15.4.1. 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.
15.4.1.1. 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.
15.4.1.2. 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.
15.4.1.3. 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.
15.4.2. 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)
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)
The following sections describe how each of these are used.
15.4.2.1. 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
15.4.2.2. 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.
15.4.2.3. 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.
15.4.2.4. 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.
15.4.2.5. 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.
Note
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.
15.4.2.7. 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.
15.4.2.8. 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.
15.4.2.9. 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()
15.4.2.10. 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.
15.4.2.11. 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
15.4.3. 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().
The following sections describe how each of these are used.
15.4.3.1. 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
15.4.3.2. 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' using for storing the values True and False
respectively. In addition, they create default values of False and True
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(bar=False, baz=True, foo=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 to None.) 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=[<type 'str'>, <type '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. See ArgumentParser for details of how the
output is created.
'version' - This expects a version= keyword argument in the
add_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.
15.4.3.3. 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=<open file 'input.txt', mode 'r' at 0x...>,
outfile=<open file 'output.txt', mode 'w' at 0x...>)
>>> parser.parse_args([])
Namespace(infile=<open file '<stdin>', mode 'r' at 0x...>,
outfile=<open file '<stdout>', mode 'w' at 0x...>)
'*'. 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
with nargs='*', but multiple optional arguments with nargs='*' 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.
15.4.3.4. 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 with
action='store_const' or action='append_const'. These actions add the
const value to one of the attributes of the object returned by
parse_args(). See the action description for examples.
- When
add_argument() is called with option strings
(like -f or --foo) and nargs='?'. 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 of const 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.
15.4.3.5. 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')
15.4.3.6. 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=file)
>>> parser.parse_args('2 temp.txt'.split())
Namespace(bar=<open file 'temp.txt', mode 'r' at 0x...>, 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= and bufsize= arguments of the
file object. 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=<open file 'out.txt', mode 'w' at 0x...>)
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=xrange(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.
15.4.3.7. 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.
15.4.3.8. 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.
Note
Required options are generally considered bad form because users expect
options to be optional, and thus they should be avoided when possible.
15.4.3.9. 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
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
15.4.3.11. 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')
15.4.3.12. 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 - The Namespace object that will be returned by
parse_args(). Most actions add an attribute to this
object using setattr().
values - The associated command-line arguments, with any type conversions
applied. Type conversions are specified with the type keyword argument to
add_argument().
option_string - The option string that was used to invoke this action.
The option_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.
15.4.4. 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 for
add_argument() for details.
- args - List of strings to parse. The default is taken from
sys.argv.
- namespace - An object to take the attributes. The default is a new empty
Namespace object.
15.4.4.1. 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')
15.4.4.2. 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
15.4.4.3. 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)
15.4.4.4. Argument abbreviations (prefix matching)
The parse_args() method 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.
15.4.4.5. 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=xrange(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])
15.4.4.6. 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(object):
... pass
...
>>> c = C()
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo')
>>> parser.parse_args(args=['--foo', 'BAR'], namespace=c)
>>> c.foo
'BAR'