Deprecated since version 3.2:Theoptparse module is deprecated and will not be developed further;development will continue with theargparse module.
Source code:Lib/optparse.py
optparse is a more convenient, flexible, and powerful library for parsingcommand-line options than the oldgetopt 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 usingoptparse in a simple script:
fromoptparseimportOptionParser[...]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 byparse_args() based on user-suppliedcommand-line values. Whenparse_args() returns from parsing this commandline,options.filename will be"outfile" andoptions.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>-foutfile--quiet<yourscript>--quiet--fileoutfile<yourscript>-q-foutfile<yourscript>-qfoutfile
Additionally, users can run one of
<yourscript>-h<yourscript>--help
andoptparse 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 ofyourscript is determined at runtime (normally fromsys.argv[0]).
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.
a string entered on the command-line, and passed by the shell toexecl()orexecv(). In Python, arguments are elements ofsys.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:]”.
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 byoptparse.
Some other option syntaxes that the world has seen include:
These option syntaxes are not supported byoptparse, 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.
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.
For example, consider this hypothetical command-line:
prog-v--reportreport.txtfoobar
-v and--report are both options. Assuming that--reporttakes one argument,report.txt is an option argument.foo andbar are positional arguments.
Options are used to provide extra information to tune or customize the executionof a program. In case it wasn’t clear, options are usuallyoptional. 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 arefind,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’snot 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 humblecputility, 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:
cpSOURCEDESTcpSOURCE...DEST-DIR
You can get pretty far with just that. Mostcp 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.
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 matterhow 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.
Whileoptparse is quite flexible and powerful, it’s also straightforwardto use in most cases. This section covers the code patterns that are common toanyoptparse-based program.
First, you need to import the OptionParser class; then, early in the mainprogram, create an OptionParser instance:
fromoptparseimportOptionParser[...]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 telloptparse 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 toOptionParser.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,optparseencountersoption strings and looks up options from them.
Once all of your options are defined, instructoptparse to parse yourprogram’s command line:
(options,args)=parser.parse_args()
(If you like, you can pass a custom argument list toparse_args(), butthat’s rarely necessary: by default it usessys.argv[1:].)
parse_args() returns two values:
This tutorial section only covers the four most important option attributes:action,type,dest(destination), andhelp. Of these,action is themost fundamental.
Actions telloptparse what to do when it encounters an option on thecommand line. There is a fixed set of actions hard-coded intooptparse;adding new actions is an advanced topic covered in sectionExtending optparse. Most actions telloptparse to storea value in some variable—for example, take a string from the command line andstore it in an attribute ofoptions.
If you don’t specify an option action,optparse defaults tostore.
The most common option action isstore, which tellsoptparse 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.
For example:
parser.add_option("-f","--file",action="store",type="string",dest="filename")
Now let’s make up a fake command line and askoptparse to parse it:
args=["-f","foo.txt"](options,args)=parser.parse_args(args)
Whenoptparse sees the option string-f, it consumes the nextargument,foo.txt, and stores it inoptions.filename. So, after thiscall toparse_args(),options.filename is"foo.txt".
Some other option types supported byoptparse areint andfloat.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 isstore.
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-n42 (two arguments), the code
(options,args)=parser.parse_args(["-n42"])print(options.num)
will print42.
If you don’t specify a type,optparse assumesstring. Combined withthe fact that the default action isstore, 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 isfoo_bar. If there are nolong option strings,optparse looks at the first short option string: thedefault destination for-f isf.
optparse also includes the built-incomplex type. Addingtypes is covered in sectionExtending optparse.
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 andstore_false. For example, you might have averboseflag 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 default values—see below.)
Whenoptparse encounters-v on the command line, it setsoptions.verbose toTrue; when it encounters-q,options.verbose is set toFalse.
Some other actions supported byoptparse are:
These are covered in sectionReference Guide, Reference Guideand sectionOption Callbacks.
