Argparse 0.3.0 tutorial

Creating a parser
Parsing command line arguments
Configuring parser
Adding arguments
- Setting number of arguments
Adding options
Mutually exclusive groups
Commands
- Adding elements to commands
- Making a command optional
Default values
- Default mode
Converters
- Table converters
Actions
Miscellaneous

Creating a parser

The module is a function which, when called, creates an instance of the Parser class.

-- script.lua
local argparse = require "argparse"
local parser = argparse()

parser is now an empty parser which does not recognize any command line arguments or options.

Parsing command line arguments

:parse([cmdline]) method of the Parser class returns a table with processed data from the command line or cmdline array.

local args = parser:parse()
for k, v in pairs(args) do print(k, v) end

When executed, this script prints nothing because the parser is empty and no command line arguments were supplied.

$ lua script.lua

Error handling

If the provided command line arguments are not recognized by the parser, it will print an error message and call os.exit(1).

$ lua script.lua foo

Usage: script.lua [-h]

Error: too many arguments

If halting the program is undesirable, :pparse([cmdline]) method should be used. It returns boolean flag indicating success of parsing and result or error message.

An error can raised manually using :error() method.

parser:error("manual argument validation failed")

Usage: script.lua [-h]

Error: manual argument validation failed

Help option

As the automatically generated usage message states, there is a help option -h added to any parser by default.

When a help option is used, parser will print a help message and call os.exit(0).

$ lua script.lua -h

Usage: script.lua [-h]

Options: 
   -h, --help            Show this help message and exit.

Typo autocorrection

When an option is not recognized by the parser, but there is an option with a similar name, a suggestion is automatically added to the error message.

$ lua script.lua --hepl

Usage: script.lua [-h]

Error: unknown option '--hepl'
Did you mean '--help'?

Configuring parser

Parsers have several fields affecting their behavior. For example, description field sets the text to be displayed in the help message between the usage message and the listings of options and arguments. Another is name, which overwrites the name of the program which is used in the usage message(default value is inferred from command line arguments).

There are several ways to set fields. The first is to call a parser with a table containing some fields.

local parser = argparse() {
   name = "script",
   description = "A testing script. "
}

The second is to chain setter methods of Parser object.

local parser = argparse()
   :name "script"
   :description "A testing script. "

As a special case, name field can be set by calling a parser with a string.

local parser = argparse "script"
   :description "A testing script. "

Adding arguments

Positional arguments can be added using :argument() method. It returns an Argument instance, which can be configured in the same way as Parsers. The name field is required.

parser:argument "input" -- sugar for :argument():name "input"

$ lua script.lua foo

input   foo

The data passed to the argument is stored in the result table at index input because it is the argument's name. The index can be changed using target field.

Setting number of arguments

args field sets how many command line arguments the argument consumes. Its value is interpreted as follows:

Value	Interpretation
Number `N`	Exactly `N` arguments
String `"A-B"`, where `A` and `B` are numbers	From `A` to `B` arguments
String `"N+"`, where `N` is a number	`N` or more arguments
String `"?"`	An optional argument
String `"*"`	Any number of arguments
String `"+"`	At least one argument

If more than one argument can be passed, a table is used to store the data.

parser:argument "pair"
   :args(2)
   :description "A pair of arguments. "
parser:argument "optional"
   :args "?"
   :description "An optional argument. "

$ lua script.lua foo bar

pair    {foo, bar}

$ lua script.lua foo bar baz

pair    {foo, bar}
optional    baz

Adding options

Options can be added using :option() method. It returns an Option instance, which can be configured in the same way as Parsers. The name field is required. An option can have several aliases, which can be set using aliases field or by continuously calling the Option instance.

parser:option "-f" "--from"

$ lua script.lua --from there
$ lua script.lua --from=there
$ lua script.lua -f there
$ lua script.lua -fthere

from    there

For an option, default index used to store arguments passed to it is the first 'long' alias (an alias starting with two control characters) or just the first alias, without control characters. Hyphens in the default index are replaced with underscores. In the following table it is assumed that local args = parser:parse() have been executed.

Option's aliases	Location of option's arguments
`-o`	`args.o`
`-o` `--output`	`args.output`
`-s` `--from-server`	`args.from_server`

As with arguments, the index can be explicitly set using target field.

Flags

Flags are almost identical to options, except that they don't take an argument by default.

parser:flag "-q" "--quiet"

$ lua script.lua -q

quiet   true

Control characters

The first characters of all aliases of all options of a parser form the set of control characters, used to distinguish options from arguments. Typically the set only consists of a hyphen.

