optparse —- 解析器的命令行选项

源代码：Lib/optparse.py

3.2 版后已移除: The optparse module is deprecated and will not be developed further;development will continue with the argparse module.

optparse is a more convenient, flexible, and powerful library for parsingcommand-line options than the old getopt module. optparse uses amore declarative style of command-line parsing: you create an instance ofOptionParser, populate it with options, and parse the commandline. optparse allows users to specify options in the conventionalGNU/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 theoptions object returned by parse_args() based on user-suppliedcommand-line values. When parse_args() returns from parsing this commandline, options.filename will be "outfile" and options.verbose will beFalse. optparse supports both long and short options, allows shortoptions to be merged together, and allows options to be associated with theirarguments in a variety of ways. Thus, the following command lines are allequivalent 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 fromsys.argv[0]).

背景

optparse was explicitly designed to encourage the creation of programswith straightforward, conventional command-line interfaces. To that end, itsupports only the most common command-line syntax and semantics conventionallyused under Unix. If you are unfamiliar with these conventions, read thissection to acquaint yourself with them.

术语

• 参数
• 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 shellsalso use the term "word".

It is occasionally desirable to substitute an argument list other thansys.argv[1:], so you should read "argument" as "an element ofsys.argv[1:], or of some other list provided as a substitute forsys.argv[1:]".

• 选项
• an argument used to supply extra information to guide or customize theexecution of a program. There are many different syntaxes for options; thetraditional Unix syntax is a hyphen ("-") followed by a single letter,e.g. -x or -F. Also, traditional Unix syntax allows multipleoptions to be merged into a single argument, e.g. -x -F is equivalentto -xF. The GNU project introduced — followed by a series ofhyphen-separated words, e.g. —file or —dry-run. These are theonly 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 sameas multiple options merged into a single argument)

• a hyphen followed by a whole word, e.g. -file (this is technicallyequivalent to the previous syntax, but they aren't usually seen in the sameprogram)

• 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 neverwill be. This is deliberate: the first three are non-standard on anyenvironment, and the last only makes sense if you're exclusively targetingVMS, MS-DOS, and/or Windows.

• 可选参数:
• an argument that follows an option, is closely associated with that option,and is consumed from the argument list when that option is. Withoptparse, option arguments may either be in a separate argument fromtheir 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 ofpeople want an "optional option arguments" feature, meaning that some optionswill take an argument if they see it, and won't if they don't. This issomewhat controversial, because it makes parsing ambiguous: if -a takesan optional argument and -b is another option entirely, how do weinterpret -ab? Because of this ambiguity, optparse does notsupport this feature.

• positional 参数

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

• 必选选项

• an option that must be supplied on the command-line; note that the phrase"required option" is self-contradictory in English. optparse doesn'tprevent you from implementing required options, but doesn't give you muchhelp 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 —reporttakes one argument, report.txt is an option argument. foo andbar are positional arguments.

What are options for?

Options are used to provide extra information to tune or customize the executionof a program. In case it wasn't clear, options are usually optional. Aprogram should be able to run just fine with no options whatsoever. (Pick arandom program from the Unix or GNU toolsets. Can it run without any options atall and still make sense? The main exceptions are find, tar, anddd—-all of which are mutant oddballs that have been rightly criticizedfor 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 informationthat your program absolutely requires in order to run successfully, that's whatpositional arguments are for.

As an example of good command-line interface design, consider the humble cputility, for copying files. It doesn't make much sense to try to copy fileswithout supplying a destination and at least one source. Hence, cp fails ifyou run it with no arguments. However, it has a flexible, useful syntax thatdoes 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 abunch of options to tweak exactly how the files are copied: you can preservemode and modification time, avoid following symlinks, ask before clobberingexisting files, etc. But none of this distracts from the core mission ofcp, which is to copy either one file to another, or several files to anotherdirectory.

位置位置

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

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

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

教程

While optparse is quite flexible and powerful, it's also straightforwardto use in most cases. This section covers the code patterns that are common toany optparse-based program.

First, you need to import the OptionParser class; then, early in the mainprogram, 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 whatto do when it encounters that option on the command line.

