15.5. "optparse" — Parser for command line options
**************************************************

New in version 2.3.

Deprecated since version 2.7: The "optparse" module is deprecated and
will not be developed further; development will continue with the
"argparse" module.

**Source code:** Lib/optparse.py

======================================================================

"optparse" is a more convenient, flexible, and powerful library for
parsing command-line options than the old "getopt" module.  "optparse"
uses a more declarative style of command-line parsing: you create an
instance of "OptionParser", populate it with options, and parse the
command line. "optparse" allows users to specify options in the
conventional GNU/POSIX syntax, and additionally generates usage and
help messages for you.

Here’s an example of using "optparse" in a simple script:

   from optparse import OptionParser
   ...
   parser = OptionParser()
   parser.add_option("-f", "--file", dest="filename",
                     help="write report to FILE", metavar="FILE")
   parser.add_option("-q", "--quiet",
                     action="store_false", dest="verbose", default=True,
                     help="don't print status messages to stdout")

   (options, args) = parser.parse_args()

With these few lines of code, users of your script can now do the
“usual thing” on the command-line, for example:

   <yourscript> --file=outfile -q

As it parses the command line, "optparse" sets attributes of the
"options" object returned by "parse_args()" based on user-supplied
command-line values.  When "parse_args()" returns from parsing this
command line, "options.filename" will be ""outfile"" and
"options.verbose" will be "False".  "optparse" supports both long and
short options, allows short options to be merged together, and allows
options to be associated with their arguments in a variety of ways.
Thus, the following command lines are all equivalent to the above
example:

   <yourscript> -f outfile --quiet
   <yourscript> --quiet --file outfile
   <yourscript> -q -foutfile
   <yourscript> -qfoutfile

Additionally, users can run one of

   <yourscript> -h
   <yourscript> --help

and "optparse" will print out a brief summary of your script’s
options:

   Usage: <yourscript> [options]

   Options:
     -h, --help            show this help message and exit
     -f FILE, --file=FILE  write report to FILE
     -q, --quiet           don't print status messages to stdout

where the value of *yourscript* is determined at runtime (normally
from "sys.argv[0]").


15.5.1. Background
==================

"optparse" was explicitly designed to encourage the creation of
programs with straightforward, conventional command-line interfaces.
To that end, it supports only the most common command-line syntax and
semantics conventionally used under Unix.  If you are unfamiliar with
these conventions, read this section to acquaint yourself with them.


15.5.1.1. Terminology
---------------------

argument
   a string entered on the command-line, and passed by the shell to
   "execl()" or "execv()".  In Python, arguments are elements of
   "sys.argv[1:]" ("sys.argv[0]" is the name of the program being
   executed).  Unix shells also use the term “word”.

   It is occasionally desirable to substitute an argument list other
   than "sys.argv[1:]", so you should read “argument” as “an element
   of "sys.argv[1:]", or of some other list provided as a substitute
   for "sys.argv[1:]"”.

option
   an argument used to supply extra information to guide or customize
   the execution of a program.  There are many different syntaxes for
   options; the traditional Unix syntax is a hyphen (“-“) followed by
   a single letter, e.g. "-x" or "-F".  Also, traditional Unix syntax
   allows multiple options to be merged into a single argument, e.g.
   "-x -F" is equivalent to "-xF".  The GNU project introduced "--"
   followed by a series of hyphen-separated words, e.g. "--file" or "
   --dry-run".  These are the only two option syntaxes provided by
   "optparse".

   Some other option syntaxes that the world has seen include:

   * a hyphen followed by a few letters, e.g. "-pf" (this is *not*
     the same as multiple options merged into a single argument)

   * a hyphen followed by a whole word, e.g. "-file" (this is
     technically equivalent to the previous syntax, but they aren’t
     usually seen in the same program)

   * a plus sign followed by a single letter, or a few letters, or a
     word, e.g. "+f", "+rgb"

   * a slash followed by a letter, or a few letters, or a word, e.g.
     "/f", "/file"

   These option syntaxes are not supported by "optparse", and they
   never will be.  This is deliberate: the first three are non-
   standard on any environment, and the last only makes sense if
   you’re exclusively targeting VMS, MS-DOS, and/or Windows.

option argument
   an argument that follows an option, is closely associated with that
   option, and is consumed from the argument list when that option is.
   With "optparse", option arguments may either be in a separate
   argument from their option:

      -f foo
      --file foo

   or included in the same argument:

      -ffoo
      --file=foo

   Typically, a given option either takes an argument or it doesn’t.
   Lots of people want an “optional option arguments” feature, meaning
   that some options will take an argument if they see it, and won’t
   if they don’t.  This is somewhat controversial, because it makes
   parsing ambiguous: if "-a" takes an optional argument and "-b" is
   another option entirely, how do we interpret "-ab"?  Because of
   this ambiguity, "optparse" does not support this feature.

positional argument
   something leftover in the argument list after options have been
   parsed, i.e. after options and their arguments have been parsed and
   removed from the argument list.

required option
   an option that must be supplied on the command-line; note that the
   phrase “required option” is self-contradictory in English.
   "optparse" doesn’t prevent you from implementing required options,
   but doesn’t give you much help at it either.

For example, consider this hypothetical command-line:

   prog -v --report report.txt foo bar

"-v" and "--report" are both options.  Assuming that "--report" takes
one argument, "report.txt" is an option argument.  "foo" and "bar" are
positional arguments.


15.5.1.2. What are options for?
-------------------------------

Options are used to provide extra information to tune or customize the
execution of a program.  In case it wasn’t clear, options are usually
*optional*.  A program should be able to run just fine with no options
whatsoever.  (Pick a random program from the Unix or GNU toolsets.
Can it run without any options at all and still make sense?  The main
exceptions are "find", "tar", and "dd"—all of which are mutant
oddballs that have been rightly criticized for their non-standard
syntax and confusing interfaces.)

Lots of people want their programs to have “required options”.  Think
about it. If it’s required, then it’s *not optional*!  If there is a
piece of information that your program absolutely requires in order to
run successfully, that’s what positional arguments are for.

As an example of good command-line interface design, consider the
humble "cp" utility, for copying files.  It doesn’t make much sense to
try to copy files without supplying a destination and at least one
source. Hence, "cp" fails if you run it with no arguments.  However,
it has a flexible, useful syntax that does not require any options at
all:

   cp SOURCE DEST
   cp SOURCE ... DEST-DIR

You can get pretty far with just that.  Most "cp" implementations
provide a bunch of options to tweak exactly how the files are copied:
you can preserve mode and modification time, avoid following symlinks,
ask before clobbering existing files, etc.  But none of this distracts
from the core mission of "cp", which is to copy either one file to
another, or several files to another directory.


15.5.1.3. What are positional arguments for?
--------------------------------------------

Positional arguments are for those pieces of information that your
program absolutely, positively requires to run.

A good user interface should have as few absolute requirements as
possible.  If your program requires 17 distinct pieces of information
in order to run successfully, it doesn’t much matter *how* you get
that information from the user—most people will give up and walk away
before they successfully run the program.  This applies whether the
user interface is a command-line, a configuration file, or a GUI: if
you make that many demands on your users, most of them will simply
give up.

In short, try to minimize the amount of information that users are
absolutely required to supply—use sensible defaults whenever possible.
Of course, you also want to make your programs reasonably flexible.
That’s what options are for.  Again, it doesn’t matter if they are
entries in a config file, widgets in the “Preferences” dialog of a
GUI, or command-line options—the more options you implement, the more
flexible your program is, and the more complicated its implementation
becomes.  Too much flexibility has drawbacks as well, of course; too
many options can overwhelm users and make your code much harder to
maintain.