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 toNone. 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 wantoptparse to setverbose toTrue 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 thedestination 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)
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 forverbose will beTrue: the last defaultvalue supplied for any particular destination is the one that counts.
A clearer way to specify default values is theset_defaults() method ofOptionParser, which you can call at any time before callingparse_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.
optparse‘s ability to generate help and usage text automatically isuseful for creating user-friendly command-line interfaces. All you have to dois supply ahelp 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]")
Ifoptparse encounters either-h or--help on thecommand-line, or if you just callparser.print_help(), it prints thefollowing to standard output:
Usage: <yourscript> [options] arg1 arg2Options: -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 helpoptparse 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 about line-wrapping—optparse takes care of wrapping lines and making the help output lookgood.
options that take a value indicate this fact in their automatically-generatedhelp message, e.g. for the “mode” option:
-mMODE,--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 setsmetavar="FILE",resulting in this automatically-generated option description:
-fFILE,--filename=FILE
This is important for more than just saving space, though: the manuallywritten help text uses the meta-variableFILE to clue the user in thatthere’s a connection between the semi-formal syntax-fFILE 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 withstr() of the option’sdefault value. If an option has no default value (or the default value isNone),%default expands tonone.
When dealing with many options, it is convenient to group these options forbetter help output. AnOptionParser can contain several option groups,each of which can contain several options.
An option group is obtained using the classOptionGroup:
where
OptionGroup inherits fromOptionContainer (likeOptionParser) and so theadd_option() method can be used to addan option to the group.
Once all the options are declared, using theOptionParser 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 arg2Options: -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 arg2Options: -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:
Return theOptionGroup to which the short or long optionstringopt_str (e.g.'-o' or'--option') belongs. Ifthere’s no suchOptionGroup, returnNone.
Similar to the brief usage string,optparse can also print a versionstring for your program. You have to supply the string as theversionargument to OptionParser:
parser=OptionParser(usage="%prog [-f] [-q]",version="%prog 1.0")
%prog is expanded just like it is inusage. 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 yourversion string (byreplacing%prog), prints it to stdout, and exits.
For example, if your script is called/usr/bin/foo:
$ /usr/bin/foo --versionfoo 1.0
The following two methods can be used to print and get theversion string:
Print the version message for the current program (self.version) tofile (default stdout). As withprint_usage(), any occurrenceof%prog inself.version is replaced with the name of the currentprogram. Does nothing ifself.version is empty or undefined.
Same asprint_version() but returns the version string instead ofprinting it.
There are two broad classes of errors thatoptparse has to worry about:programmer errors and user errors. Programmer errors are usually erroneouscalls toOptionParser.add_option(), e.g. invalid option strings, unknownoption attributes, missing option attributes, etc. These are dealt with in theusual way: raise an exception (eitheroptparse.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-n4x 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 callOptionParser.error() to signal an application-defined errorcondition:
(options,args)=parser.parse_args()[...]ifoptions.aandoptions.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 passes4x to an optionthat takes an integer:
$ /usr/bin/foo -n 4xUsage: foo [options]foo: error: option -n: invalid integer value: '4x'
Or, where the user fails to pass a value at all:
$ /usr/bin/foo -nUsage: 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.
Ifoptparse‘s default error-handling behaviour does not suit your needs,you’ll need to subclass OptionParser and override itsexit()and/orerror() methods.
Here’s whatoptparse-based scripts usually look like:
fromoptparseimportOptionParser[...]defmain():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()iflen(args)!=1:parser.error("incorrect number of arguments")ifoptions.verbose:print("reading %s..."%options.filename)[...]if__name__=="__main__":main()
The first step in usingoptparse is to create an OptionParser instance.
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.
There are several ways to populate the parser with options. The preferred wayis by usingOptionParser.add_option(), as shown in sectionTutorial.add_option() can be called in one of two ways:
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, andmake_option()will pick the right class to instantiate. Do not instantiate Option directly.)
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 anOption instance is with theadd_option() method ofOptionParser.
