo FOO]
optional arguments:
--foo FOO foo help
The help option is typically -h/--help
. The exception to this is
if the prefix_chars=
is specified and does not include -
, in
which case -h
and --help
are not valid options. In
this case, the first character in prefix_chars
is used to prefix
the help options:
>>> parser = argparse.ArgumentParser(prog='PROG', prefix_chars='+/')
>>> parser.print_help()
usage: PROG [+h]
optional arguments:
+h, ++help show this help message and exit
16.4.3. The add_argument() method¶
-
ArgumentParser.
add_argument
(name or flags...[, action][, nargs][, const][, default][, type][, choices][, required][, help][, metavar][, dest])¶ Define how a single command-line argument should be parsed. Each parameter has its own more detailed description below, but in short they are:
- 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 - Either a name or a list of option strings, e.g.
The following sections describe how each of these are used.
16.4.3.1. name or flags¶
The add_argument()
method must know whether an optional
argument, like -f
or --foo
, or a positional argument, like a list of
filenames, is expected. The first arguments passed to
add_argument()
must therefore be either a series of
flags, or a simple argument name. For example, an optional argument could
be created like:
>>> parser.add_argument('-f', '--foo')
while a positional argument could be created like:
>>> parser.add_argument('bar')
When parse_args()
is called, optional arguments will be
identified by the -
prefix, and the remaining arguments will be assumed to
be positional:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-f', '--foo')
>>> parser.add_argument('bar')
>>> parser.parse_args(['BAR'])
Namespace(bar='BAR', foo=None)
>>> parser.parse_args(['BAR', '--foo', 'FOO'])
Namespace(bar='BAR', foo='FOO')
>>> parser.parse_args(['--foo', 'FOO'])
usage: PROG [-h] [-f FOO] bar
PROG: error: too few arguments
16.4.3.2. action¶
ArgumentParser
objects associate command-line arguments with actions. These
actions can do just about anything with the command-line arguments associated with
them, though most actions simply add an attribute to the object returned by
parse_args()
. 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. This is the default action. For example:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo') >>> parser.parse_args('--foo 1'.split()) Namespace(foo='1')
'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. For example:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', action='store_const', const=42) >>> parser.parse_args(['--foo']) Namespace(foo=42)
'store_true'
and'store_false'
- These are special cases of'store_const'
used for storing the valuesTrue
andFalse
respectively. In addition, they create default values ofFalse
andTrue
respectively. For example:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', action='store_true') >>> parser.add_argument('--bar', action='store_false') >>> parser.add_argument('--baz', action='store_false') >>> parser.parse_args('--foo --bar'.split()) Namespace(foo=True, bar=False, baz=True)
'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. Example usage:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', action='append') >>> parser.parse_args('--foo 1 --foo 2'.split()) Namespace(foo=['1', '2'])
'append_const'
- This stores a list, and appends the value specified by the const keyword argument to the list. (Note that the const keyword argument defaults toNone
.) The'append_const'
action is typically useful when multiple arguments need to store constants to the same list. For example:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--str', dest='types', action='append_const', const=str) >>> parser.add_argument('--int', dest='types', action='append_const', const=int) >>> parser.parse_args('--str --int'.split()) Namespace(types=[<class 'str'>, <class 'int'>])
'count'
- This counts the number of times a keyword argument occurs. For example, this is useful for increasing verbosity levels:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--verbose', '-v', action='count') >>> parser.parse_args(['-vvv']) Namespace(verbose=3)
'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. SeeArgumentParser
for details of how the output is created.'version'
- This expects aversion=
keyword argument in theadd_argument()
call, and prints version information and exits when invoked:>>> import argparse >>> parser = argparse.ArgumentParser(prog='PROG') >>> parser.add_argument('--version', action='version', version='%(prog)s 2.0') >>> parser.parse_args(['--version']) PROG 2.0
You may also specify an arbitrary action by passing an Action subclass or
other object that implements the same interface. The recommended way to do
this is to extend Action
, overriding the __call__
method
and optionally the __init__
method.
An example of a custom action:
>>> class FooAction(argparse.Action):
... def __init__(self, option_strings, dest, nargs=None, **kwargs):
... if nargs is not None:
... raise ValueError("nargs not allowed")
... super(FooAction, self).__init__(option_strings, dest, **kwargs)
... def __call__(self, parser, namespace, values, option_string=None):
... print('%r %r %r' % (namespace, values, option_string))
... setattr(namespace, self.dest, values)
...
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action=FooAction)
>>> parser.add_argument('bar', action=FooAction)
>>> args = parser.parse_args('1 --foo 2'.split())
Namespace(bar=None, foo=None) '1' None
Namespace(bar='1', foo=None) '2' '--foo'
>>> args
Namespace(bar='1', foo='2')
For more details, see Action
.