15.5.2. Tutorial
================

While "optparse" is quite flexible and powerful, it’s also
straightforward to use in most cases.  This section covers the code
patterns that are common to any "optparse"-based program.

First, you need to import the OptionParser class; then, early in the
main program, create an OptionParser instance:

   from optparse import OptionParser
   ...
   parser = OptionParser()

Then you can start defining options.  The basic syntax is:

   parser.add_option(opt_str, ...,
                     attr=value, ...)

Each option has one or more option strings, such as "-f" or "--file",
and several option attributes that tell "optparse" what to expect and
what to do when it encounters that option on the command line.

Typically, each option will have one short option string and one long
option string, e.g.:

   parser.add_option("-f", "--file", ...)

You’re free to define as many short option strings and as many long
option strings as you like (including zero), as long as there is at
least one option string overall.

The option strings passed to "OptionParser.add_option()" are
effectively labels for the option defined by that call.  For brevity,
we will frequently refer to *encountering an option* on the command
line; in reality, "optparse" encounters *option strings* and looks up
options from them.

Once all of your options are defined, instruct "optparse" to parse
your program’s command line:

   (options, args) = parser.parse_args()

(If you like, you can pass a custom argument list to "parse_args()",
but that’s rarely necessary: by default it uses "sys.argv[1:]".)

"parse_args()" returns two values:

* "options", an object containing values for all of your
  options—e.g. if "--file" takes a single string argument, then
  "options.file" will be the filename supplied by the user, or "None"
  if the user did not supply that option

* "args", the list of positional arguments leftover after parsing
  options

This tutorial section only covers the four most important option
attributes: "action", "type", "dest" (destination), and "help". Of
these, "action" is the most fundamental.


15.5.2.1. Understanding option actions
--------------------------------------

Actions tell "optparse" what to do when it encounters an option on the
command line.  There is a fixed set of actions hard-coded into
"optparse"; adding new actions is an advanced topic covered in section
Extending optparse.  Most actions tell "optparse" to store a value in
some variable—for example, take a string from the command line and
store it in an attribute of "options".

If you don’t specify an option action, "optparse" defaults to "store".


15.5.2.2. The store action
--------------------------

The most common option action is "store", which tells "optparse" to
take the next argument (or the remainder of the current argument),
ensure that it is of the correct type, and store it to your chosen
destination.

For example:

   parser.add_option("-f", "--file",
                     action="store", type="string", dest="filename")

Now let’s make up a fake command line and ask "optparse" to parse it:

   args = ["-f", "foo.txt"]
   (options, args) = parser.parse_args(args)

When "optparse" sees the option string "-f", it consumes the next
argument, "foo.txt", and stores it in "options.filename".  So, after
this call to "parse_args()", "options.filename" is ""foo.txt"".

Some other option types supported by "optparse" are "int" and "float".
Here’s an option that expects an integer argument:

   parser.add_option("-n", type="int", dest="num")

Note that this option has no long option string, which is perfectly
acceptable. Also, there’s no explicit action, since the default is
"store".

Let’s parse another fake command-line.  This time, we’ll jam the
option argument right up against the option: since "-n42" (one
argument) is equivalent to "-n 42" (two arguments), the code

   (options, args) = parser.parse_args(["-n42"])
   print options.num

will print "42".

If you don’t specify a type, "optparse" assumes "string".  Combined
with the fact that the default action is "store", that means our first
example can be a lot shorter:

   parser.add_option("-f", "--file", dest="filename")

If you don’t supply a destination, "optparse" figures out a sensible
default from the option strings: if the first long option string is "
--foo-bar", then the default destination is "foo_bar".  If there are
no long option strings, "optparse" looks at the first short option
string: the default destination for "-f" is "f".

"optparse" also includes built-in "long" and "complex" types.  Adding
types is covered in section Extending optparse.


15.5.2.3. Handling boolean (flag) options
-----------------------------------------

Flag options—set a variable to true or false when a particular option
is seen —are quite common.  "optparse" supports them with two separate
actions, "store_true" and "store_false".  For example, you might have
a "verbose" flag that is turned on with "-v" and off with "-q":

   parser.add_option("-v", action="store_true", dest="verbose")
   parser.add_option("-q", action="store_false", dest="verbose")

Here we have two different options with the same destination, which is
perfectly OK.  (It just means you have to be a bit careful when
setting default values— see below.)

When "optparse" encounters "-v" on the command line, it sets
"options.verbose" to "True"; when it encounters "-q",
"options.verbose" is set to "False".


15.5.2.4. Other actions
-----------------------

Some other actions supported by "optparse" are:

""store_const""
   store a constant value

""append""
   append this option’s argument to a list

""count""
   increment a counter by one

""callback""
   call a specified function

These are covered in section Reference Guide, Reference Guide and
section Option Callbacks.


15.5.2.5. Default values
------------------------

All of the above examples involve setting some variable (the
“destination”) when certain command-line options are seen.  What
happens if those options are never seen?  Since we didn’t supply any
defaults, they are all set to "None".  This is usually fine, but
sometimes you want more control.  "optparse" lets you supply a default
value for each destination, which is assigned before the command line
is parsed.

First, consider the verbose/quiet example.  If we want "optparse" to
set "verbose" to "True" unless "-q" is seen, then we can do this:

   parser.add_option("-v", action="store_true", dest="verbose", default=True)
   parser.add_option("-q", action="store_false", dest="verbose")

Since default values apply to the *destination* rather than to any
particular option, and these two options happen to have the same
destination, this is exactly equivalent:

   parser.add_option("-v", action="store_true", dest="verbose")
   parser.add_option("-q", action="store_false", dest="verbose", default=True)

Consider this:

   parser.add_option("-v", action="store_true", dest="verbose", default=False)
   parser.add_option("-q", action="store_false", dest="verbose", default=True)

Again, the default value for "verbose" will be "True": the last
default value supplied for any particular destination is the one that
counts.

A clearer way to specify default values is the "set_defaults()" method
of OptionParser, which you can call at any time before calling
"parse_args()":

   parser.set_defaults(verbose=True)
   parser.add_option(...)
   (options, args) = parser.parse_args()

As before, the last value specified for a given option destination is
the one that counts.  For clarity, try to use one method or the other
of setting default values, not both.


15.5.2.6. Generating help
-------------------------

"optparse"’s ability to generate help and usage text automatically is
useful for creating user-friendly command-line interfaces.  All you
have to do is supply a "help" value for each option, and optionally a
short usage message for your whole program.  Here’s an OptionParser
populated with user-friendly (documented) options:

   usage = "usage: %prog [options] arg1 arg2"
   parser = OptionParser(usage=usage)
   parser.add_option("-v", "--verbose",
                     action="store_true", dest="verbose", default=True,
                     help="make lots of noise [default]")
   parser.add_option("-q", "--quiet",
                     action="store_false", dest="verbose",
                     help="be vewwy quiet (I'm hunting wabbits)")
   parser.add_option("-f", "--filename",
                     metavar="FILE", help="write output to FILE")
   parser.add_option("-m", "--mode",
                     default="intermediate",
                     help="interaction mode: novice, intermediate, "
                          "or expert [default: %default]")

If "optparse" encounters either "-h" or "--help" on the command-line,
or if you just call "parser.print_help()", it prints the following to
standard output:

   Usage: <yourscript> [options] arg1 arg2

   Options:
     -h, --help            show this help message and exit
     -v, --verbose         make lots of noise [default]
     -q, --quiet           be vewwy quiet (I'm hunting wabbits)
     -f FILE, --filename=FILE
                           write output to FILE
     -m MODE, --mode=MODE  interaction mode: novice, intermediate, or
                           expert [default: intermediate]

(If the help output is triggered by a help option, "optparse" exits
after printing the help text.)

