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argparse - argparse tutorial

ARGPARSE(1) argparse ARGPARSE(1)

NAME

argparse - argparse tutorial
 
Contents:

CREATING AND USING PARSERS

The argparse 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([args]) method of the Parser class returns a table with processed data from the command line or args array.
 
local args = parser:parse()
print(args)  -- Assuming print is patched to handle tables nicely.


 
When executed, this script prints {} because the parser is empty and no command line arguments were supplied.

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([args]) 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 parsers

Parsers have several properties affecting their behavior. For example, description and epilog properties set the text to be displayed in the help message after the usage message and after the listings of options and arguments, respectively. 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 properties. The first is to chain setter methods of Parser object.
 
local parser = argparse()
   :name "script"
   :description "A testing script."
   :epilog "For more info, see http://example.com"


 
The second is to call a parser with a table containing some properties.
 
local parser = argparse() {
   name = "script",
   description = "A testing script.",
   epilog "For more info, see http://example.com."
}


 
Finally, name. description and epilog properties can be passed as arguments when calling a parser.
 
local parser = argparse("script", "A testing script.", "For more info, see http://example.com.")


ADDING AND CONFIGURING ARGUMENTS

Positional arguments can be added using :argument(name, description, default, convert, args) method. It returns an Argument instance, which can be configured in the same way as Parsers. The name property is required.
 
parser:argument "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 property.

Setting number of consumed arguments

args property 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 consumed, a table is used to store the data.
 
parser:argument("pair", "A pair of arguments.")
   :args(2)
parser:argument("optional", "An optional argument.")
   :args "?"


 
$ lua script.lua foo bar


 
{
   pair = {"foo", "bar"}
}


 
$ lua script.lua foo bar baz


 
{
   pair = {"foo", "bar"},
   optional = "baz"
}


ADDING AND CONFIGURING OPTIONS

Options can be added using :option(name, description, default, convert, args, count) method. It returns an Option instance, which can be configured in the same way as Parsers. The name property is required. An option can have several aliases, which can be set as space separated substrings in its name or by continuously setting name property.
 
-- These lines are equivalent:
parser:option "-f" "--from"
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, typically hyphens) 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() has 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 property.

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 consumed 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 property to set how many times an option can be invoked. The value of the property 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 the associated field of the result table.
 
parser:flag("-v --verbose", "Sets verbosity level.")
   :count "0-2"
   :target "verbosity"


 
$ lua script.lua -vv


 
{
   verbosity = 2
}


MUTUALLY EXCLUSIVE GROUPS

A group of options can be marked as mutually exclusive using :mutex(option, ...) 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'


ADDING AND CONFIGURING COMMANDS

A command is a subparser invoked when its name is passed as an argument. For example, in git CLI add, commit, push, etc. are commands. Each command has its own set of arguments and options, but inherits options of its parent.
 
Commands can be added using :command(name, description, epilog) 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'?


Getting name of selected command

Use command_target property of the parser to store the name of used command in a field of the result table.
 
parser:command_target("command")
parser:command("install")
parser:command("remove")


 
$ lua script.lua install


 
{
   install = true,
   command = "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 require_command property.
 
local parser = argparse()
   :require_command(false)
parser:command "install"


DEFAULT VALUES

For elements such as arguments and options, if default property is set to a string, its value is stored in case the element was not used (if it's not a string, it'll be used as init property instead, see actions).
 
parser:option("-o --output", "Output file.", "a.out")
-- Equivalent:
parser:option "-o" "--output"
   :description "Output file."
   :default "a.out"


 
$ lua script.lua


 
{
   output = "a.out"
}


 
The existence of a default value is reflected in help message, unless show_default property is set to false.
 
$ lua script.lua --help


 
Usage: script.lua [-o <output>] [-h]
Options: -o <output>, --output <output> Output file. (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.lua [-o <output>] [-h]
Error: too few arguments


Default mode

defmode property 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.lua [-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.lua [-o <o>] [-p [<p>]] [-h]
Error: too few arguments


CALLBACKS

Converters

argparse can perform automatic validation and conversion on arguments. If convert property 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_object,
   times = 5
}


 
$ 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 property of an element is 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

Argument and option actions

argparse uses action callbacks to process invocations of arguments and options. Default actions simply put passed arguments into the result table as a single value or insert into an array depending on number of arguments the option can take and how many times it can be used.
 
A custom action can be set using action property. An action must be a function. and will be called after each invocation of the option or the argument it is assigned to. Four arguments are passed: result table, target index in that table, an argument or an array of arguments passed by user, and overwrite flag used when an option is invoked too many times.
 