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 isaction, and it largelydetermines which other attributes are relevant or required. If you passirrelevant option attributes, or fail to pass required ones,optparseraises anOptionError exception explaining your mistake.
An option’saction determines whatoptparse does when it encountersthis option on the command-line. The standard option actions hard-coded intooptparse are:
(If you don’t supply an action, the default is"store". For this action,you may also supplytype anddest optionattributes; seeStandard 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 ofoptparse.Values). Optionarguments (and various other values) are stored as attributes of this object,according to thedest (destination) option attribute.
For example, when you call
parser.parse_args()
one of the first thingsoptparse does is create theoptions 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-ffoo--file=foo--filefoo
thenoptparse, on seeing this option, will do the equivalent of
options.filename="foo"
Thetype anddest option attributes are almostas important asaction, butaction is the onlyone that makes sense forall options.
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 raisesOptionError.
(default:"store")
Determinesoptparse‘s behaviour when this option is seen on thecommand line; the available options are documentedhere.
(default:"string")
The argument type expected by this option (e.g.,"string" or"int");the available option types are documentedhere.
(default: derived from option strings)
If the option’s action implies writing or modifying a value somewhere, thistellsoptparse where to write it:dest names anattribute of theoptions object thatoptparse builds as it parsesthe command line.
The value to use for this option’s destination if the option is not seen onthe command line. See alsoOptionParser.set_defaults().
(default: 1)
How many arguments of typetype should be consumed when thisoption is seen. If > 1,optparse will store a tuple of values todest.
For actions that store a constant value, the constant value to store.
For options of type"choice", the list of strings the user may choosefrom.
For options with action"callback", the callable to call when this optionis seen. See sectionOption Callbacks for detail on thearguments passed to the callable.
Additional positional and keyword arguments to pass tocallback after thefour standard callback arguments.
The various option actions all have slightly different requirements and effects.Most actions have several relevant option attributes which you may specify toguideoptparse‘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 totype and stored indest. Ifnargs > 1, multiple arguments will be consumed from thecommand line; all will be converted according totype andstored todest as a tuple. See theStandard option types section.
Ifchoices is supplied (a list or tuple of strings), the typedefaults to"choice".
Iftype is not supplied, it defaults to"string".
Ifdest 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 impliesf).
Example:
parser.add_option("-f")parser.add_option("-p",type="float",nargs=3,dest="point")
As it parses the command line
-ffoo.txt-p1-3.54-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 valueconst is stored indest.
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 todest.
"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 indest. If no default value fordest issupplied, an empty list is automatically created whenoptparse firstencounters this option on the command-line. Ifnargs > 1,multiple arguments are consumed, and a tuple of lengthnargsis appended todest.
The defaults fortype anddest are the same asfor the"store" action.
Example:
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"))
Theappend action calls theappend method on the current value of theoption. This means that any default value specified must have anappendmethod. 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 valueconst 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 atdest. If no default value issupplied,dest is set to zero before being incremented thefirst time.
Example:
parser.add_option("-v",action="count",dest="verbosity")
The first time-v is seen on the command line,optparse does theequivalent of:
options.verbosity=0options.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 bycallback, which is called as
func(option,opt_str,value,parser,*args,**kwargs)
See sectionOption Callbacks for more detail.
"help"
Prints a complete help message for all the options in the current optionparser. The help message is constructed from theusage string passed toOptionParser’s constructor and thehelp string passed to everyoption.
If nohelp 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 ahelp option to allOptionParsers, so you do not normally need to create one.
Example:
fromoptparseimportOptionParser,SUPPRESS_HELP# usually, a help option is added automatically, but that can# be suppressed using the add_help_option argumentparser=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)
Ifoptparse 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 createversion options,sinceoptparse automatically adds them when needed.
optparse has five built-in option types:"string","int","choice","float" and"complex". If you need to add newoption types, see sectionExtending 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:
The conversion is done by callingint() with the appropriate base (2, 8,10, or 16). If this fails, so willoptparse, although with a more usefulerror message.
"float" and"complex" option arguments are converted directly withfloat() andcomplex(), 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.