There’s a lot going on here to help "optparse" generate the best
possible help message:

* the script defines its own usage message:

     usage = "usage: %prog [options] arg1 arg2"

  "optparse" expands "%prog" in the usage string to the name of the
  current program, i.e. "os.path.basename(sys.argv[0])".  The expanded
  string is then printed before the detailed option help.

  If you don’t supply a usage string, "optparse" uses a bland but
  sensible default: ""Usage: %prog [options]"", which is fine if your
  script doesn’t take any positional arguments.

* every option defines a help string, and doesn’t worry about line-
  wrapping— "optparse" takes care of wrapping lines and making the
  help output look good.

* options that take a value indicate this fact in their
  automatically- generated help message, e.g. for the “mode” option:

     -m MODE, --mode=MODE

  Here, “MODE” is called the meta-variable: it stands for the argument
  that the user is expected to supply to "-m"/"--mode".  By default,
  "optparse" converts the destination variable name to uppercase and
  uses that for the meta-variable.  Sometimes, that’s not what you
  want—for example, the "--filename" option explicitly sets
  "metavar="FILE"", resulting in this automatically-generated option
  description:

     -f FILE, --filename=FILE

  This is important for more than just saving space, though: the
  manually written help text uses the meta-variable "FILE" to clue the
  user in that there’s a connection between the semi-formal syntax "-f
  FILE" and the informal semantic description “write output to FILE”.
  This is a simple but effective way to make your help text a lot
  clearer and more useful for end users.

New in version 2.4: Options that have a default value can include
"%default" in the help string—"optparse" will replace it with "str()"
of the option’s default value.  If an option has no default value (or
the default value is "None"), "%default" expands to "none".


15.5.2.6.1. Grouping Options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When dealing with many options, it is convenient to group these
options for better help output.  An "OptionParser" can contain several
option groups, each of which can contain several options.

An option group is obtained using the class "OptionGroup":

class optparse.OptionGroup(parser, title, description=None)

   where

   * parser is the "OptionParser" instance the group will be
     insterted in to

   * title is the group title

   * description, optional, is a long description of the group

"OptionGroup" inherits from "OptionContainer" (like "OptionParser")
and so the "add_option()" method can be used to add an option to the
group.

Once all the options are declared, using the "OptionParser" method
"add_option_group()" the group is added to the previously defined
parser.

Continuing with the parser defined in the previous section, adding an
"OptionGroup" to a parser is easy:

   group = OptionGroup(parser, "Dangerous Options",
                       "Caution: use these options at your own risk.  "
                       "It is believed that some of them bite.")
   group.add_option("-g", action="store_true", help="Group option.")
   parser.add_option_group(group)

This would result in the following help output:

   Usage: <yourscript> [options] arg1 arg2

   Options:
     -h, --help            show this help message and exit
     -v, --verbose         make lots of noise [default]
     -q, --quiet           be vewwy quiet (I'm hunting wabbits)
     -f FILE, --filename=FILE
                           write output to FILE
     -m MODE, --mode=MODE  interaction mode: novice, intermediate, or
                           expert [default: intermediate]

     Dangerous Options:
       Caution: use these options at your own risk.  It is believed that some
       of them bite.

       -g                  Group option.

A bit more complete example might involve using more than one group:
still extending the previous example:

   group = OptionGroup(parser, "Dangerous Options",
                       "Caution: use these options at your own risk.  "
                       "It is believed that some of them bite.")
   group.add_option("-g", action="store_true", help="Group option.")
   parser.add_option_group(group)

   group = OptionGroup(parser, "Debug Options")
   group.add_option("-d", "--debug", action="store_true",
                    help="Print debug information")
   group.add_option("-s", "--sql", action="store_true",
                    help="Print all SQL statements executed")
   group.add_option("-e", action="store_true", help="Print every action done")
   parser.add_option_group(group)

that results in the following output:

   Usage: <yourscript> [options] arg1 arg2

   Options:
     -h, --help            show this help message and exit
     -v, --verbose         make lots of noise [default]
     -q, --quiet           be vewwy quiet (I'm hunting wabbits)
     -f FILE, --filename=FILE
                           write output to FILE
     -m MODE, --mode=MODE  interaction mode: novice, intermediate, or expert
                           [default: intermediate]

     Dangerous Options:
       Caution: use these options at your own risk.  It is believed that some
       of them bite.

       -g                  Group option.

     Debug Options:
       -d, --debug         Print debug information
       -s, --sql           Print all SQL statements executed
       -e                  Print every action done

Another interesting method, in particular when working
programmatically with option groups is:

OptionParser.get_option_group(opt_str)

   Return the "OptionGroup" to which the short or long option string
   *opt_str* (e.g. "'-o'" or "'--option'") belongs. If there’s no such
   "OptionGroup", return "None".


15.5.2.7. Printing a version string
-----------------------------------

Similar to the brief usage string, "optparse" can also print a version
string for your program.  You have to supply the string as the
"version" argument to OptionParser:

   parser = OptionParser(usage="%prog [-f] [-q]", version="%prog 1.0")

"%prog" is expanded just like it is in "usage".  Apart from that,
"version" can contain anything you like.  When you supply it,
"optparse" automatically adds a "--version" option to your parser. If
it encounters this option on the command line, it expands your
"version" string (by replacing "%prog"), prints it to stdout, and
exits.

For example, if your script is called "/usr/bin/foo":

   $ /usr/bin/foo --version
   foo 1.0

The following two methods can be used to print and get the "version"
string:

OptionParser.print_version(file=None)

   Print the version message for the current program ("self.version")
   to *file* (default stdout).  As with "print_usage()", any
   occurrence of "%prog" in "self.version" is replaced with the name
   of the current program.  Does nothing if "self.version" is empty or
   undefined.

OptionParser.get_version()

   Same as "print_version()" but returns the version string instead of
   printing it.


15.5.2.8. How "optparse" handles errors
---------------------------------------

There are two broad classes of errors that "optparse" has to worry
about: programmer errors and user errors.  Programmer errors are
usually erroneous calls to "OptionParser.add_option()", e.g. invalid
option strings, unknown option attributes, missing option attributes,
etc.  These are dealt with in the usual way: raise an exception
(either "optparse.OptionError" or "TypeError") and let the program
crash.

Handling user errors is much more important, since they are guaranteed
to happen no matter how stable your code is.  "optparse" can
automatically detect some user errors, such as bad option arguments
(passing "-n 4x" where "-n" takes an integer argument), missing
arguments ("-n" at the end of the command line, where "-n" takes an
argument of any type).  Also, you can call "OptionParser.error()" to
signal an application-defined error condition:

   (options, args) = parser.parse_args()
   ...
   if options.a and options.b:
       parser.error("options -a and -b are mutually exclusive")

In either case, "optparse" handles the error the same way: it prints
the program’s usage message and an error message to standard error and
exits with error status 2.

