There is frequently a need for programs to parse a UNIX-like
command line program: options preceded by - or
--, sometimes followed by a parameter, followed by
a list of arguments. The twisted.python.usage provides a class,
Options, to facilitate such parsing.
While Python has the getopt module for doing
this, it provides a very low level of abstraction for options.
Twisted has a higher level of abstraction, in the class twisted.python.usage.Options. It uses
Python's reflection facilities to provide an easy to use yet
flexible interface to the command line. While most command line
processors either force the application writer to write her own
loops, or have arbitrary limitations on the command line (the
most common one being not being able to have more then one
instance of a specific option, thus rendering the idiom
program -v -v -v impossible), Twisted allows the
programmer to decide how much control she wants.
The Options class is used by subclassing. Since
a lot of time it will be used in the twisted.tap package, where the local
conventions require the specific options parsing class to also
be called Options, it is usually imported with
from twisted.python import usage
For simple boolean options, define the attribute
optFlags like this:
class Options(usage.Options):
optFlags = [["fast", "f"], ["safe", "s"]]
optFlags should be a list of 2-lists. The first
element is the long name, and will be used on the command line
as --fast. The second one is the short name, and
will be used on the command line as -f. The long
name also determines the name of the key that will be set on
the Options instance. Its value will be 1 if the option was seen, 0
otherwise. Here is an example for usage:
class Options(usage.Options):
optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]
command_line = ["-g", "--fast"]
options = Options()
try:
options.parseOptions(command_line)
except usage.UsageError, errortext:
print '%s: %s' % (sys.argv[0], errortext)
print '%s: Try --help for usage details.' % (sys.argv[0])
sys.exit(1)
if options['fast']:
print "fast",
if options['good']:
print "good",
if options['cheap']:
print "cheap",
print
The above will print fast good.
Note here that Options fully supports the mapping interface. You can access it mostly just like you can access any other dict. Options are stored as mapping items in the Options instance: parameters as 'paramname': 'value' and flags as 'flagname': 1 or 0.
Sometimes there is a need for several option processors with
a unifying core. Perhaps you want all your commands to
understand -q/--quiet means to be
quiet, or something similar. On the face of it, this looks
impossible: in Python, the subclass's optFlags
would shadow the superclass's. However,
usage.Options uses special reflection code to get
all of the optFlags defined in the hierarchy. So
the following:
class BaseOptions(usage.Options):
optFlags = [["quiet", "q"]]
class SpecificOptions(BaseOptions):
optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]
Is the same as:
class SpecificOptions(BaseOptions):
optFlags = [["quiet", "q"], ["fast", "f"], ["good", "g"], ["cheap", "c"]]
Parameters are specified using the attribute
optParameters. They must be given a
default. If you want to make sure you got the parameter from
the command line, give a non-string default. Since the command
line only has strings, this is completely reliable.
Here is an example:
from twisted.python import usage
class Options(usage.Options):
optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]
optParameters = [["user", "u", None]]
try:
config.parseOptions() # When given no argument, parses sys.argv[1:]
except usage.UsageError, errortext:
print '%s: %s' % (sys.argv[0], errortext)
print '%s: Try --help for usage details.' % (sys.argv[0])
sys.exit(1)
if config['user'] is not None:
print "Hello", config['user']
print "So, you want it:"
if config['fast']:
print "fast",
if config['good']:
print "good",
if config['cheap']:
print "cheap",
print
Like optFlags, optParameters works
smoothly with inheritance.
It is useful, on occassion, to group a set of options together based
on the logical action
to which they belong. For this, the
usage.Options class allows you to define a set of
subcommands
, each of which can provide its own
usage.Options instance to handle its particular
options.
Here is an example for an Options class that might parse options like those the cvs program takes
from twisted.python import usage
class ImportOptions(usage.Options):
optParameters = [['module', 'm', None], ['vendor', 'v', None],
['release', 'r', None]]
class CheckoutOptions(usage.Options):
optParameters = [['module', 'm', None], ['tag', 'r', None]]
class Options(usage.Options):
subCommands = [['import', None, ImportOptions],
['checkout', None, CheckoutOptions]]
optParameters = [['compression', 'z', 0], ['repository', 'r', None]]
The subCommands attribute of Options
directs the parser to the two other Options subclasses
when the strings "import" or "checkout" are
present on the command
line. All options after the given command string are passed to the
specified Options subclass for further parsing. Only one subcommand
may be specified at a time. After parsing has completed, the Options
instance has two new attributes - subCommand and
subOptions - which hold the command string and the Options
instance used to parse the remaining options.
Sometimes, just setting an attribute on the basis of the
options is not flexible enough. In those cases, Twisted does
not even attempt to provide abstractions such as counts
or
lists
, but rathers lets you call your own method, which will
be called whenever the option is encountered.
Here is an example of counting verbosity
from twisted.python import usage
class Options(usage.Options):
def __init__(self):
usage.Options.__init__(self)
self['verbosity'] = 0 # default
def opt_verbose(self):
self['verbosity'] = self['verbosity']+1
def opt_quiet(self):
self['verbosity'] = self['verbosity']-1
opt_v = opt_verbose
opt_q = opt_quiet
Command lines that look like
command -v -v -v -v will
increase verbosity to 4, while
command -q -q -q will decrease
verbosity to -3.
The usage.Options
class knows that these are
parameter-less options, since the methods do not receive an
argument. Here is an example for a method with a parameter:
from twisted.python import usage
class Options(usage.Options):
def __init__(self):
usage.Options.__init__(self)
self['symbols'] = []
def opt_define(self, symbol):
self['symbols'].append(symbol)
opt_D = opt_define
This example is useful for the common idiom of having
command -DFOO -DBAR to define symbols.
usage.Options does not stop helping when the
last parameter is gone. All the other arguments are sent into a
function which should deal with them. Here is an example for a
cmp like command.
from twisted.python import usage
class Options(usage.Options):
optParameters = [["max_differences", "d", 1]]
def parseArgs(self, origin, changed):
self['origin'] = origin
self['changed'] = changed
The command should look like command origin
changed.
If you want to have a variable number of left-over
arguments, just use def parseArgs(self, *args):.
This is useful for commands like the UNIX
cat(1).
Sometimes, you want to perform post processing of options to patch up inconsistencies, and the like. Here is an example:
from twisted.python import usage
class Options(usage.Options):
optFlags = [["fast", "f"], ["good", "g"], ["cheap", "c"]]
def postOptions(self):
if self['fast'] and self['good'] and self['cheap']:
raise usage.UsageError, "can't have it all, brother"