The whole point of creating and populating an OptionParser is to call itsparse_args() method:
(options,args)=parser.parse_args(args=None,values=None)
where the input parameters are
and the return values are
The most common usage is to supply neither keyword argument. If you supplyvalues, it will be modified with repeatedsetattr() calls (roughly onefor every option argument stored to an option destination) and returned byparse_args().
Ifparse_args() encounters any errors in the argument list, it calls theOptionParser’serror() 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).
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:
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-aarg1-barg2
and treats it as equivalent to
prog-a-barg1arg2
To disable this feature, calldisable_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.
Set parsing to not stop on the first non-option, allowing interspersingswitches with command arguments. This is the default behavior.
Returns the Option instance with the option stringopt_str, orNone ifno options have that option string.
Return true if the OptionParser has an option with option stringopt_str(e.g.,-q or--verbose).
If theOptionParser has an option corresponding toopt_str, thatoption is removed. If that option provided any other option strings, all ofthose option strings become invalid. Ifopt_str does not occur in anyoption belonging to thisOptionParser, raisesValueError.
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" (default)
- assume option conflicts are a programming error and raiseOptionConflictError
- "resolve"
- resolve option conflicts intelligently (see below)
As an example, let’s define anOptionParser 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. Sinceconflict_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-rundonoharm[...]-n,--noisybenoisy
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, sooptparse removes it, leaving this help text:
Options:[...]-n,--noisybenoisy--dry-runnewdry-runoption
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 callingdestroy() on yourOptionParser once you are done with it. This is particularly useful inlong-running applications where large object graphs are reachable from yourOptionParser.
OptionParser supports several other public methods:
Set the usage string according to the rules described above for theusageconstructor keyword argument. PassingNone sets the default usagestring; useoptparse.SUPPRESS_USAGE to suppress a usage message.
Print the usage message for the current program (self.usage) tofile(default stdout). Any occurrence of the string%prog inself.usageis replaced with the name of the current program. Does nothing ifself.usage is empty or not defined.
Same asprint_usage() but returns the usage string instead ofprinting it.
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 belowparser.add_option("--novice",action="store_const",dest="mode",const="novice",default="advanced")# overrides above setting
To avoid this confusion, useset_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")
Whenoptparse‘s built-in actions and types aren’t quite enough for yourneeds, you have two choices: extendoptparse or define a callback option.Extendingoptparse 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:
As always, the easiest way to define a callback option is by using theOptionParser.add_option() method. Apart fromaction, theonly option attribute you must specify iscallback, 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 alreadydefinedmy_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 andcallback_kwargs. Thus, theminimal callback function signature is:
defmy_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:
All callbacks are called as follows:
func(option,opt_str,value,parser,*args,**kwargs)
where
is the OptionParser instance driving the whole thing, mainly useful becauseyou can access some other interesting data through its instance attributes:
The callback function should raiseOptionValueError 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 he did wrong.
Here’s an example of a callback option that takes no arguments, and simplyrecords that the option was seen:
defrecord_foo_seen(option,opt_str,value,parser):parser.values.saw_foo=Trueparser.add_option("--foo",action="callback",callback=record_foo_seen)
Of course, you could do that with the"store_true" action.
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.
defcheck_order(option,opt_str,value,parser):ifparser.values.b:raiseOptionValueError("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")
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.
defcheck_order(option,opt_str,value,parser):ifparser.values.b:raiseOptionValueError("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')
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:
defcheck_moon(option,opt_str,value,parser):ifis_moon_full():raiseOptionValueError("%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 ofis_moon_full() is left as an exercise for the reader.)
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 definenargs, then theoption takesnargs arguments.
Here’s an example that just emulates the standard"store" action:
defstore_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 thatoptparse 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.)
Things get hairy when you want an option to take a variable number of arguments.For this case, you must write a callback, asoptparse doesn’t provide anybuilt-in capabilities for it. And you have to deal with certain intricacies ofconventional Unix command-line parsing thatoptparse normally handles foryou. In particular, callbacks should implement the conventional rules for bare-- and- arguments:
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 whyoptparse doesn’t support this sort of thingdirectly).