Setting number of arguments

Just as arguments, options can be configured to take several command line arguments.

parser:option "--pair"
   :args(2)
parser:option "--optional"
   :args "?"

$ lua script.lua --pair foo bar

pair    {foo, bar}

$ lua script.lua --pair foo bar --optional

pair    {foo, bar}
optional    {}

$ lua script.lua --optional=baz

optional    {baz}

Note that the data passed to optional option is stored in an array. That is necessary to distinguish whether the option was invoked without an argument or it was not invoked at all.

Setting number of invocations

For options, it is possible to control how many times they can be used. argparse uses count field to set how many times an option can be invoked. The value of the field is interpreted in the same way args is.

parser:option "-e" "--exclude"
   :count "*"

$ lua script.lua -eFOO -eBAR

exclude {FOO, BAR}

If an option can be used more than once and it can consume more than one argument, the data is stored as an array of invocations, each being an array of arguments.

As a special case, if an option can be used more than once and it consumes no arguments(e.g. it's a flag), than the number of invocations is stored in associated field of the result table.

parser:flag "-v" "--verbose"
   :count "0-2"
   :target "verbosity"
   :description [[Sets verbosity level. 
-v: Report all warning. 
-vv: Report all debugging information. ]]

$ lua script.lua -vv

verbosity   2

Mutually exclusive groups

A group of options can be marked as mutually exclusive using :mutex() method of the Parser class.

parser:mutex(
   parser:flag "-q" "--quiet",
   parser:flag "-v" "--verbose"
)

If more than one element of a mutually exclusive group is used, an error is raised.

$ lua script.lua -qv

Usage: script.lua ([-q] | [-v]) [-h]

Error: option '-v' can not be used together with option '-q'

Commands

A command is a subparser invoked when its name is passed as an argument. For example, in luarocks CLI install, make, build, etc. are commands. Each command has its own set of arguments and options.

Commands can be added using :command() method. Just as options, commands can have several aliases.

parser:command "install" "i"

If a command it used, true is stored in the corresponding field of the result table.

$ lua script.lua install

install true

A typo will result in an appropriate error message:

$ lua script.lua instal

Usage: script.lua [-h] <command> ...

Error: unknown command 'instal'
Did you mean 'install'?

Adding elements to commands

The Command class is a subclass of the Parser class, so all the Parser's methods for adding elements work on commands, too.

local install = parser:command "install"
install:argument "rock"
install:option "-f" "--from"

$ lua script.lua install foo --from=bar

install true
rock    foo
from    bar

Commands have their own usage and help messages.

$ lua script.lua install

Usage: script.lua install [-f <from>] [-h] <rock>

Error: too few arguments

$ lua script.lua install --help

Usage: script.lua install [-f <from>] [-h] <rock>

Arguments: 
   rock

Options: 
   -f <from>, --from <from>
   -h, --help            Show this help message and exit.

Making a command optional

By default, if a parser has commands, using one of them is obligatory.

local parser = argparse()
parser:command "install"

$ lua script.lua

Usage: script.lua [-h] <command> ...

Error: a command is required

This can be changed using the require_command field.

local parser = argparse()
   :require_command(false)
parser:command "install"

Now not using a command is not an error:

$ lua script.lua

produces nothing.

Default values

For elements such as arguments and options, if default field is set, its value is stored in case the element was not used.

parser:option "-o" "--output"
   :default "a.out"

$ lua script.lua

output  a.out

The existence of a default value is reflected in help message.

$ lua script.lua --help

Usage: script [-o <output>] [-h]

Options: 
   -o <output>, --output <output>
                         default: a.out
   -h, --help            Show this help message and exit.

Note that invocation without required arguments is still an error.

$ lua script.lua -o

Usage: script [-o <output>] [-h]

Error: too few arguments

Default mode

The defmode field regulates how argparse should use the default value of an element.

If defmode contains "u"(for unused), the default value will be automatically passed to the element if it was not invoked at all. This is the default behavior.

If defmode contains "a"(for argument), the default value will be automatically passed to the element if not enough arguments were passed, or not enough invocations were made.

Consider the difference:

parser:option "-o"
   :default "a.out"
parser:option "-p" 
   :default "password"
   :defmode "arg"

$ lua script.lua -h

Usage: script [-o <o>] [-p [<p>]] [-h]

Options: 
   -o <o>                default: a.out
   -p [<p>]              default: password
   -h, --help            Show this help message and exit.