16.4.3.3. nargs¶
ArgumentParser objects usually associate a single command-line argument with a
single action to be taken. The nargs
keyword argument associates a
different number of command-line arguments with a single action. The supported
values are:
N
(an integer).N
arguments from the command line will be gathered together into a list. For example:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', nargs=2) >>> parser.add_argument('bar', nargs=1) >>> parser.parse_args('c --foo a b'.split()) Namespace(bar=['c'], foo=['a', 'b'])
Note that
nargs=1
produces a list of one item. This is different from the default, in which the item is produced by itself.'?'
. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced. Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line argument. In this case the value from const will be produced. Some examples to illustrate this:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', nargs='?', const='c', default='d') >>> parser.add_argument('bar', nargs='?', default='d') >>> parser.parse_args(['XX', '--foo', 'YY']) Namespace(bar='XX', foo='YY') >>> parser.parse_args(['XX', '--foo']) Namespace(bar='XX', foo='c') >>> parser.parse_args([]) Namespace(bar='d', foo='d')
One of the more common uses of
nargs='?'
is to allow optional input and output files:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('infile', nargs='?', type=argparse.FileType('r'), ... default=sys.stdin) >>> parser.add_argument('outfile', nargs='?', type=argparse.FileType('w'), ... default=sys.stdout) >>> parser.parse_args(['input.txt', 'output.txt']) Namespace(infile=<_io.TextIOWrapper name='input.txt' encoding='UTF-8'>, outfile=<_io.TextIOWrapper name='output.txt' encoding='UTF-8'>) >>> parser.parse_args([]) Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>, outfile=<_io.TextIOWrapper name='<stdout>' encoding='UTF-8'>)
'*'
. All command-line arguments present are gathered into a list. Note that it generally doesn’t make much sense to have more than one positional argument withnargs='*'
, but multiple optional arguments withnargs='*'
is possible. For example:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', nargs='*') >>> parser.add_argument('--bar', nargs='*') >>> parser.add_argument('baz', nargs='*') >>> parser.parse_args('a b --foo x y --bar 1 2'.split()) Namespace(bar=['1', '2'], baz=['a', 'b'], foo=['x', 'y'])
'+'
. Just like'*'
, all command-line args present are gathered into a list. Additionally, an error message will be generated if there wasn’t at least one command-line argument present. For example:>>> parser = argparse.ArgumentParser(prog='PROG') >>> parser.add_argument('foo', nargs='+') >>> parser.parse_args(['a', 'b']) Namespace(foo=['a', 'b']) >>> parser.parse_args([]) usage: PROG [-h] foo [foo ...] PROG: error: too few arguments
argparse.REMAINDER
. All the remaining command-line arguments are gathered into a list. This is commonly useful for command line utilities that dispatch to other command line utilities:>>> parser = argparse.ArgumentParser(prog='PROG') >>> parser.add_argument('--foo') >>> parser.add_argument('command') >>> parser.add_argument('args', nargs=argparse.REMAINDER) >>> print(parser.parse_args('--foo B cmd --arg1 XX ZZ'.split())) Namespace(args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')
If the nargs
keyword argument is not provided, the number of arguments consumed
is determined by the action. Generally this means a single command-line argument
will be consumed and a single item (not a list) will be produced.
16.4.3.4. 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. The two most common uses of it are:
- When
add_argument()
is called withaction='store_const'
oraction='append_const'
. These actions add theconst
value to one of the attributes of the object returned byparse_args()
. See the action description for examples. - When
add_argument()
is called with option strings (like-f
or--foo
) andnargs='?'
. This creates an optional argument that can be followed by zero or one command-line arguments. When parsing the command line, if the option string is encountered with no command-line argument following it, the value ofconst
will be assumed instead. See the nargs description for examples.
With the 'store_const'
and 'append_const'
actions, the const
keyword argument must be given. For other actions, it defaults to None
.
16.4.3.5. default¶
All optional arguments and some positional arguments may be omitted at the
command line. The default
keyword argument of
add_argument()
, whose value defaults to None
,
specifies what value should be used if the command-line argument is not present.
For optional arguments, the default
value is used when the option string
was not present at the command line:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', default=42)
>>> parser.parse_args(['--foo', '2'])
Namespace(foo='2')
>>> parser.parse_args([])
Namespace(foo=42)
If the default
value is a string, the parser parses the value as if it
were a command-line argument. In particular, the parser applies any type
conversion argument, if provided, before setting the attribute on the
Namespace
return value. Otherwise, the parser uses the value as is:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--length', default='10', type=int)
>>> parser.add_argument('--width', default=10.5, type=int)
>>> parser.parse_args()
Namespace(length=10, width=10.5)
For positional arguments with nargs equal to ?
or *
, the default
value
is used when no command-line argument was present:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', nargs='?', default=42)
>>> parser.parse_args(['a'])
Namespace(foo='a')
>>> parser.parse_args([])
Namespace(foo=42)
Providing default=argparse.SUPPRESS
causes no attribute to be added if the
command-line argument was not present.:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', default=argparse.SUPPRESS)
>>> parser.parse_args([])
Namespace()
>>> parser.parse_args(['--foo', '1'])
Namespace(foo='1')
16.4.3.6. type¶
By default, ArgumentParser
objects read command-line arguments in as simple
strings. However, quite often the command-line string should instead be
interpreted as another type, like a float
or int
. The
type
keyword argument of add_argument()
allows any
necessary type-checking and type conversions to be performed. Common built-in
types and functions can be used directly as the value of the type
argument:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('foo', type=int)
>>> parser.add_argument('bar', type=open)
>>> parser.parse_args('2 temp.txt'.split())
Namespace(bar=<_io.TextIOWrapper name='temp.txt' encoding='UTF-8'>, foo=2)
See the section on the default keyword argument for information on when the
type
argument is applied to default arguments.