Consider the first example above, where the user passes "4x" to an
option that takes an integer:

   $ /usr/bin/foo -n 4x
   Usage: foo [options]

   foo: error: option -n: invalid integer value: '4x'

Or, where the user fails to pass a value at all:

   $ /usr/bin/foo -n
   Usage: foo [options]

   foo: error: -n option requires an argument

"optparse"-generated error messages take care always to mention the
option involved in the error; be sure to do the same when calling
"OptionParser.error()" from your application code.

If "optparse"’s default error-handling behaviour does not suit your
needs, you’ll need to subclass OptionParser and override its "exit()"
and/or "error()" methods.


15.5.2.9. Putting it all together
---------------------------------

Here’s what "optparse"-based scripts usually look like:

   from optparse import OptionParser
   ...
   def main():
       usage = "usage: %prog [options] arg"
       parser = OptionParser(usage)
       parser.add_option("-f", "--file", dest="filename",
                         help="read data from FILENAME")
       parser.add_option("-v", "--verbose",
                         action="store_true", dest="verbose")
       parser.add_option("-q", "--quiet",
                         action="store_false", dest="verbose")
       ...
       (options, args) = parser.parse_args()
       if len(args) != 1:
           parser.error("incorrect number of arguments")
       if options.verbose:
           print "reading %s..." % options.filename
       ...

   if __name__ == "__main__":
       main()


15.5.3. Reference Guide
=======================


15.5.3.1. Creating the parser
-----------------------------

The first step in using "optparse" is to create an OptionParser
instance.

class optparse.OptionParser(...)

   The OptionParser constructor has no required arguments, but a
   number of optional keyword arguments.  You should always pass them
   as keyword arguments, i.e. do not rely on the order in which the
   arguments are declared.

   "usage" (default: ""%prog [options]"")
      The usage summary to print when your program is run incorrectly
      or with a help option.  When "optparse" prints the usage string,
      it expands "%prog" to "os.path.basename(sys.argv[0])" (or to
      "prog" if you passed that keyword argument).  To suppress a
      usage message, pass the special value "optparse.SUPPRESS_USAGE".

   "option_list" (default: "[]")
      A list of Option objects to populate the parser with.  The
      options in "option_list" are added after any options in
      "standard_option_list" (a class attribute that may be set by
      OptionParser subclasses), but before any version or help
      options. Deprecated; use "add_option()" after creating the
      parser instead.

   "option_class" (default: optparse.Option)
      Class to use when adding options to the parser in
      "add_option()".

   "version" (default: "None")
      A version string to print when the user supplies a version
      option. If you supply a true value for "version", "optparse"
      automatically adds a version option with the single option
      string "--version".  The substring "%prog" is expanded the same
      as for "usage".

   "conflict_handler" (default: ""error"")
      Specifies what to do when options with conflicting option
      strings are added to the parser; see section Conflicts between
      options.

   "description" (default: "None")
      A paragraph of text giving a brief overview of your program.
      "optparse" reformats this paragraph to fit the current terminal
      width and prints it when the user requests help (after "usage",
      but before the list of options).

   "formatter" (default: a new "IndentedHelpFormatter")
      An instance of optparse.HelpFormatter that will be used for
      printing help text.  "optparse" provides two concrete classes
      for this purpose: IndentedHelpFormatter and TitledHelpFormatter.

   "add_help_option" (default: "True")
      If true, "optparse" will add a help option (with option strings
      "-h" and "--help") to the parser.

   "prog"
      The string to use when expanding "%prog" in "usage" and
      "version" instead of "os.path.basename(sys.argv[0])".

   "epilog" (default: "None")
      A paragraph of help text to print after the option help.


15.5.3.2. Populating the parser
-------------------------------

There are several ways to populate the parser with options.  The
preferred way is by using "OptionParser.add_option()", as shown in
section Tutorial.  "add_option()" can be called in one of two ways:

* pass it an Option instance (as returned by "make_option()")

* pass it any combination of positional and keyword arguments that
  are acceptable to "make_option()" (i.e., to the Option constructor),
  and it will create the Option instance for you

The other alternative is to pass a list of pre-constructed Option
instances to the OptionParser constructor, as in:

   option_list = [
       make_option("-f", "--filename",
                   action="store", type="string", dest="filename"),
       make_option("-q", "--quiet",
                   action="store_false", dest="verbose"),
       ]
   parser = OptionParser(option_list=option_list)

("make_option()" is a factory function for creating Option instances;
currently it is an alias for the Option constructor.  A future version
of "optparse" may split Option into several classes, and
"make_option()" will pick the right class to instantiate.  Do not
instantiate Option directly.)


15.5.3.3. Defining options
--------------------------

Each Option instance represents a set of synonymous command-line
option strings, e.g. "-f" and "--file".  You can specify any number of
short or long option strings, but you must specify at least one
overall option string.

The canonical way to create an "Option" instance is with the
"add_option()" method of "OptionParser".

OptionParser.add_option(option)
OptionParser.add_option(*opt_str, attr=value, ...)

   To define an option with only a short option string:

      parser.add_option("-f", attr=value, ...)

   And to define an option with only a long option string:

      parser.add_option("--foo", attr=value, ...)

   The keyword arguments define attributes of the new Option object.
   The most important option attribute is "action", and it largely
   determines which other attributes are relevant or required.  If you
   pass irrelevant option attributes, or fail to pass required ones,
   "optparse" raises an "OptionError" exception explaining your
   mistake.

   An option’s *action* determines what "optparse" does when it
   encounters this option on the command-line.  The standard option
   actions hard-coded into "optparse" are:

   ""store""
      store this option’s argument (default)

   ""store_const""
      store a constant value

   ""store_true""
      store a true value

   ""store_false""
      store a false value

   ""append""
      append this option’s argument to a list

   ""append_const""
      append a constant value to a list

   ""count""
      increment a counter by one

   ""callback""
      call a specified function

   ""help""
      print a usage message including all options and the
      documentation for them

   (If you don’t supply an action, the default is ""store"".  For this
   action, you may also supply "type" and "dest" option attributes;
   see Standard option actions.)

As you can see, most actions involve storing or updating a value
somewhere. "optparse" always creates a special object for this,
conventionally called "options" (it happens to be an instance of
"optparse.Values").  Option arguments (and various other values) are
stored as attributes of this object, according to the "dest"
(destination) option attribute.

For example, when you call

   parser.parse_args()

one of the first things "optparse" does is create the "options"
object:

   options = Values()

If one of the options in this parser is defined with

   parser.add_option("-f", "--file", action="store", type="string", dest="filename")

and the command-line being parsed includes any of the following:

   -ffoo
   -f foo
   --file=foo
   --file foo

then "optparse", on seeing this option, will do the equivalent of

   options.filename = "foo"

The "type" and "dest" option attributes are almost as important as
"action", but "action" is the only one that makes sense for *all*
options.


15.5.3.4. Option attributes
---------------------------

The following option attributes may be passed as keyword arguments to
"OptionParser.add_option()".  If you pass an option attribute that is
not relevant to a particular option, or fail to pass a required option
attribute, "optparse" raises "OptionError".

Option.action

   (default: ""store"")

   Determines "optparse"’s behaviour when this option is seen on the
   command line; the available options are documented here.

Option.type

   (default: ""string"")

   The argument type expected by this option (e.g., ""string"" or
   ""int""); the available option types are documented here.

Option.dest

   (default: derived from option strings)

   If the option’s action implies writing or modifying a value
   somewhere, this tells "optparse" where to write it: "dest" names an
   attribute of the "options" object that "optparse" builds as it
   parses the command line.

Option.default

   The value to use for this option’s destination if the option is not
   seen on the command line.  See also "OptionParser.set_defaults()".