$ lua script.lua

o   a.out

$ lua script.lua -p

o   a.out
p   password

$ lua script.lua -o

Usage: script [-o <o>] [-p [<p>]] [-h]

Error: too few arguments

Converters

argparse can perform automatic validation and conversion on arguments. If convert field of an element is a function, it will be applied to all the arguments passed to it. The function should return nil and, optionally, an error message if conversion failed. Standard tonumber and io.open functions work exactly like that.

parser:argument "input"
   :convert(io.open)
parser:option "-t" "--times"
   :convert(tonumber)

$ lua script.lua foo.txt -t5

input   file (0xaddress)
times   5 (number)

$ lua script.lua nonexistent.txt

Usage: script.lua [-t <times>] [-h] <input>

Error: nonexistent.txt: No such file or directory

$ lua script.lua foo.txt --times=many

Usage: script.lua [-t <times>] [-h] <input>

Error: malformed argument 'many'

Table converters

If convert field of an element contains a table, arguments passed to it will be used as keys. If a key is missing, an error is raised.

parser:argument "choice"
   :convert {
      foo = "Something foo-related",
      bar = "Something bar-related"
   }

$ lua script.lua bar

choice  Something bar-related

$ lua script.lua baz

Usage: script.lua [-h] <choice>

Error: malformed argument 'baz'

Actions

argparse can trigger a callback when an option or a command is encountered. The callback can be set using action field. Actions are called regardless of whether the rest of command line arguments are correct.

parser:flag "-v" "--version"
   :description "Show version info and exit. "
   :action(function()
      print("script.lua v1.0.0")
      os.exit(0)
   end)

$ lua script.lua -v

script.lua v1.0.0

This example would work even if the script had mandatory arguments.

Miscellaneous

Overwriting default help option

If the field add_help of a parser is set to false, no help option will be added to it. Otherwise, the value of the field will be used to configure it.

local parser = argparse()
   :add_help "/?"

$ lua script.lua /?

Usage: script.lua [/?]

Options: 
   /?                    Show this help message and exit.

Configuring usage and help messages

Setting description and epilog

The value of description field of a parser is placed between the usage message and the argument list in the help message.

The value of epilog field is appended to the help message.

local parser = argparse "script"
   :description "A description. "
   :epilog "An epilog. "

$ lua script.lua --help

Usage: script [-h]

A description. 

Options: 
   -h, --help            Show this help message and exit. 

An epilog.

Setting argument placeholder

For options and arguments, argname field controls the placeholder for the argument in the usage message.

parser:option "-f" "--from"
   :argname "<server>"

$ lua script.lua --help

Usage: script.lua [-f <server>] [-h]

Options: 
   -f <server>, --from <server>
   -h, --help            Show this help message and exit.

argname can be an array of placeholders.

parser:option "--pair"
   :args(2)
   :argname{"<key>", "<value>"}

$ lua script.lua --help

Usage: script.lua [--pair <key> <value>] [-h]

Options: 
   --pair <key> <value>
   -h, --help            Show this help message and exit.

Hiding default value

If an option has a default value, argparse will automatically append (default: <value>>) to its usage message. This can be disabled using show_default field:

parser:option "-o" "--output"
   :default "a.out"
   :show_default(false)

$ lua script.lua --help

Usage: script [-o <output>] [-h]

Options: 
   -o <output>, --output <output>
   -h, --help            Show this help message and exit.

Prohibiting overuse of options

By default, if an option is invoked too many times, latest invocations overwrite the data passed earlier.

parser:option "-o" "--output"

$ lua script.lua -oFOO -oBAR

output   BAR

Set the overwrite field to false to prohibit this behavior.

parser:option "-o" "--output"
   :overwrite(false)

$ lua script.lua -oFOO -oBAR

Usage: script.lua [-o <output>] [-h]

Error: option '-o' must be used at most 1 time

Generating usage and help messages

:get_help() and :get_usage() methods of Parser and Command classes can be used to generate their help and usage messages.

Parsing algorithm

argparse interprets command line arguments in the following way:

Argument	Interpretation
`foo`	An argument of an option or a positional argument.
`--foo`	An option.
`--foo=bar`	An option and its argument. The option must be able to take arguments.
`-f`	An option.
`-abcdef`	Letters are interpreted as options. If one of them can take an argument, the rest of the string is passed to it.
`--`	The rest of the command line arguments will be interpreted as positional arguments.