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

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

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

The option strings passed to OptionParser.add_option() are effectivelylabels for theoption defined by that call. For brevity, we will frequently refer toencountering an option on the command line; in reality, optparseencounters option strings and looks up options from them.

Once all of your options are defined, instruct optparse to parse yourprogram's command line:

(options, args) = parser.parse_args()

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

parse_args() 返回两个值：

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

• 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 themost fundamental.

Understanding option actions

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

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

The store action

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

例如:

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 nextargument, foo.txt, and stores it in options.filename. So, after thiscall 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 argumentright 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 withthe fact that the default action is store, that means our first example canbe a lot shorter:

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

If you don't supply a destination, optparse figures out a sensibledefault from the option strings: if the first long option string is—foo-bar, then the default destination is foo_bar. If there are nolong option strings, optparse looks at the first short option string: thedefault destination for -f is f.

optparse also includes the built-in complex type. Addingtypes is covered in section Extending optparse.

Handling boolean (flag) options

Flag options—-set a variable to true or false when a particular option isseen—-are quite common. optparse supports them with two separate actions,store_true and store_false. For example, you might have a verboseflag 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 perfectlyOK. (It just means you have to be a bit careful when setting defaultvalues—-see below.)

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

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"

• 调用指定函数

These are covered in section 参考指南,and section Option Callbacks.

默认值

All of the above examples involve setting some variable (the "destination") whencertain command-line options are seen. What happens if those options are neverseen? Since we didn't supply any defaults, they are all set to None. Thisis usually fine, but sometimes you want more control. optparse lets yousupply a default value for each destination, which is assigned before thecommand line is parsed.

First, consider the verbose/quiet example. If we want optparse to setverbose 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 particularoption, and these two options happen to have the same destination, this isexactly equivalent:

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

考虑一下：

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 defaultvalue supplied for any particular destination is the one that counts.

A clearer way to specify default values is the set_defaults() method ofOptionParser, 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 onethat counts. For clarity, try to use one method or the other of setting defaultvalues, not both.

Generating help

optparse's ability to generate help and usage text automatically isuseful for creating user-friendly command-line interfaces. All you have to dois supply a help value for each option, and optionally a shortusage message for your whole program. Here's an OptionParser populated withuser-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 thecommand-line, or if you just call parser.print_help(), it prints thefollowing 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 afterprinting the help text.)

There's a lot going on here to help optparse generate the best possiblehelp 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 thecurrent program, i.e. os.path.basename(sys.argv[0]). The expanded stringis then printed before the detailed option help.

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

• every option defines a help string, and doesn't worry aboutline-wrapping—-optparse takes care of wrapping lines and makingthe help output look good.

• options that take a value indicate this fact in their automatically-generatedhelp message, e.g. for the "mode" option:

-m MODE, --mode=MODE

Here, "MODE" is called the meta-variable: it stands for the argument that theuser is expected to supply to -m/—mode. By default,optparse converts the destination variable name to uppercase and usesthat for the meta-variable. Sometimes, that's not what you want—-forexample, 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 manuallywritten help text uses the meta-variable FILE to clue the user in thatthere's a connection between the semi-formal syntax -f FILE and the informalsemantic description "write output to FILE". This is a simple but effectiveway to make your help text a lot clearer and more useful for end users.

• options that have a default value can include %default in the helpstring—-optparse will replace it with str() of the option'sdefault value. If an option has no default value (or the default value isNone), %default expands to none.

Grouping Options

When dealing with many options, it is convenient to group these options forbetter 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 inserted into

  • title is the group title

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

OptionGroup inherits from OptionContainer (likeOptionParser) and so the add_option() method can be used to addan option to the group.

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

Continuing with the parser defined in the previous section, adding anOptionGroup 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: stillextending 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 withoption groups is:

• OptionParser.getoption_group(_opt_str)
• Return the OptionGroup to which the short or long optionstring opt_str (e.g. '-o' or '—option') belongs. Ifthere's no such OptionGroup, return None.