To ease the use of various types of files, the argparse module provides the
factory FileType which takes the mode=
, bufsize=
, encoding=
and
errors=
arguments of the open()
function. For example,
FileType('w')
can be used to create a writable file:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('bar', type=argparse.FileType('w'))
>>> parser.parse_args(['out.txt'])
Namespace(bar=<_io.TextIOWrapper name='out.txt' encoding='UTF-8'>)
type=
can take any callable that takes a single string argument and returns
the converted value:
>>> def perfect_square(string):
... value = int(string)
... sqrt = math.sqrt(value)
... if sqrt != int(sqrt):
... msg = "%r is not a perfect square" % string
... raise argparse.ArgumentTypeError(msg)
... return value
...
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', type=perfect_square)
>>> parser.parse_args(['9'])
Namespace(foo=9)
>>> parser.parse_args(['7'])
usage: PROG [-h] foo
PROG: error: argument foo: '7' is not a perfect square
The choices keyword argument may be more convenient for type checkers that simply check against a range of values:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('foo', type=int, choices=range(5, 10))
>>> parser.parse_args(['7'])
Namespace(foo=7)
>>> parser.parse_args(['11'])
usage: PROG [-h] {5,6,7,8,9}
PROG: error: argument foo: invalid choice: 11 (choose from 5, 6, 7, 8, 9)
See the choices section for more details.
16.4.3.7. 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:
>>> parser = argparse.ArgumentParser(prog='game.py')
>>> parser.add_argument('move', choices=['rock', 'paper', 'scissors'])
>>> parser.parse_args(['rock'])
Namespace(move='rock')
>>> parser.parse_args(['fire'])
usage: game.py [-h] {rock,paper,scissors}
game.py: error: argument move: invalid choice: 'fire' (choose from 'rock',
'paper', 'scissors')
Note that inclusion in the choices container is checked after any type conversions have been performed, so the type of the objects in the choices container should match the type specified:
>>> parser = argparse.ArgumentParser(prog='doors.py')
>>> parser.add_argument('door', type=int, choices=range(1, 4))
>>> print(parser.parse_args(['3']))
Namespace(door=3)
>>> parser.parse_args(['4'])
usage: doors.py [-h] {1,2,3}
doors.py: error: argument door: invalid choice: 4 (choose from 1, 2, 3)
Any object that supports the in
operator can be passed as the choices
value, so dict
objects, set
objects, custom containers,
etc. are all supported.
16.4.3.8. required¶
In general, the argparse
module assumes that flags like -f
and --bar
indicate optional arguments, which can always be omitted at the command line.
To make an option required, True
can be specified for the required=
keyword argument to add_argument()
:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', required=True)
>>> parser.parse_args(['--foo', 'BAR'])
Namespace(foo='BAR')
>>> parser.parse_args([])
usage: argparse.py [-h] [--foo FOO]
argparse.py: error: option --foo is required
As the example shows, if an option is marked as required
,
parse_args()
will report an error if that option is not
present at the command line.
Nota
Required options are generally considered bad form because users expect options to be optional, and thus they should be avoided when possible.
16.4.3.9. 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:
>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', action='store_true',
... help='foo the bars before frobbling')
>>> parser.add_argument('bar', nargs='+',
... help='one of the bars to be frobbled')
>>> parser.parse_args(['-h'])
usage: frobble [-h] [--foo] bar [bar ...]
positional arguments:
bar one of the bars to be frobbled
optional arguments:
-h, --help show this help message and exit
--foo foo the bars before frobbling
The help
strings can include various format specifiers to avoid repetition
of things like the program name or the argument default. The available
specifiers include the program name, %(prog)s
and most keyword arguments to
add_argument()
, e.g. %(default)s
, %(type)s
, etc.:
>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('bar', nargs='?', type=int, default=42,
... help='the bar to %(prog)s (default: %(default)s)')
>>> parser.print_help()
usage: frobble [-h] [bar]
positional arguments:
bar the bar to frobble (default: 42)
optional arguments:
-h, --help show this help message and exit
As the help string supports %-formatting, if you want a literal %
to appear
in the help string, you must escape it as %%
.