Option.nargs

   (default: 1)

   How many arguments of type "type" should be consumed when this
   option is seen.  If > 1, "optparse" will store a tuple of values to
   "dest".

Option.const

   For actions that store a constant value, the constant value to
   store.

Option.choices

   For options of type ""choice"", the list of strings the user may
   choose from.

Option.callback

   For options with action ""callback"", the callable to call when
   this option is seen.  See section Option Callbacks for detail on
   the arguments passed to the callable.

Option.callback_args
Option.callback_kwargs

   Additional positional and keyword arguments to pass to "callback"
   after the four standard callback arguments.

Option.help

   Help text to print for this option when listing all available
   options after the user supplies a "help" option (such as "--help").
   If no help text is supplied, the option will be listed without help
   text.  To hide this option, use the special value
   "optparse.SUPPRESS_HELP".

Option.metavar

   (default: derived from option strings)

   Stand-in for the option argument(s) to use when printing help text.
   See section Tutorial for an example.


15.5.3.5. Standard option actions
---------------------------------

The various option actions all have slightly different requirements
and effects. Most actions have several relevant option attributes
which you may specify to guide "optparse"’s behaviour; a few have
required attributes, which you must specify for any option using that
action.

* ""store"" [relevant: "type", "dest", "nargs", "choices"]

  The option must be followed by an argument, which is converted to a
  value according to "type" and stored in "dest".  If "nargs" > 1,
  multiple arguments will be consumed from the command line; all will
  be converted according to "type" and stored to "dest" as a tuple.
  See the Standard option types section.

  If "choices" is supplied (a list or tuple of strings), the type
  defaults to ""choice"".

  If "type" is not supplied, it defaults to ""string"".

  If "dest" is not supplied, "optparse" derives a destination from the
  first long option string (e.g., "--foo-bar" implies "foo_bar"). If
  there are no long option strings, "optparse" derives a destination
  from the first short option string (e.g., "-f" implies "f").

  Example:

     parser.add_option("-f")
     parser.add_option("-p", type="float", nargs=3, dest="point")

  As it parses the command line

     -f foo.txt -p 1 -3.5 4 -fbar.txt

  "optparse" will set

     options.f = "foo.txt"
     options.point = (1.0, -3.5, 4.0)
     options.f = "bar.txt"

* ""store_const"" [required: "const"; relevant: "dest"]

  The value "const" is stored in "dest".

  Example:

     parser.add_option("-q", "--quiet",
                       action="store_const", const=0, dest="verbose")
     parser.add_option("-v", "--verbose",
                       action="store_const", const=1, dest="verbose")
     parser.add_option("--noisy",
                       action="store_const", const=2, dest="verbose")

  If "--noisy" is seen, "optparse" will set

     options.verbose = 2

* ""store_true"" [relevant: "dest"]

  A special case of ""store_const"" that stores a true value to
  "dest".

* ""store_false"" [relevant: "dest"]

  Like ""store_true"", but stores a false value.

  Example:

     parser.add_option("--clobber", action="store_true", dest="clobber")
     parser.add_option("--no-clobber", action="store_false", dest="clobber")

* ""append"" [relevant: "type", "dest", "nargs", "choices"]

  The option must be followed by an argument, which is appended to the
  list in "dest".  If no default value for "dest" is supplied, an
  empty list is automatically created when "optparse" first encounters
  this option on the command-line.  If "nargs" > 1, multiple arguments
  are consumed, and a tuple of length "nargs" is appended to "dest".

  The defaults for "type" and "dest" are the same as for the ""store""
  action.

  Example:

     parser.add_option("-t", "--tracks", action="append", type="int")

  If "-t3" is seen on the command-line, "optparse" does the equivalent
  of:

     options.tracks = []
     options.tracks.append(int("3"))

  If, a little later on, "--tracks=4" is seen, it does:

     options.tracks.append(int("4"))

  The "append" action calls the "append" method on the current value
  of the option.  This means that any default value specified must
  have an "append" method.  It also means that if the default value is
  non-empty, the default elements will be present in the parsed value
  for the option, with any values from the command line appended after
  those default values:

     >>> parser.add_option("--files", action="append", default=['~/.mypkg/defaults'])
     >>> opts, args = parser.parse_args(['--files', 'overrides.mypkg'])
     >>> opts.files
     ['~/.mypkg/defaults', 'overrides.mypkg']

* ""append_const"" [required: "const"; relevant: "dest"]

  Like ""store_const"", but the value "const" is appended to "dest";
  as with ""append"", "dest" defaults to "None", and an empty list is
  automatically created the first time the option is encountered.

* ""count"" [relevant: "dest"]

  Increment the integer stored at "dest".  If no default value is
  supplied, "dest" is set to zero before being incremented the first
  time.

  Example:

     parser.add_option("-v", action="count", dest="verbosity")

  The first time "-v" is seen on the command line, "optparse" does the
  equivalent of:

     options.verbosity = 0
     options.verbosity += 1

  Every subsequent occurrence of "-v" results in

     options.verbosity += 1

* ""callback"" [required: "callback"; relevant: "type", "nargs",
  "callback_args", "callback_kwargs"]

  Call the function specified by "callback", which is called as

     func(option, opt_str, value, parser, *args, **kwargs)

  See section Option Callbacks for more detail.

* ""help""

  Prints a complete help message for all the options in the current
  option parser.  The help message is constructed from the "usage"
  string passed to OptionParser’s constructor and the "help" string
  passed to every option.

  If no "help" string is supplied for an option, it will still be
  listed in the help message.  To omit an option entirely, use the
  special value "optparse.SUPPRESS_HELP".

  "optparse" automatically adds a "help" option to all OptionParsers,
  so you do not normally need to create one.

  Example:

     from optparse import OptionParser, SUPPRESS_HELP

     # usually, a help option is added automatically, but that can
     # be suppressed using the add_help_option argument
     parser = OptionParser(add_help_option=False)

     parser.add_option("-h", "--help", action="help")
     parser.add_option("-v", action="store_true", dest="verbose",
                       help="Be moderately verbose")
     parser.add_option("--file", dest="filename",
                       help="Input file to read data from")
     parser.add_option("--secret", help=SUPPRESS_HELP)

  If "optparse" sees either "-h" or "--help" on the command line, it
  will print something like the following help message to stdout
  (assuming "sys.argv[0]" is ""foo.py""):

     Usage: foo.py [options]

     Options:
       -h, --help        Show this help message and exit
       -v                Be moderately verbose
       --file=FILENAME   Input file to read data from

  After printing the help message, "optparse" terminates your process
  with "sys.exit(0)".

* ""version""

  Prints the version number supplied to the OptionParser to stdout and
  exits. The version number is actually formatted and printed by the
  "print_version()" method of OptionParser.  Generally only relevant
  if the "version" argument is supplied to the OptionParser
  constructor.  As with "help" options, you will rarely create
  "version" options, since "optparse" automatically adds them when
  needed.


15.5.3.6. Standard option types
-------------------------------

"optparse" has six built-in option types: ""string"", ""int"",
""long"", ""choice"", ""float"" and ""complex"".  If you need to add
new option types, see section Extending optparse.

Arguments to string options are not checked or converted in any way:
the text on the command line is stored in the destination (or passed
to the callback) as-is.