Converters are applied before actions.
 
Initial value to be stored at target index in the result table can be set using init property, or also using default property if the value is not a string.
 
parser:option("--exceptions"):args("*"):action(function(args, _, exceptions)
   for _, exception in ipairs(exceptions) do
      table.insert(args.exceptions, exception)
   end
end):init({"foo", "bar"})
parser:flag("--no-exceptions"):action(function() args.exceptions = {} end)


 
$ lua script.lua --exceptions x y --exceptions z t


 
{
   exceptions = {
      "foo",
      "bar",
      "x",
      "y",
      "z",
      "t"
   }
}


 
$ lua script.lua --exceptions x y --no-exceptions


 
{
   exceptions = {}
}


 
Actions can also be used when a flag needs to print some message and exit without parsing remaining arguments.
 
parser:flag("-v --version"):action(function()
   print("script v1.0.0")
   os.exit(0)
end)


 
$ lua script.lua -v


 
script v1.0.0


Built-in actions

These actions can be referred to by their string names when setting action property:
Name Description
store Stores argument or arguments at target index.
store_true Stores true at target index.
store_false Stores false at target index.
count Increments number at target index.
append Appends argument or arguments to table at target index.
concat Appends arguments one by one to table at target index.
 
Examples using store_false and concat actions:
 
parser:flag("--candy")
parser:flag("--no-candy"):target("candy"):action("store_false")
parser:flag("--rain", "Enable rain", false)
parser:option("--exceptions"):args("*"):action("concat"):init({"foo", "bar"})


 
$ lua script.lua


 
{
   rain = false
}


 
$ lua script.lua --candy


 
{
   candy = true,
   rain = false
}


 
$ lua script.lua --no-candy --rain


 
{
   candy = false,
   rain = true
}


 
$ lua script.lua --exceptions x y --exceptions z t


 
{
   exceptions = {
      "foo",
      "bar",
      "x",
      "y",
      "z",
      "t"
   },
   rain = false
}


Command actions

Actions for parsers and commands are simply callbacks invoked after parsing, with result table and command name as the arguments. Actions for nested commands are called first.
 
local install = parser:command("install"):action(function(args, name)
   print("Running " .. name)
   -- Use args here
)
parser:action(function(args) print("Callbacks are fun!") end)


 
$ lua script.lua install


 
Running install
Callbacks are fun!


MISCELLANEOUS

Generating and overwriting help and usage messages

The usage and help messages of parsers and commands can be generated on demand using :get_usage() and :get_help() methods, and overridden using help and usage properties.

Overwriting default help option

If the property 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.


Setting argument placeholder

For options and arguments, argname property 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.


Disabling option handling

When handle_options property of a parser or a command is set to false, all options will be passed verbatim to the argument list, as if the input included double-hyphens.
 
parser:handle_options(false)
parser:argument "input"
   :args "*"
parser:option "-f" "--foo"
   :args "*"


 
$ lua script.lua bar -f --foo bar


 
{
   input = {"bar", "-f", "--foo", "bar"}
}


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 overwrite property 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


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.

Property lists

Parser properties

Properties that can be set as arguments when calling or constructing a parser, in this order:
Property Type
name String
description String
epilog String
 
Other properties:
Property Type
usage String
help String
require_command Boolean
handle_options Boolean
add_help Boolean or string or table
command_target String

Command properties

Properties that can be set as arguments when calling or constructing a command, in this order:
Property Type
name String
description String
epilog String
 
Other properties:
Property Type
target String
usage String
help String
require_command Boolean
handle_options Boolean
action Function
add_help Boolean or string or table
command_target String

Argument properties

Properties that can be set as arguments when calling or constructing an argument, in this order:
Property Type
name String
description String
default Any
convert Function or table
args Number or string
 
Other properties:
Property Type
target String
defmode String
show_default Boolean
argname String or table
action Function or string
init Any

Option and flag properties

Properties that can be set as arguments when calling or constructing an option or a flag, in this order:
Property Type
name String
description String
default Any
convert Function or table
args Number or string
count Number or string
 
Other properties:
Property Type
target String
defmode String
show_default Boolean
overwrite Booleans
argname String or table
action Function or string
init Any
 
This is a tutorial for argparse, a feature-rich command line parser for Lua.

AUTHOR

Peter Melnichenko 2017 - 2015, Peter Melnichenko
May 22, 2017 0.5.0