argparse
supports silencing the help entry for certain options, by
setting the help
value to argparse.SUPPRESS
:
>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', help=argparse.SUPPRESS)
>>> parser.print_help()
usage: frobble [-h]
optional arguments:
-h, --help show this help message and exit
16.4.3.10. 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. By default, for positional argument
actions, the dest value is used directly, and for optional argument actions,
the dest value is uppercased. So, a single positional argument with
dest='bar'
will be referred to as bar
. A single
optional argument --foo
that should be followed by a single command-line argument
will be referred to as FOO
. An example:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.add_argument('bar')
>>> parser.parse_args('X --foo Y'.split())
Namespace(bar='X', foo='Y')
>>> parser.print_help()
usage: [-h] [--foo FOO] bar
positional arguments:
bar
optional arguments:
-h, --help show this help message and exit
--foo FOO
An alternative name can be specified with metavar
:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', metavar='YYY')
>>> parser.add_argument('bar', metavar='XXX')
>>> parser.parse_args('X --foo Y'.split())
Namespace(bar='X', foo='Y')
>>> parser.print_help()
usage: [-h] [--foo YYY] XXX
positional arguments:
XXX
optional arguments:
-h, --help show this help message and exit
--foo YYY
Note that metavar
only changes the displayed name - the name of the
attribute on the parse_args()
object is still determined
by the dest value.
Different values of nargs
may cause the metavar to be used multiple times.
Providing a tuple to metavar
specifies a different display for each of the
arguments:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x', nargs=2)
>>> parser.add_argument('--foo', nargs=2, metavar=('bar', 'baz'))
>>> parser.print_help()
usage: PROG [-h] [-x X X] [--foo bar baz]
optional arguments:
-h, --help show this help message and exit
-x X X
--foo bar baz
16.4.3.11. 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()
:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('bar')
>>> parser.parse_args(['XXX'])
Namespace(bar='XXX')
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. The examples below illustrate this
behavior:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('-f', '--foo-bar', '--foo')
>>> parser.add_argument('-x', '-y')
>>> parser.parse_args('-f 1 -x 2'.split())
Namespace(foo_bar='1', x='2')
>>> parser.parse_args('--foo 1 -y 2'.split())
Namespace(foo_bar='1', x='2')
dest
allows a custom attribute name to be provided:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', dest='bar')
>>> parser.parse_args('--foo XXX'.split())
Namespace(bar='XXX')
16.4.3.12. Action classes¶
Action classes implement the Action API, a callable which returns a callable
which processes arguments from the command-line. Any object which follows
this API may be passed as the action
parameter to
add_argument()
.
-
class
argparse.
Action
(option_strings, dest, nargs=None, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None)¶
Action objects are used by an ArgumentParser to represent the information
needed to parse a single argument from one or more strings from the
command line. The Action class must accept the two positional arguments
plus any keyword arguments passed to ArgumentParser.add_argument()
except for the action
itself.
Instances of Action (or return value of any callable to the action
parameter) should have attributes «dest», «option_strings», «default», «type»,
«required», «help», etc. defined. The easiest way to ensure these attributes
are defined is to call Action.__init__
.
Action instances should be callable, so subclasses must override the
__call__
method, which should accept four parameters:
parser
- The ArgumentParser object which contains this action.namespace
- TheNamespace
object that will be returned byparse_args()
. Most actions add an attribute to this object usingsetattr()
.values
- The associated command-line arguments, with any type conversions applied. Type conversions are specified with the type keyword argument toadd_argument()
.option_string
- The option string that was used to invoke this action. Theoption_string
argument is optional, and will be absent if the action is associated with a positional argument.
The __call__
method may perform arbitrary actions, but will typically set
attributes on the namespace
based on dest
and values
.
16.4.4. The parse_args() method¶
-
ArgumentParser.
parse_args
(args=None, namespace=None)¶ Convert argument strings to objects and assign them as attributes of the namespace. Return the populated namespace.
Previous calls to
add_argument()
determine exactly what objects are created and how they are assigned. See the documentation foradd_argument()
for details.By default, the argument strings are taken from
sys.argv
, and a new emptyNamespace
object is created for the attributes.
16.4.4.1. Option value syntax¶
The parse_args()
method supports several ways of
specifying the value of an option (if it takes one). In the simplest case, the
option and its value are passed as two separate arguments:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x')
>>> parser.add_argument('--foo')
>>> parser.parse_args(['-x', 'X'])
Namespace(foo=None, x='X')
>>> parser.parse_args(['--foo', 'FOO'])
Namespace(foo='FOO', x=None)
For long options (options with names longer than a single character), the option
and value can also be passed as a single command-line argument, using =
to
separate them:
>>> parser.parse_args(['--foo=FOO'])
Namespace(foo='FOO', x=None)
For short options (options only one character long), the option and its value can be concatenated:
>>> parser.parse_args(['-xX'])
Namespace(foo=None, x='X')
Several short options can be joined together, using only a single -
prefix,
as long as only the last option (or none of them) requires a value:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x', action='store_true')
>>> parser.add_argument('-y', action='store_true')
>>> parser.add_argument('-z')
>>> parser.parse_args(['-xyzZ'])
Namespace(x=True, y=True, z='Z')
16.4.4.2. Invalid arguments¶
While parsing the command line, parse_args()
checks for a
variety of errors, including ambiguous options, invalid types, invalid options,
wrong number of positional arguments, etc. When it encounters such an error,
it exits and prints the error along with a usage message:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', type=int)
>>> parser.add_argument('bar', nargs='?')