Integer arguments (type ""int"" or ""long"") are parsed as follows:

* if the number starts with "0x", it is parsed as a hexadecimal
  number

* if the number starts with "0", it is parsed as an octal number

* if the number starts with "0b", it is parsed as a binary number

* otherwise, the number is parsed as a decimal number

The conversion is done by calling either "int()" or "long()" with the
appropriate base (2, 8, 10, or 16).  If this fails, so will
"optparse", although with a more useful error message.

""float"" and ""complex"" option arguments are converted directly with
"float()" and "complex()", with similar error-handling.

""choice"" options are a subtype of ""string"" options.  The "choices"
option attribute (a sequence of strings) defines the set of allowed
option arguments.  "optparse.check_choice()" compares user-supplied
option arguments against this master list and raises
"OptionValueError" if an invalid string is given.


15.5.3.7. Parsing arguments
---------------------------

The whole point of creating and populating an OptionParser is to call
its "parse_args()" method:

   (options, args) = parser.parse_args(args=None, values=None)

where the input parameters are

"args"
   the list of arguments to process (default: "sys.argv[1:]")

"values"
   an "optparse.Values" object to store option arguments in (default:
   a new instance of "Values") – if you give an existing object, the
   option defaults will not be initialized on it

and the return values are

"options"
   the same object that was passed in as "values", or the
   optparse.Values instance created by "optparse"

"args"
   the leftover positional arguments after all options have been
   processed

The most common usage is to supply neither keyword argument.  If you
supply "values", it will be modified with repeated "setattr()" calls
(roughly one for every option argument stored to an option
destination) and returned by "parse_args()".

If "parse_args()" encounters any errors in the argument list, it calls
the OptionParser’s "error()" method with an appropriate end-user error
message. This ultimately terminates your process with an exit status
of 2 (the traditional Unix exit status for command-line errors).


15.5.3.8. Querying and manipulating your option parser
------------------------------------------------------

The default behavior of the option parser can be customized slightly,
and you can also poke around your option parser and see what’s there.
OptionParser provides several methods to help you out:

OptionParser.disable_interspersed_args()

   Set parsing to stop on the first non-option.  For example, if "-a"
   and "-b" are both simple options that take no arguments, "optparse"
   normally accepts this syntax:

      prog -a arg1 -b arg2

   and treats it as equivalent to

      prog -a -b arg1 arg2

   To disable this feature, call "disable_interspersed_args()".  This
   restores traditional Unix syntax, where option parsing stops with
   the first non-option argument.

   Use this if you have a command processor which runs another command
   which has options of its own and you want to make sure these
   options don’t get confused.  For example, each command might have a
   different set of options.

OptionParser.enable_interspersed_args()

   Set parsing to not stop on the first non-option, allowing
   interspersing switches with command arguments.  This is the default
   behavior.

OptionParser.get_option(opt_str)

   Returns the Option instance with the option string *opt_str*, or
   "None" if no options have that option string.

OptionParser.has_option(opt_str)

   Return true if the OptionParser has an option with option string
   *opt_str* (e.g., "-q" or "--verbose").

OptionParser.remove_option(opt_str)

   If the "OptionParser" has an option corresponding to *opt_str*,
   that option is removed.  If that option provided any other option
   strings, all of those option strings become invalid. If *opt_str*
   does not occur in any option belonging to this "OptionParser",
   raises "ValueError".


15.5.3.9. Conflicts between options
-----------------------------------

If you’re not careful, it’s easy to define options with conflicting
option strings:

   parser.add_option("-n", "--dry-run", ...)
   ...
   parser.add_option("-n", "--noisy", ...)

(This is particularly true if you’ve defined your own OptionParser
subclass with some standard options.)

Every time you add an option, "optparse" checks for conflicts with
existing options.  If it finds any, it invokes the current conflict-
handling mechanism. You can set the conflict-handling mechanism either
in the constructor:

   parser = OptionParser(..., conflict_handler=handler)

or with a separate call:

   parser.set_conflict_handler(handler)

The available conflict handlers are:

   ""error"" (default)
      assume option conflicts are a programming error and raise
      "OptionConflictError"

   ""resolve""
      resolve option conflicts intelligently (see below)

As an example, let’s define an "OptionParser" that resolves conflicts
intelligently and add conflicting options to it:

   parser = OptionParser(conflict_handler="resolve")
   parser.add_option("-n", "--dry-run", ..., help="do no harm")
   parser.add_option("-n", "--noisy", ..., help="be noisy")

At this point, "optparse" detects that a previously-added option is
already using the "-n" option string.  Since "conflict_handler" is
""resolve"", it resolves the situation by removing "-n" from the
earlier option’s list of option strings.  Now "--dry-run" is the only
way for the user to activate that option.  If the user asks for help,
the help message will reflect that:

   Options:
     --dry-run     do no harm
     ...
     -n, --noisy   be noisy

It’s possible to whittle away the option strings for a previously-
added option until there are none left, and the user has no way of
invoking that option from the command-line.  In that case, "optparse"
removes that option completely, so it doesn’t show up in help text or
anywhere else. Carrying on with our existing OptionParser:

   parser.add_option("--dry-run", ..., help="new dry-run option")

At this point, the original "-n"/"--dry-run" option is no longer
accessible, so "optparse" removes it, leaving this help text:

   Options:
     ...
     -n, --noisy   be noisy
     --dry-run     new dry-run option


15.5.3.10. Cleanup
------------------

OptionParser instances have several cyclic references.  This should
not be a problem for Python’s garbage collector, but you may wish to
break the cyclic references explicitly by calling "destroy()" on your
OptionParser once you are done with it.  This is particularly useful
in long-running applications where large object graphs are reachable
from your OptionParser.


15.5.3.11. Other methods
------------------------

OptionParser supports several other public methods:

OptionParser.set_usage(usage)

   Set the usage string according to the rules described above for the
   "usage" constructor keyword argument.  Passing "None" sets the
   default usage string; use "optparse.SUPPRESS_USAGE" to suppress a
   usage message.

OptionParser.print_usage(file=None)

   Print the usage message for the current program ("self.usage") to
   *file* (default stdout).  Any occurrence of the string "%prog" in
   "self.usage" is replaced with the name of the current program.
   Does nothing if "self.usage" is empty or not defined.

OptionParser.get_usage()

   Same as "print_usage()" but returns the usage string instead of
   printing it.

OptionParser.set_defaults(dest=value, ...)

   Set default values for several option destinations at once.  Using
   "set_defaults()" is the preferred way to set default values for
   options, since multiple options can share the same destination.
   For example, if several “mode” options all set the same
   destination, any one of them can set the default, and the last one
   wins:

      parser.add_option("--advanced", action="store_const",
                        dest="mode", const="advanced",
                        default="novice")    # overridden below
      parser.add_option("--novice", action="store_const",
                        dest="mode", const="novice",
                        default="advanced")  # overrides above setting

   To avoid this confusion, use "set_defaults()":

      parser.set_defaults(mode="advanced")
      parser.add_option("--advanced", action="store_const",
                        dest="mode", const="advanced")
      parser.add_option("--novice", action="store_const",
                        dest="mode", const="novice")


15.5.4. Option Callbacks
========================

When "optparse"’s built-in actions and types aren’t quite enough for
your needs, you have two choices: extend "optparse" or define a
callback option. Extending "optparse" is more general, but overkill
for a lot of simple cases.  Quite often a simple callback is all you
need.