Printing a version string

Similar to the brief usage string, optparse can also print a versionstring for your program. You have to supply the string as the versionargument 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, optparseautomatically adds a —version option to your parser. If it encountersthis option on the command line, it expands your version string (byreplacing %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.printversion(_file=None)

• Print the version message for the current program (self.version) tofile (default stdout). As with print_usage(), any occurrenceof %prog in self.version is replaced with the name of the currentprogram. Does nothing if self.version is empty or undefined.

• OptionParser.get_version()

• Same as print_version() but returns the version string instead ofprinting it.

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 erroneouscalls to OptionParser.add_option(), e.g. invalid option strings, unknownoption attributes, missing option attributes, etc. These are dealt with in theusual way: raise an exception (either optparse.OptionError orTypeError) and let the program crash.

Handling user errors is much more important, since they are guaranteed to happenno matter how stable your code is. optparse can automatically detectsome user errors, such as bad option arguments (passing -n 4x where-n takes an integer argument), missing arguments (-n at the endof the command line, where -n takes an argument of any type). Also,you can call OptionParser.error() to signal an application-defined errorcondition:

(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 theprogram's usage message and an error message to standard error and exits witherror status 2.

Consider the first example above, where the user passes 4x to an optionthat 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 theoption involved in the error; be sure to do the same when callingOptionParser.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.

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()

参考指南

创建解析器

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 ofoptional keyword arguments. You should always pass them as keywordarguments, i.e. do not rely on the order in which the arguments are declared.

  • usage (默认: "%prog [options]")

  • The usage summary to print when your program is run incorrectly or with ahelp option. When optparse prints the usage string, it expands%prog to os.path.basename(sys.argv[0]) (or to prog if youpassed that keyword argument). To suppress a usage message, pass thespecial value optparse.SUPPRESS_USAGE.

  • option_list (默认: [])

  • A list of Option objects to populate the parser with. The options inoption_list are added after any options in standard_option_list (aclass attribute that may be set by OptionParser subclasses), but beforeany version or help options. Deprecated; use add_option() aftercreating the parser instead.

  • option_class (默认: optparse.Option)

  • Class to use when adding options to the parser in add_option().

  • version (默认: None)

  • A version string to print when the user supplies a version option. If yousupply a true value for version, optparse automatically adds aversion option with the single option string —version. Thesubstring %prog is expanded the same as for usage.

  • conflict_handler (默认: "error")

  • Specifies what to do when options with conflicting option strings areadded to the parser; see sectionConflicts between options.

  • description (默认: None)

  • A paragraph of text giving a brief overview of your program.optparse reformats this paragraph to fit the current terminal widthand prints it when the user requests help (after usage, but before thelist of options).

  • formatter (default: a new IndentedHelpFormatter)

  • An instance of optparse.HelpFormatter that will be used for printing helptext. optparse provides two concrete classes for this purpose:IndentedHelpFormatter and TitledHelpFormatter.

  • add_help_option (默认: True)

  • If true, optparse will add a help option (with option strings -hand —help) to the parser.

  • prog

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

  • epilog (默认: None)

  • A paragraph of help text to print after the option help.

填充解析器

There are several ways to populate the parser with options. The preferred wayis by using OptionParser.add_option(), as shown in section教程. 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 areacceptable to make_option() (i.e., to the Option constructor), and itwill create the Option instance for you

The other alternative is to pass a list of pre-constructed Option instances tothe 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 ofoptparse may split Option into several classes, and make_option()will pick the right class to instantiate. Do not instantiate Option directly.)

定义选项

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

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

• OptionParser.addoption(_option)
• OptionParser.addoption(*optstr, 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 mostimportant option attribute is action, and it largelydetermines which other attributes are relevant or required. If you passirrelevant option attributes, or fail to pass required ones, optparseraises an OptionError exception explaining your mistake.