>>> # invalid type
>>> parser.parse_args(['--foo', 'spam'])
usage: PROG [-h] [--foo FOO] [bar]
PROG: error: argument --foo: invalid int value: 'spam'
>>> # invalid option
>>> parser.parse_args(['--bar'])
usage: PROG [-h] [--foo FOO] [bar]
PROG: error: no such option: --bar
>>> # wrong number of arguments
>>> parser.parse_args(['spam', 'badger'])
usage: PROG [-h] [--foo FOO] [bar]
PROG: error: extra arguments found: badger
16.4.4.3. Arguments containing -
¶
The parse_args()
method attempts to give errors whenever
the user has clearly made a mistake, but some situations are inherently
ambiguous. For example, the command-line argument -1
could either be an
attempt to specify an option or an attempt to provide a positional argument.
The parse_args()
method is cautious here: positional
arguments may only begin with -
if they look like negative numbers and
there are no options in the parser that look like negative numbers:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-x')
>>> parser.add_argument('foo', nargs='?')
>>> # no negative number options, so -1 is a positional argument
>>> parser.parse_args(['-x', '-1'])
Namespace(foo=None, x='-1')
>>> # no negative number options, so -1 and -5 are positional arguments
>>> parser.parse_args(['-x', '-1', '-5'])
Namespace(foo='-5', x='-1')
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-1', dest='one')
>>> parser.add_argument('foo', nargs='?')
>>> # negative number options present, so -1 is an option
>>> parser.parse_args(['-1', 'X'])
Namespace(foo=None, one='X')
>>> # negative number options present, so -2 is an option
>>> parser.parse_args(['-2'])
usage: PROG [-h] [-1 ONE] [foo]
PROG: error: no such option: -2
>>> # negative number options present, so both -1s are options
>>> parser.parse_args(['-1', '-1'])
usage: PROG [-h] [-1 ONE] [foo]
PROG: error: argument -1: expected one argument
If you have positional arguments that must begin with -
and don’t look
like negative numbers, you can insert the pseudo-argument '--'
which tells
parse_args()
that everything after that is a positional
argument:
>>> parser.parse_args(['--', '-f'])
Namespace(foo='-f', one=None)
16.4.4.4. Argument abbreviations (prefix matching)¶
The parse_args()
method by default
allows long options to be abbreviated to a prefix, if the abbreviation is
unambiguous (the prefix matches a unique option):
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('-bacon')
>>> parser.add_argument('-badger')
>>> parser.parse_args('-bac MMM'.split())
Namespace(bacon='MMM', badger=None)
>>> parser.parse_args('-bad WOOD'.split())
Namespace(bacon=None, badger='WOOD')
>>> parser.parse_args('-ba BA'.split())
usage: PROG [-h] [-bacon BACON] [-badger BADGER]
PROG: error: ambiguous option: -ba could match -badger, -bacon
An error is produced for arguments that could produce more than one options.
This feature can be disabled by setting allow_abbrev to False
.
16.4.4.5. Beyond sys.argv
¶
Sometimes it may be useful to have an ArgumentParser parse arguments other than those
of sys.argv
. This can be accomplished by passing a list of strings to
parse_args()
. This is useful for testing at the
interactive prompt:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument(
... 'integers', metavar='int', type=int, choices=range(10),
... nargs='+', help='an integer in the range 0..9')
>>> parser.add_argument(
... '--sum', dest='accumulate', action='store_const', const=sum,
... default=max, help='sum the integers (default: find the max)')
>>> parser.parse_args(['1', '2', '3', '4'])
Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])
>>> parser.parse_args(['1', '2', '3', '4', '--sum'])
Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])
16.4.4.6. The Namespace object¶
-
class
argparse.
Namespace
¶ Simple class used by default by
parse_args()
to create an object holding attributes and return it.
This class is deliberately simple, just an object
subclass with a
readable string representation. If you prefer to have dict-like view of the
attributes, you can use the standard Python idiom, vars()
:
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> args = parser.parse_args(['--foo', 'BAR'])
>>> vars(args)
{'foo': 'BAR'}
It may also be useful to have an ArgumentParser
assign attributes to an
already existing object, rather than a new Namespace
object. This can
be achieved by specifying the namespace=
keyword argument:
>>> class C:
... pass
...