Nevertheless, here’s a stab at a callback for an option with variablearguments:
defvararg_callback(option,opt_str,value,parser):assertvalueisNonevalue=[]deffloatable(str):try:float(str)returnTrueexceptValueError:returnFalseforarginparser.rargs:# stop on --foo like optionsifarg[:2]=="--"andlen(arg)>2:break# stop on -a, but not on -3 or -3.0ifarg[:1]=="-"andlen(arg)>1andnotfloatable(arg):breakvalue.append(arg)delparser.rargs[:len(value)]setattr(parser.values,option.dest,value)[...]parser.add_option("-c","--callback",dest="vararg_attr",action="callback",callback=vararg_callback)
Since the two major controlling factors in howoptparse interpretscommand-line options are the action and type of each option, the most likelydirection of extension is to add new actions and new types.
To add new types, you need to define your own subclass ofoptparse‘sOption class. This class has a couple of attributes that defineoptparse‘s types:TYPES andTYPE_CHECKER.
A tuple of type names; in your subclass, simply define a new tupleTYPES that builds on the standard one.
A dictionary mapping type names to type-checking functions. A type-checkingfunction has the following signature:
defcheck_mytype(option,opt,value)
whereoption is anOption instance,opt is an option string(e.g.,-f), andvalue is the string from the command line that mustbe checked and converted to your desired type.check_mytype() shouldreturn an object of the hypothetical typemytype. The value returned bya type-checking function will wind up in the OptionValues instance returnedbyOptionParser.parse_args(), or be passed to a callback as thevalue parameter.
Your type-checking function should raiseOptionValueError if itencounters any problems.OptionValueError takes a single stringargument, which is passed as-is toOptionParser‘serror()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, becauseoptparse 1.3 added built-in support forcomplex numbers, but never mind.)
First, the necessary imports:
fromcopyimportcopyfromoptparseimportOption,OptionValueError
You need to define your type-checker first, since it’s referred to later (in theTYPE_CHECKER class attribute of your Option subclass):
defcheck_complex(option,opt,value):try:returncomplex(value)exceptValueError:raiseOptionValueError("option %s: invalid complex value: %r"%(opt,value))
Finally, the Option subclass:
classMyOption(Option):TYPES=Option.TYPES+("complex",)TYPE_CHECKER=copy(Option.TYPE_CHECKER)TYPE_CHECKER["complex"]=check_complex
(If we didn’t make acopy() ofOption.TYPE_CHECKER, we would endup modifying theTYPE_CHECKER attribute ofoptparse‘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 otheroptparse-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 useadd_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 is a bit trickier, because you have to understand thatoptparse has a couple of classifications for actions:
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):
All actions must be listed in ACTIONS.
“store” actions are additionally listed here.
“typed” actions are additionally listed here.
Actions that always take a type (i.e. whose options always take a value) areadditionally listed here. The only effect of this is thatoptparseassigns the default type,"string", to options with no explicit typewhose action is listed inALWAYS_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--namesblah--namesding,dong
would result in a list
["foo","bar","blah","ding","dong"]
Again we define a subclass of Option:
classMyOption(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",)deftake_action(self,action,dest,opt,value,values,parser):ifaction=="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 bothSTORE_ACTIONS andTYPED_ACTIONS.
to ensure thatoptparse 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 toOption.take_action() for the standardoptparseactions.
values is an instance of the optparse_parser.Values class, which providesthe very usefulensure_value() method.ensure_value() isessentiallygetattr() with a safety valve; it is called as
values.ensure_value(attr,value)
If theattr attribute ofvalues doesn’t exist or is None, thenensure_value() first sets it tovalue, 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 andensure_value() will take care ofgetting it right when it’s needed.
16.4.argparse — Parser for command-line options, arguments and sub-commands
16.6.getopt — C-style parser for command line options
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