There are two steps to defining a callback option:

* define the option itself using the ""callback"" action

* write the callback; this is a function (or method) that takes at
  least four arguments, as described below


15.5.4.1. Defining a callback option
------------------------------------

As always, the easiest way to define a callback option is by using the
"OptionParser.add_option()" method.  Apart from "action", the only
option attribute you must specify is "callback", the function to call:

   parser.add_option("-c", action="callback", callback=my_callback)

"callback" is a function (or other callable object), so you must have
already defined "my_callback()" when you create this callback option.
In this simple case, "optparse" doesn’t even know if "-c" takes any
arguments, which usually means that the option takes no arguments—the
mere presence of "-c" on the command-line is all it needs to know.  In
some circumstances, though, you might want your callback to consume an
arbitrary number of command-line arguments.  This is where writing
callbacks gets tricky; it’s covered later in this section.

"optparse" always passes four particular arguments to your callback,
and it will only pass additional arguments if you specify them via
"callback_args" and "callback_kwargs".  Thus, the minimal callback
function signature is:

   def my_callback(option, opt, value, parser):

The four arguments to a callback are described below.

There are several other option attributes that you can supply when you
define a callback option:

"type"
   has its usual meaning: as with the ""store"" or ""append"" actions,
   it instructs "optparse" to consume one argument and convert it to
   "type".  Rather than storing the converted value(s) anywhere,
   though, "optparse" passes it to your callback function.

"nargs"
   also has its usual meaning: if it is supplied and > 1, "optparse"
   will consume "nargs" arguments, each of which must be convertible
   to "type".  It then passes a tuple of converted values to your
   callback.

"callback_args"
   a tuple of extra positional arguments to pass to the callback

"callback_kwargs"
   a dictionary of extra keyword arguments to pass to the callback


15.5.4.2. How callbacks are called
----------------------------------

All callbacks are called as follows:

   func(option, opt_str, value, parser, *args, **kwargs)

where

"option"
   is the Option instance that’s calling the callback

"opt_str"
   is the option string seen on the command-line that’s triggering the
   callback. (If an abbreviated long option was used, "opt_str" will
   be the full, canonical option string—e.g. if the user puts "--foo"
   on the command-line as an abbreviation for "--foobar", then
   "opt_str" will be ""--foobar"".)

"value"
   is the argument to this option seen on the command-line.
   "optparse" will only expect an argument if "type" is set; the type
   of "value" will be the type implied by the option’s type.  If
   "type" for this option is "None" (no argument expected), then
   "value" will be "None".  If "nargs" > 1, "value" will be a tuple of
   values of the appropriate type.

"parser"
   is the OptionParser instance driving the whole thing, mainly useful
   because you can access some other interesting data through its
   instance attributes:

   "parser.largs"
      the current list of leftover arguments, ie. arguments that have
      been consumed but are neither options nor option arguments. Feel
      free to modify "parser.largs", e.g. by adding more arguments to
      it.  (This list will become "args", the second return value of
      "parse_args()".)

   "parser.rargs"
      the current list of remaining arguments, ie. with "opt_str" and
      "value" (if applicable) removed, and only the arguments
      following them still there.  Feel free to modify "parser.rargs",
      e.g. by consuming more arguments.

   "parser.values"
      the object where option values are by default stored (an
      instance of optparse.OptionValues).  This lets callbacks use the
      same mechanism as the rest of "optparse" for storing option
      values; you don’t need to mess around with globals or closures.
      You can also access or modify the value(s) of any options
      already encountered on the command-line.

"args"
   is a tuple of arbitrary positional arguments supplied via the
   "callback_args" option attribute.

"kwargs"
   is a dictionary of arbitrary keyword arguments supplied via
   "callback_kwargs".


15.5.4.3. Raising errors in a callback
--------------------------------------

The callback function should raise "OptionValueError" if there are any
problems with the option or its argument(s).  "optparse" catches this
and terminates the program, printing the error message you supply to
stderr.  Your message should be clear, concise, accurate, and mention
the option at fault. Otherwise, the user will have a hard time
figuring out what they did wrong.


15.5.4.4. Callback example 1: trivial callback
----------------------------------------------

Here’s an example of a callback option that takes no arguments, and
simply records that the option was seen:

   def record_foo_seen(option, opt_str, value, parser):
       parser.values.saw_foo = True

   parser.add_option("--foo", action="callback", callback=record_foo_seen)

Of course, you could do that with the ""store_true"" action.


15.5.4.5. Callback example 2: check option order
------------------------------------------------

Here’s a slightly more interesting example: record the fact that "-a"
is seen, but blow up if it comes after "-b" in the command-line.

   def check_order(option, opt_str, value, parser):
       if parser.values.b:
           raise OptionValueError("can't use -a after -b")
       parser.values.a = 1
   ...
   parser.add_option("-a", action="callback", callback=check_order)
   parser.add_option("-b", action="store_true", dest="b")


15.5.4.6. Callback example 3: check option order (generalized)
--------------------------------------------------------------

If you want to re-use this callback for several similar options (set a
flag, but blow up if "-b" has already been seen), it needs a bit of
work: the error message and the flag that it sets must be generalized.

   def check_order(option, opt_str, value, parser):
       if parser.values.b:
           raise OptionValueError("can't use %s after -b" % opt_str)
       setattr(parser.values, option.dest, 1)
   ...
   parser.add_option("-a", action="callback", callback=check_order, dest='a')
   parser.add_option("-b", action="store_true", dest="b")
   parser.add_option("-c", action="callback", callback=check_order, dest='c')


15.5.4.7. Callback example 4: check arbitrary condition
-------------------------------------------------------

Of course, you could put any condition in there—you’re not limited to
checking the values of already-defined options.  For example, if you
have options that should not be called when the moon is full, all you
have to do is this:

   def check_moon(option, opt_str, value, parser):
       if is_moon_full():
           raise OptionValueError("%s option invalid when moon is full"
                                  % opt_str)
       setattr(parser.values, option.dest, 1)
   ...
   parser.add_option("--foo",
                     action="callback", callback=check_moon, dest="foo")

(The definition of "is_moon_full()" is left as an exercise for the
reader.)


15.5.4.8. Callback example 5: fixed arguments
---------------------------------------------

Things get slightly more interesting when you define callback options
that take a fixed number of arguments.  Specifying that a callback
option takes arguments is similar to defining a ""store"" or
""append"" option: if you define "type", then the option takes one
argument that must be convertible to that type; if you further define
"nargs", then the option takes "nargs" arguments.

Here’s an example that just emulates the standard ""store"" action:

   def store_value(option, opt_str, value, parser):
       setattr(parser.values, option.dest, value)
   ...
   parser.add_option("--foo",
                     action="callback", callback=store_value,
                     type="int", nargs=3, dest="foo")

Note that "optparse" takes care of consuming 3 arguments and
converting them to integers for you; all you have to do is store them.
(Or whatever; obviously you don’t need a callback for this example.)


15.5.4.9. Callback example 6: variable arguments
------------------------------------------------

Things get hairy when you want an option to take a variable number of
arguments. For this case, you must write a callback, as "optparse"
doesn’t provide any built-in capabilities for it.  And you have to
deal with certain intricacies of conventional Unix command-line
parsing that "optparse" normally handles for you.  In particular,
callbacks should implement the conventional rules for bare "--" and
"-" arguments:

* either "--" or "-" can be option arguments

* bare "--" (if not the argument to some option): halt command-line
  processing and discard the "--"

* bare "-" (if not the argument to some option): halt command-line
  processing but keep the "-" (append it to "parser.largs")

If you want an option that takes a variable number of arguments, there
are several subtle, tricky issues to worry about.  The exact
implementation you choose will be based on which trade-offs you’re
willing to make for your application (which is why "optparse" doesn’t
support this sort of thing directly).