>>> c = C()
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.parse_args(args=['--foo', 'BAR'], namespace=c)
>>> c.foo
'BAR'
16.4.5. Other utilities¶
16.4.5.1. Sub-commands¶
-
ArgumentParser.
add_subparsers
([title][, description][, prog][, parser_class][, action][, option_string][, dest][, help][, metavar])¶ Many programs split up their functionality into a number of sub-commands, for example, the
svn
program can invoke sub-commands likesvn checkout
,svn update
, andsvn commit
. Splitting up functionality this way can be a particularly good idea when a program performs several different functions which require different kinds of command-line arguments.ArgumentParser
supports the creation of such sub-commands with theadd_subparsers()
method. Theadd_subparsers()
method is normally called with no arguments and returns a special action object. This object has a single method,add_parser()
, which takes a command name and anyArgumentParser
constructor arguments, and returns anArgumentParser
object that can be modified as usual.Description of parameters:
- title - title for the sub-parser group in help output; by default «subcommands» if description is provided, otherwise uses title for positional arguments
- description - description for the sub-parser group in help output, by
default
None
- prog - usage information that will be displayed with sub-command help, by default the name of the program and any positional arguments before the subparser argument
- parser_class - class which will be used to create sub-parser instances, by default the class of the current parser (e.g. ArgumentParser)
- action - the basic type of action to be taken when this argument is encountered at the command line
- dest - name of the attribute under which sub-command name will be
stored; by default
None
and no value is stored - help - help for sub-parser group in help output, by default
None
- metavar - string presenting available sub-commands in help; by default it
is
None
and presents sub-commands in form {cmd1, cmd2, ..}
Some example usage:
>>> # create the top-level parser >>> parser = argparse.ArgumentParser(prog='PROG') >>> parser.add_argument('--foo', action='store_true', help='foo help') >>> subparsers = parser.add_subparsers(help='sub-command help') >>> >>> # create the parser for the "a" command >>> parser_a = subparsers.add_parser('a', help='a help') >>> parser_a.add_argument('bar', type=int, help='bar help') >>> >>> # create the parser for the "b" command >>> parser_b = subparsers.add_parser('b', help='b help') >>> parser_b.add_argument('--baz', choices='XYZ', help='baz help') >>> >>> # parse some argument lists >>> parser.parse_args(['a', '12']) Namespace(bar=12, foo=False) >>> parser.parse_args(['--foo', 'b', '--baz', 'Z']) Namespace(baz='Z', foo=True)
Note that the object returned by
parse_args()
will only contain attributes for the main parser and the subparser that was selected by the command line (and not any other subparsers). So in the example above, when thea
command is specified, only thefoo
andbar
attributes are present, and when theb
command is specified, only thefoo
andbaz
attributes are present.Similarly, when a help message is requested from a subparser, only the help for that particular parser will be printed. The help message will not include parent parser or sibling parser messages. (A help message for each subparser command, however, can be given by supplying the
help=
argument toadd_parser()
as above.)>>> parser.parse_args(['--help']) usage: PROG [-h] [--foo] {a,b} ... positional arguments: {a,b} sub-command help a a help b b help optional arguments: -h, --help show this help message and exit --foo foo help >>> parser.parse_args(['a', '--help']) usage: PROG a [-h] bar positional arguments: bar bar help optional arguments: -h, --help show this help message and exit >>> parser.parse_args(['b', '--help']) usage: PROG b [-h] [--baz {X,Y,Z}] optional arguments: -h, --help show this help message and exit --baz {X,Y,Z} baz help
The
add_subparsers()
method also supportstitle
anddescription
keyword arguments. When either is present, the subparser’s commands will appear in their own group in the help output. For example:>>> parser = argparse.ArgumentParser() >>> subparsers = parser.add_subparsers(title='subcommands', ... description='valid subcommands', ... help='additional help') >>> subparsers.add_parser('foo') >>> subparsers.add_parser('bar') >>> parser.parse_args(['-h']) usage: [-h] {foo,bar} ... optional arguments: -h, --help show this help message and exit subcommands: valid subcommands {foo,bar} additional help
Furthermore,
add_parser
supports an additionalaliases
argument, which allows multiple strings to refer to the same subparser. This example, likesvn
, aliasesco
as a shorthand forcheckout
:>>> parser = argparse.ArgumentParser() >>> subparsers = parser.add_subparsers() >>> checkout = subparsers.add_parser('checkout', aliases=['co']) >>> checkout.add_argument('foo') >>> parser.parse_args(['co', 'bar']) Namespace(foo='bar')
One particularly effective way of handling sub-commands is to combine the use of the
add_subparsers()
method with calls toset_defaults()
so that each subparser knows which Python function it should execute. For example:>>> # sub-command functions >>> def foo(args): ... print(args.x * args.y) ... >>> def bar(args): ... print('((%s))' % args.z) ... >>> # create the top-level parser >>> parser = argparse.ArgumentParser() >>> subparsers = parser.add_subparsers() >>> >>> # create the parser for the "foo" command >>> parser_foo = subparsers.add_parser('foo') >>> parser_foo.add_argument('-x', type=int, default=1) >>> parser_foo.add_argument('y', type=float) >>> parser_foo.set_defaults(func=foo) >>> >>> # create the parser for the "bar" command >>> parser_bar = subparsers.add_parser('bar') >>> parser_bar.add_argument('z') >>> parser_bar.set_defaults(func=bar) >>> >>> # parse the args and call whatever function was selected >>> args = parser.parse_args('foo 1 -x 2'.split()) >>> args.func(args) 2.0 >>> >>> # parse the args and call whatever function was selected >>> args = parser.parse_args('bar XYZYX'.split()) >>> args.func(args) ((XYZYX))
This way, you can let
parse_args()
do the job of calling the appropriate function after argument parsing is complete. Associating functions with actions like this is typically the easiest way to handle the different actions for each of your subparsers. However, if it is necessary to check the name of the subparser that was invoked, thedest
keyword argument to theadd_subparsers()
call will work:>>> parser = argparse.ArgumentParser() >>> subparsers = parser.add_subparsers(dest='subparser_name') >>> subparser1 = subparsers.add_parser('1') >>> subparser1.add_argument('-x') >>> subparser2 = subparsers.add_parser('2') >>> subparser2.add_argument('y') >>> parser.parse_args(['2', 'frobble']) Namespace(subparser_name='2', y='frobble')
16.4.5.2. FileType objects¶
-
class
argparse.