An option's action determines what optparse does when it encountersthis option on the command-line. The standard option actions hard-coded intooptparse are:

• "store"

• 存储此选项的参数（默认）

• "store_const"

• store a constant value

• "store_true"

• store True

• "store_false"

• store False

• "append"

• append this option's argument to a list

• "append_const"

• 将常量值附加到列表

• "count"

• increment a counter by one

• "callback"

• 调用指定函数

• "help"

• 打印用法消息，包括所有选项和文档

(If you don't supply an action, the default is "store". For this action,you may also supply type and dest optionattributes; 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 calledoptions (it happens to be an instance of optparse.Values). Optionarguments (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 almostas important as action, but action is the onlyone that makes sense for all options.

Option attributes

The following option attributes may be passed as keyword arguments toOptionParser.add_option(). If you pass an option attribute that is notrelevant to a particular option, or fail to pass a required option attribute,optparse raises OptionError.

• Option.action
• (默认: "store")

Determines optparse's behaviour when this option is seen on thecommand line; the available options are documented here.

• Option.type
• (默认: "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, thistells optparse where to write it: dest names anattribute of the options object that optparse builds as it parsesthe command line.

• Option.default

• The value to use for this option's destination if the option is not seen onthe command line. See also OptionParser.set_defaults().

• Option.nargs

• (默认: 1)

How many arguments of type type should be consumed when thisoption is seen. If > 1, optparse will store a tuple of values todest.

• 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 choosefrom.

• Option.callback

• For options with action "callback", the callable to call when this optionis seen. See section Option Callbacks for detail on thearguments passed to the callable.

• Option.callback_args

• Option.callback_kwargs

• Additional positional and keyword arguments to pass to callback after thefour standard callback arguments.

• Option.help

• Help text to print for this option when listing all available options afterthe user supplies a help option (such as —help). Ifno help text is supplied, the option will be listed without help text. Tohide 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. Seesection 教程 for an example.

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 toguide optparse's behaviour; a few have required attributes, which youmust 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 valueaccording to type and stored in dest. Ifnargs > 1, multiple arguments will be consumed from thecommand line; all will be converted according to type andstored to dest as a tuple. See theStandard option types section.

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

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

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

示例:

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.

示例:

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 True todest.

• "store_false" [relevant: dest]

Like "store_true", but stores False.

示例:

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 indest. If no default value for dest issupplied, an empty list is automatically created when optparse firstencounters this option on the command-line. If nargs > 1,multiple arguments are consumed, and a tuple of length nargsis appended to dest.

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

示例:

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

If -t3 is seen on the command-line, optparse does the equivalentof:

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 theoption. This means that any default value specified must have an appendmethod. It also means that if the default value is non-empty, the defaultelements will be present in the parsed value for the option, with any valuesfrom 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 todest; as with "append", dest defaults toNone, and an empty list is automatically created the first time the optionis encountered.

• "count" [relevant: dest]

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

示例:

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

The first time -v is seen on the command line, optparse does theequivalent 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 optionparser. The help message is constructed from the usage string passed toOptionParser's constructor and the help string passed to everyoption.

If no help string is supplied for an option, it will still belisted in the help message. To omit an option entirely, use the special valueoptparse.SUPPRESS_HELP.

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

示例:

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 (assumingsys.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 withsys.exit(0).

• "version"

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

Standard option types

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

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

Integer arguments (type "int") 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 int() with the appropriate base (2, 8,10, or 16). If this fails, so will optparse, although with a more usefulerror message.

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

"choice" options are a subtype of "string" options. Thechoices option attribute (a sequence of strings) defines theset of allowed option arguments. optparse.check_choice() comparesuser-supplied option arguments against this master list and raisesOptionValueError if an invalid string is given.

解析参数

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

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

输入参数的位置

• args

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

• values

• an optparse.Values object to store option arguments in (default: anew instance of Values) — if you give an existing object, theoption 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.Valuesinstance 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 supplyvalues, it will be modified with repeated setattr() calls (roughly onefor every option argument stored to an option destination) and returned byparse_args().

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

Querying and manipulating your option parser

The default behavior of the option parser can be customized slightly, and youcan also poke around your option parser and see what's there. OptionParserprovides 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, optparsenormally 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(). Thisrestores traditional Unix syntax, where option parsing stops with the firstnon-option argument.