Nevertheless, here’s a stab at a callback for an option with variable
arguments:

   def vararg_callback(option, opt_str, value, parser):
       assert value is None
       value = []

       def floatable(str):
           try:
               float(str)
               return True
           except ValueError:
               return False

       for arg in parser.rargs:
           # stop on --foo like options
           if arg[:2] == "--" and len(arg) > 2:
               break
           # stop on -a, but not on -3 or -3.0
           if arg[:1] == "-" and len(arg) > 1 and not floatable(arg):
               break
           value.append(arg)

       del parser.rargs[:len(value)]
       setattr(parser.values, option.dest, value)

   ...
   parser.add_option("-c", "--callback", dest="vararg_attr",
                     action="callback", callback=vararg_callback)


15.5.5. Extending "optparse"
============================

Since the two major controlling factors in how "optparse" interprets
command-line options are the action and type of each option, the most
likely direction of extension is to add new actions and new types.


15.5.5.1. Adding new types
--------------------------

To add new types, you need to define your own subclass of "optparse"’s
"Option" class.  This class has a couple of attributes that define
"optparse"’s types: "TYPES" and "TYPE_CHECKER".

Option.TYPES

   A tuple of type names; in your subclass, simply define a new tuple
   "TYPES" that builds on the standard one.

Option.TYPE_CHECKER

   A dictionary mapping type names to type-checking functions.  A
   type-checking function has the following signature:

      def check_mytype(option, opt, value)

   where "option" is an "Option" instance, "opt" is an option string
   (e.g., "-f"), and "value" is the string from the command line that
   must be checked and converted to your desired type.
   "check_mytype()" should return an object of the hypothetical type
   "mytype".  The value returned by a type-checking function will wind
   up in the OptionValues instance returned by
   "OptionParser.parse_args()", or be passed to a callback as the
   "value" parameter.

   Your type-checking function should raise "OptionValueError" if it
   encounters any problems.  "OptionValueError" takes a single string
   argument, which is passed as-is to "OptionParser"’s "error()"
   method, which in turn prepends the program name and the string
   ""error:"" and prints everything to stderr before terminating the
   process.

Here’s a silly example that demonstrates adding a ""complex"" option
type to parse Python-style complex numbers on the command line.  (This
is even sillier than it used to be, because "optparse" 1.3 added
built-in support for complex numbers, but never mind.)

First, the necessary imports:

   from copy import copy
   from optparse import Option, OptionValueError

You need to define your type-checker first, since it’s referred to
later (in the "TYPE_CHECKER" class attribute of your Option subclass):

   def check_complex(option, opt, value):
       try:
           return complex(value)
       except ValueError:
           raise OptionValueError(
               "option %s: invalid complex value: %r" % (opt, value))

Finally, the Option subclass:

   class MyOption (Option):
       TYPES = Option.TYPES + ("complex",)
       TYPE_CHECKER = copy(Option.TYPE_CHECKER)
       TYPE_CHECKER["complex"] = check_complex

(If we didn’t make a "copy()" of "Option.TYPE_CHECKER", we would end
up modifying the "TYPE_CHECKER" attribute of "optparse"’s Option
class.  This being Python, nothing stops you from doing that except
good manners and common sense.)

That’s it!  Now you can write a script that uses the new option type
just like any other "optparse"-based script, except you have to
instruct your OptionParser to use MyOption instead of Option:

   parser = OptionParser(option_class=MyOption)
   parser.add_option("-c", type="complex")

Alternately, you can build your own option list and pass it to
OptionParser; if you don’t use "add_option()" in the above way, you
don’t need to tell OptionParser which option class to use:

   option_list = [MyOption("-c", action="store", type="complex", dest="c")]
   parser = OptionParser(option_list=option_list)


15.5.5.2. Adding new actions
----------------------------

Adding new actions is a bit trickier, because you have to understand
that "optparse" has a couple of classifications for actions:

“store” actions
   actions that result in "optparse" storing a value to an attribute
   of the current OptionValues instance; these options require a
   "dest" attribute to be supplied to the Option constructor.

“typed” actions
   actions that take a value from the command line and expect it to be
   of a certain type; or rather, a string that can be converted to a
   certain type. These options require a "type" attribute to the
   Option constructor.

These are overlapping sets: some default “store” actions are
""store"", ""store_const"", ""append"", and ""count"", while the
default “typed” actions are ""store"", ""append"", and ""callback"".

When you add an action, you need to categorize it by listing it in at
least one of the following class attributes of Option (all are lists
of strings):

Option.ACTIONS

   All actions must be listed in ACTIONS.

Option.STORE_ACTIONS

   “store” actions are additionally listed here.

Option.TYPED_ACTIONS

   “typed” actions are additionally listed here.

Option.ALWAYS_TYPED_ACTIONS

   Actions that always take a type (i.e. whose options always take a
   value) are additionally listed here.  The only effect of this is
   that "optparse" assigns the default type, ""string"", to options
   with no explicit type whose action is listed in
   "ALWAYS_TYPED_ACTIONS".

In order to actually implement your new action, you must override
Option’s "take_action()" method and add a case that recognizes your
action.

For example, let’s add an ""extend"" action.  This is similar to the
standard ""append"" action, but instead of taking a single value from
the command-line and appending it to an existing list, ""extend"" will
take multiple values in a single comma-delimited string, and extend an
existing list with them.  That is, if "--names" is an ""extend""
option of type ""string"", the command line

   --names=foo,bar --names blah --names ding,dong

would result in a list

   ["foo", "bar", "blah", "ding", "dong"]

Again we define a subclass of Option:

   class MyOption(Option):

       ACTIONS = Option.ACTIONS + ("extend",)
       STORE_ACTIONS = Option.STORE_ACTIONS + ("extend",)
       TYPED_ACTIONS = Option.TYPED_ACTIONS + ("extend",)
       ALWAYS_TYPED_ACTIONS = Option.ALWAYS_TYPED_ACTIONS + ("extend",)

       def take_action(self, action, dest, opt, value, values, parser):
           if action == "extend":
               lvalue = value.split(",")
               values.ensure_value(dest, []).extend(lvalue)
           else:
               Option.take_action(
                   self, action, dest, opt, value, values, parser)

Features of note:

* ""extend"" both expects a value on the command-line and stores
  that value somewhere, so it goes in both "STORE_ACTIONS" and
  "TYPED_ACTIONS".

* to ensure that "optparse" assigns the default type of ""string""
  to ""extend"" actions, we put the ""extend"" action in
  "ALWAYS_TYPED_ACTIONS" as well.

* "MyOption.take_action()" implements just this one new action, and
  passes control back to "Option.take_action()" for the standard
  "optparse" actions.

* "values" is an instance of the optparse_parser.Values class, which
  provides the very useful "ensure_value()" method. "ensure_value()"
  is essentially "getattr()" with a safety valve; it is called as

     values.ensure_value(attr, value)

  If the "attr" attribute of "values" doesn’t exist or is "None", then
  ensure_value() first sets it to "value", and then returns ‘value.
  This is very handy for actions like ""extend"", ""append"", and
  ""count"", all of which accumulate data in a variable and expect
  that variable to be of a certain type (a list for the first two, an
  integer for the latter).  Using "ensure_value()" means that scripts
  using your action don’t have to worry about setting a default value
  for the option destinations in question; they can just leave the
  default as "None" and "ensure_value()" will take care of getting it
  right when it’s needed.