FileType
(mode='r', bufsize=-1, encoding=None, errors=None)¶ The
FileType
factory creates objects that can be passed to the type argument ofArgumentParser.add_argument()
. Arguments that haveFileType
objects as their type will open command-line arguments as files with the requested modes, buffer sizes, encodings and error handling (see theopen()
function for more details):>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--raw', type=argparse.FileType('wb', 0)) >>> parser.add_argument('out', type=argparse.FileType('w', encoding='UTF-8')) >>> parser.parse_args(['--raw', 'raw.dat', 'file.txt']) Namespace(out=<_io.TextIOWrapper name='file.txt' mode='w' encoding='UTF-8'>, raw=<_io.FileIO name='raw.dat' mode='wb'>)
FileType objects understand the pseudo-argument
'-'
and automatically convert this intosys.stdin
for readableFileType
objects andsys.stdout
for writableFileType
objects:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('infile', type=argparse.FileType('r')) >>> parser.parse_args(['-']) Namespace(infile=<_io.TextIOWrapper name='<stdin>' encoding='UTF-8'>)
Nuevo en la versión 3.4: The encodings and errors keyword arguments.
16.4.5.3. Argument groups¶
-
ArgumentParser.
add_argument_group
(title=None, description=None)¶ By default,
ArgumentParser
groups command-line arguments into «positional arguments» and «optional arguments» when displaying help messages. When there is a better conceptual grouping of arguments than this default one, appropriate groups can be created using theadd_argument_group()
method:>>> parser = argparse.ArgumentParser(prog='PROG', add_help=False) >>> group = parser.add_argument_group('group') >>> group.add_argument('--foo', help='foo help') >>> group.add_argument('bar', help='bar help') >>> parser.print_help() usage: PROG [--foo FOO] bar group: bar bar help --foo FOO foo help
The
add_argument_group()
method returns an argument group object which has anadd_argument()
method just like a regularArgumentParser
. When an argument is added to the group, the parser treats it just like a normal argument, but displays the argument in a separate group for help messages. Theadd_argument_group()
method accepts title and description arguments which can be used to customize this display:>>> parser = argparse.ArgumentParser(prog='PROG', add_help=False) >>> group1 = parser.add_argument_group('group1', 'group1 description') >>> group1.add_argument('foo', help='foo help') >>> group2 = parser.add_argument_group('group2', 'group2 description') >>> group2.add_argument('--bar', help='bar help') >>> parser.print_help() usage: PROG [--bar BAR] foo group1: group1 description foo foo help group2: group2 description --bar BAR bar help
Note that any arguments not in your user-defined groups will end up back in the usual «positional arguments» and «optional arguments» sections.
16.4.5.4. Mutual exclusion¶
-
ArgumentParser.
add_mutually_exclusive_group
(required=False)¶ Create a mutually exclusive group.
argparse
will make sure that only one of the arguments in the mutually exclusive group was present on the command line:>>> parser = argparse.ArgumentParser(prog='PROG') >>> group = parser.add_mutually_exclusive_group() >>> group.add_argument('--foo', action='store_true') >>> group.add_argument('--bar', action='store_false') >>> parser.parse_args(['--foo']) Namespace(bar=True, foo=True) >>> parser.parse_args(['--bar']) Namespace(bar=False, foo=False) >>> parser.parse_args(['--foo', '--bar']) usage: PROG [-h] [--foo | --bar] PROG: error: argument --bar: not allowed with argument --foo
The
add_mutually_exclusive_group()
method also accepts a required argument, to indicate that at least one of the mutually exclusive arguments is required:>>> parser = argparse.ArgumentParser(prog='PROG') >>> group = parser.add_mutually_exclusive_group(required=True) >>> group.add_argument('--foo', action='store_true') >>> group.add_argument('--bar', action='store_false') >>> parser.parse_args([]) usage: PROG [-h] (--foo | --bar) PROG: error: one of the arguments --foo --bar is required
Note that currently mutually exclusive argument groups do not support the title and description arguments of
add_argument_group()
.