Use this if you have a command processor which runs another command which hasoptions of its own and you want to make sure these options don't getconfused. 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 interspersingswitches with command arguments. This is the default behavior.

• OptionParser.getoption(_opt_str)

• Returns the Option instance with the option string opt_str, or None ifno options have that option string.

• OptionParser.hasoption(_opt_str)

• Return True if the OptionParser has an option with option string opt_str(e.g., -q or —verbose).

• OptionParser.removeoption(_opt_str)

• If the OptionParser has an option corresponding to opt_str, thatoption is removed. If that option provided any other option strings, all ofthose option strings become invalid. If opt_str does not occur in anyoption belonging to this OptionParser, raises ValueError.

Conflicts between options

If you're not careful, it's easy to define options with conflicting optionstrings:

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

(This is particularly true if you've defined your own OptionParser subclass withsome standard options.)

Every time you add an option, optparse checks for conflicts with existingoptions. 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" (默认)

assume option conflicts are a programming error and raiseOptionConflictError

"resolve"

resolve option conflicts intelligently (see below)

As an example, let's define an OptionParser that resolves conflictsintelligently 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 alreadyusing the -n option string. Since conflict_handler is "resolve",it resolves the situation by removing -n from the earlier option's list ofoption strings. Now —dry-run is the only way for the user to activatethat 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 optionuntil there are none left, and the user has no way of invoking that option fromthe 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 ourexisting OptionParser:

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

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

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

清理

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

Other methods

OptionParser supports several other public methods:

• OptionParser.setusage(_usage)

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

• OptionParser.printusage(_file=None)

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

• OptionParser.get_usage()

• Same as print_usage() but returns the usage string instead ofprinting it.

• OptionParser.setdefaults(_dest=value, …)

• Set default values for several option destinations at once. Usingset_defaults() is the preferred way to set default values for options,since multiple options can share the same destination. For example, ifseveral "mode" options all set the same destination, any one of them can setthe 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")

Option Callbacks

When optparse's built-in actions and types aren't quite enough for yourneeds, you have two choices: extend optparse or define a callback option.Extending optparse is more general, but overkill for a lot of simplecases. 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 fourarguments, as described below

Defining a callback option

As always, the easiest way to define a callback option is by using theOptionParser.add_option() method. Apart from action, theonly 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 alreadydefined my_callback() when you create this callback option. In this simplecase, 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 somecircumstances, though, you might want your callback to consume an arbitrarynumber 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 itwill only pass additional arguments if you specify them viacallback_args and callback_kwargs. Thus, theminimal 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 acallback option:

• type

• has its usual meaning: as with the "store" or "append" actions, itinstructs optparse to consume one argument and convert it totype. 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 willconsume nargs arguments, each of which must be convertible totype. It then passes a tuple of converted values to yourcallback.

• 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

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 thecommand-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 willonly expect an argument if type is set; the type of value will bethe type implied by the option's type. If type for this option isNone (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 becauseyou can access some other interesting data through its instance attributes:

  • parser.largs

  • the current list of leftover arguments, ie. arguments that have beenconsumed but are neither options nor option arguments. Feel free to modifyparser.largs, e.g. by adding more arguments to it. (This list willbecome args, the second return value of parse_args().)

  • parser.rargs

  • the current list of remaining arguments, ie. with opt_str andvalue (if applicable) removed, and only the arguments following themstill there. Feel free to modify parser.rargs, e.g. by consuming morearguments.

  • parser.values

  • the object where option values are by default stored (an instance ofoptparse.OptionValues). This lets callbacks use the same mechanism as therest of optparse for storing option values; you don't need to messaround with globals or closures. You can also access or modify thevalue(s) of any options already encountered on the command-line.

• args

• is a tuple of arbitrary positional arguments supplied via thecallback_args option attribute.

• kwargs

• is a dictionary of arbitrary keyword arguments supplied viacallback_kwargs.

Raising errors in a callback

The callback function should raise OptionValueError if there are anyproblems with the option or its argument(s). optparse catches this andterminates the program, printing the error message you supply to stderr. Yourmessage 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.

Callback example 1: trivial callback

Here's an example of a callback option that takes no arguments, and simplyrecords 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.

Callback example 2: check option order

Here's a slightly more interesting example: record the fact that -a isseen, 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")

Callback example 3: check option order (generalized)

If you want to re-use this callback for several similar options (set a flag, butblow up if -b has already been seen), it needs a bit of work: the errormessage 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')

Callback example 4: check arbitrary condition

Of course, you could put any condition in there—-you're not limited to checkingthe values of already-defined options. For example, if you have options thatshould 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.)

Callback example 5: fixed arguments

Things get slightly more interesting when you define callback options that takea fixed number of arguments. Specifying that a callback option takes argumentsis similar to defining a "store" or "append" option: if you definetype, then the option takes one argument that must beconvertible to that type; if you further define nargs, then theoption 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 convertingthem to integers for you; all you have to do is store them. (Or whatever;obviously you don't need a callback for this example.)

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 anybuilt-in capabilities for it. And you have to deal with certain intricacies ofconventional Unix command-line parsing that optparse normally handles foryou. 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-lineprocessing and discard the —

• bare - (if not the argument to some option): halt command-lineprocessing but keep the - (append it to parser.largs)

If you want an option that takes a variable number of arguments, there areseveral subtle, tricky issues to worry about. The exact implementation youchoose will be based on which trade-offs you're willing to make for yourapplication (which is why optparse doesn't support this sort of thingdirectly).

Nevertheless, here's a stab at a callback for an option with variablearguments:

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)

Extending optparse

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

Adding new types

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

• Option.TYPES

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

• Option.TYPE_CHECKER

• A dictionary mapping type names to type-checking functions. A type-checkingfunction 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 mustbe checked and converted to your desired type. check_mytype() shouldreturn an object of the hypothetical type mytype. The value returned bya type-checking function will wind up in the OptionValues instance returnedby OptionParser.parse_args(), or be passed to a callback as thevalue parameter.

Your type-checking function should raise OptionValueError if itencounters any problems. OptionValueError takes a single stringargument, 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 toparse Python-style complex numbers on the command line. (This is even sillierthan it used to be, because optparse 1.3 added built-in support forcomplex 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 theTYPE_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 endup modifying the TYPE_CHECKER attribute of optparse'sOption class. This being Python, nothing stops you from doing that except goodmanners and common sense.)

That's it! Now you can write a script that uses the new option type just likeany other optparse-based script, except you have to instruct yourOptionParser 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; ifyou don't use add_option() in the above way, you don't need to tellOptionParser which option class to use:

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

Adding new actions

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

• "store" actions

• actions that result in optparse storing a value to an attribute of thecurrent OptionValues instance; these options require a destattribute to be supplied to the Option constructor.

• "typed" actions

• actions that take a value from the command line and expect it to be of acertain type; or rather, a string that can be converted to a certain type.These options require a type attribute to the Optionconstructor.

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 oneof 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) areadditionally listed here. The only effect of this is that optparseassigns the default type, "string", to options with no explicit typewhose action is listed in ALWAYS_TYPED_ACTIONS.

In order to actually implement your new action, you must override Option'stake_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-lineand appending it to an existing list, "extend" will take multiple values ina single comma-delimited string, and extend an existing list with them. Thatis, if —names is an "extend" option of type "string", the commandline

--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 valuesomewhere, so it goes in both STORE_ACTIONS andTYPED_ACTIONS.

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

• MyOption.take_action() implements just this one new action, and passescontrol back to Option.take_action() for the standard optparseactions.

• values is an instance of the optparse_parser.Values class, which providesthe very useful ensure_value() method. ensure_value() isessentially 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, thenensure_value() first sets it to value, and then returns 'value. This isvery handy for actions like "extend", "append", and "count", allof which accumulate data in a variable and expect that variable to be of acertain type (a list for the first two, an integer for the latter). Usingensure_value() means that scripts using your action don't have to worryabout setting a default value for the option destinations in question; theycan just leave the default as None and ensure_value() will take care ofgetting it right when it's needed.