16.4.5.5. Parser defaults¶
-
ArgumentParser.
set_defaults
(**kwargs)¶ Most of the time, the attributes of the object returned by
parse_args()
will be fully determined by inspecting the command-line arguments and the argument actions.set_defaults()
allows some additional attributes that are determined without any inspection of the command line to be added:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('foo', type=int) >>> parser.set_defaults(bar=42, baz='badger') >>> parser.parse_args(['736']) Namespace(bar=42, baz='badger', foo=736)
Note that parser-level defaults always override argument-level defaults:
>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', default='bar') >>> parser.set_defaults(foo='spam') >>> parser.parse_args([]) Namespace(foo='spam')
Parser-level defaults can be particularly useful when working with multiple parsers. See the
add_subparsers()
method for an example of this type.
-
ArgumentParser.
get_default
(dest)¶ Get the default value for a namespace attribute, as set by either
add_argument()
or byset_defaults()
:>>> parser = argparse.ArgumentParser() >>> parser.add_argument('--foo', default='badger') >>> parser.get_default('foo') 'badger'
16.4.5.6. Printing help¶
In most typical applications, parse_args()
will take
care of formatting and printing any usage or error messages. However, several
formatting methods are available:
-
ArgumentParser.
print_usage
(file=None)¶ Print a brief description of how the
ArgumentParser
should be invoked on the command line. If file isNone
,sys.stdout
is assumed.
-
ArgumentParser.
print_help
(file=None)¶ Print a help message, including the program usage and information about the arguments registered with the
ArgumentParser
. If file isNone
,sys.stdout
is assumed.
There are also variants of these methods that simply return a string instead of printing it:
-
ArgumentParser.
format_usage
()¶ Return a string containing a brief description of how the
ArgumentParser
should be invoked on the command line.
-
ArgumentParser.
format_help
()¶ Return a string containing a help message, including the program usage and information about the arguments registered with the
ArgumentParser
.
16.4.5.7. Partial parsing¶
-
ArgumentParser.
parse_known_args
(args=None, namespace=None)¶
Sometimes a script may only parse a few of the command-line arguments, passing
the remaining arguments on to another script or program. In these cases, the
parse_known_args()
method can be useful. It works much like
parse_args()
except that it does not produce an error when
extra arguments are present. Instead, it returns a two item tuple containing
the populated namespace and the list of remaining argument strings.
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', action='store_true')
>>> parser.add_argument('bar')
>>> parser.parse_known_args(['--foo', '--badger', 'BAR', 'spam'])
(Namespace(bar='BAR', foo=True), ['--badger', 'spam'])
Advertencia
Prefix matching rules apply to
parse_known_args()
. The parser may consume an option even if it’s just
a prefix of one of its known options, instead of leaving it in the remaining
arguments list.
16.4.5.8. Customizing file parsing¶
-
ArgumentParser.
convert_arg_line_to_args
(arg_line)¶ Arguments that are read from a file (see the fromfile_prefix_chars keyword argument to the
ArgumentParser
constructor) are read one argument per line.convert_arg_line_to_args()
can be overridden for fancier reading.This method takes a single argument arg_line which is a string read from the argument file. It returns a list of arguments parsed from this string. The method is called once per line read from the argument file, in order.
A useful override of this method is one that treats each space-separated word as an argument. The following example demonstrates how to do this:
class MyArgumentParser(argparse.ArgumentParser): def convert_arg_line_to_args(self, arg_line): return arg_line.split()
16.4.5.9. Exiting methods¶
-
ArgumentParser.
exit
(status=0, message=None)¶ This method terminates the program, exiting with the specified status and, if given, it prints a message before that.
-
ArgumentParser.
error
(message)¶ This method prints a usage message including the message to the standard error and terminates the program with a status code of 2.
16.4.6. Upgrading optparse code¶
Originally, the argparse
module had attempted to maintain compatibility
with optparse
. However, optparse
was difficult to extend
transparently, particularly with the changes required to support the new
nargs=
specifiers and better usage messages. When most everything in
optparse
had either been copy-pasted over or monkey-patched, it no
longer seemed practical to try to maintain the backwards compatibility.
The argparse
module improves on the standard library optparse
module in a number of ways including:
- Handling positional arguments.
- Supporting sub-commands.
- Allowing alternative option prefixes like
+
and/
. - Handling zero-or-more and one-or-more style arguments.
- Producing more informative usage messages.
- Providing a much simpler interface for custom
type
andaction
.
A partial upgrade path from optparse
to argparse
:
- Replace all
optparse.OptionParser.add_option()
calls withArgumentParser.add_argument()
calls. - Replace
(options, args) = parser.parse_args()
withargs = parser.parse_args()
and add additionalArgumentParser.add_argument()
calls for the positional arguments. Keep in mind that what was previously calledoptions
, now inargparse
context is calledargs
. - Replace callback actions and the
callback_*
keyword arguments withtype
oraction
arguments. - Replace string names for
type
keyword arguments with the corresponding type objects (e.g. int, float, complex, etc). - Replace
optparse.Values
withNamespace
andoptparse.OptionError
andoptparse.OptionValueError
withArgumentError
. - Replace strings with implicit arguments such as
%default
or%prog
with the standard Python syntax to use dictionaries to format strings, that is,%(default)s
and%(prog)s
. - Replace the OptionParser constructor
version
argument with a call toparser.add_argument('--version', action='version', version='<the version>')
.