The cli_utils Module

This module provides functions and patterns for creating Command Line Interface (CLI) tools.

Common CLI Functions

Commonly used, CLI related functions.

osrf_pycommon.cli_utils.common.extract_argument_group(args, delimiting_option)[source]

Extract a group of arguments from a list of arguments using a delimiter.

Here is an example:

>>> extract_argument_group(['foo', '--args', 'bar', '--baz'], '--args')
(['foo'], ['bar', '--baz'])

The group can always be ended using the double hyphen --. In order to pass a double hyphen as arguments, use three hyphens ---. Any set of hyphens encountered after the delimiter, and up to --, which have three or more hyphens and are isolated, will be captured and reduced by one hyphen.

For example:

>> extract_argument_group(['foo',
                           '--args', 'bar', '--baz', '---', '--',
                           '--foo-option'], '--args')
(['foo', '--foo-option'], ['bar', '--baz', '--'])

In the result the -- comes from the --- in the input. The --args and the corresponding -- are removed entirely.

The delimiter and -- terminator combination can also happen multiple times, in which case the bodies of arguments are combined and returned in the order they appeared.

For example:

>> extract_argument_group(['foo',
                           '--args', 'ping', '--',
                           '--args', 'pong', '--',
                           '--args', '--'], '--args')
(['foo', 'bar', 'baz'], ['ping', 'pong'])

Note: -- cannot be used as the delimiting_option.

  • args (list) – list of strings which are ordered arguments.
  • delimiting_option (str) – option which denotes where to split the args.

tuple of arguments before and after the delimiter.

Return type:



ValueError if the delimiting_option is --.


Extracts make job flags from a list of other make flags, i.e. -j8 -l8

The input arguments are given as a string separated by whitespace. Make job flags are matched and removed from the arguments, and the Make job flags and what is left over from the input arguments are returned.

If no job flags are encountered, then an empty string is returned as the first element of the returned tuple.


>> extract_jobs_flags('-j8 -l8')
('-j8 -l8', '')
>> extract_jobs_flags('-j8 ')
('-j8', ' ')
>> extract_jobs_flags('target -j8 -l8 --some-option')
('-j8 -l8', 'target --some-option')
>> extract_jobs_flags('target --some-option')
('', 'target --some-option')
Parameters:arguments (str) – string of space separated arguments which may or may not contain make job flags
Returns:tuple of make jobs flags as a space separated string and leftover arguments as a space separated string
Return type:tuple

The Verb Pattern

The verb pattern is a pattern where a single command aggregates multiple related commands by taking a required positional argument which is the “verb” for the action you want to perform. For example, catkin build is an example of a command and verb pair, where catkin is the command and build is the verb. In this example, the catkin command groups “actions” which are related to catkin together using verbs like build which will build a workspace of catkin packages.

Command Boilerplate

This is an example boilerplate of a command which will use verbs:

from __future__ import print_function

import argparse
import sys

from osrf_pycommon.cli_utils.verb_pattern import create_subparsers
from osrf_pycommon.cli_utils.verb_pattern import list_verbs
from osrf_pycommon.cli_utils.verb_pattern import split_arguments_by_verb


VERBS_ENTRY_POINT = '{0}.verbs'.format(COMMAND_NAME)

def main(sysargs=None):
    # Assign sysargs if not set
    sysargs = sys.argv[1:] if sysargs is None else sysargs

    # Create a top level parser
    parser = argparse.ArgumentParser(
        description="{0} command".format(COMMAND_NAME)

    # Generate a list of verbs available
    verbs = list_verbs(VERBS_ENTRY_POINT)

    # Create the subparsers for each verb and collect the arg preprocessors
    argument_preprocessors, verb_subparsers = create_subparsers(

    # Determine the verb, splitting arguments into pre and post verb
    verb, pre_verb_args, post_verb_args = split_arguments_by_verb(sysargs)

    # Short circuit -h and --help
    if '-h' in pre_verb_args or '--help' in pre_verb_args:

    # Error on no verb provided
    if verb is None:
        sys.exit("Error: No verb provided.")
    # Error on unknown verb provided
    if verb not in verbs:
        sys.exit("Error: Unknown verb '{0}' provided.".format(verb))

    # Short circuit -h and --help for verbs
    if '-h' in post_verb_args or '--help' in post_verb_args:

    # First allow the verb's argument preprocessor to strip any args
    # and return any "extra" information it wants as a dict
    processed_post_verb_args, extras = \
    # Then allow argparse to process the left over post-verb arguments along
    # with the pre-verb arguments and the verb itself
    args = parser.parse_args(pre_verb_args + [verb] + processed_post_verb_args)
    # Extend the argparse result with the extras from the preprocessor
    for key, value in extras.items():
        setattr(args, key, value)

    # Finally call the subparser's main function with the processed args
    # and the extras which the preprocessor may have returned
    sys.exit(args.main(args) or 0)

This function is mostly boilerplate in that it will likely not change much between commands of different types, but it would also be less transparent to have this function created for you. If you are using this boilerplate to implement your command, then you should be careful to update COMMAND_NAME to reflect your command’s name.

This line defines the entry_point group for your command’s verbs:

VERBS_ENTRY_POINT = '{0}.verbs'.format(COMMAND_NAME)

In the case that your command is called foo then this would become foo.verbs. This name is important because it is how verbs for this command can be provided by your Python package or others. For example, each verb for your command foo will need entry in the of its containing package, like this:

        'foo.verbs': [
            'bar =',

You can see here that you are defining bar to be a an entry_point of type foo.verbs which in turn points to a module and reference and entry_point_data. At run time this verb pattern will let your command lookup all things defined as foo.verbs and load up the reference to which they point.

Adding Verbs

In order to add a verb to your command, a few things must happen.

First you must have an entry in the as described above. This allows the command to find the entry_point for your verb at run time. The entry_point for these verbs should point to a dictionary which describes the verb being added.

This is an example of an entry_point_data dictionary for a verb:

entry_point_data = dict(
    description='Builds a workspace of packages',
    # Called for execution, given parsed arguments object
    # Called first to setup argparse, given argparse parser
    # Called after prepare_arguments, but before argparse.parse_args

As you can see this dictionary describes the verb and gives references to functions which allow the command to describe the verb, hook into argparse parameter creation for the verb, and to execute the verb. The verb, description, main, and prepare_arguments keys of the dictionary are required, but the argument_preprocessor key is optional.

  • verb: This is the name of the verb, and is how the command knows which verb implementation to match to a verb on the command line.
  • description: This is used by the argument parsing to describe the verb in --help.
  • prepare_arguments: This function gets called to allow the verb to setup it’s own argparse options. This function should always take one parameter which is the argparse.ArgumentParser for this verb, to which arguments can be added. It can optionally take a second parameter which are the current command line arguments. This is not always needed, but can be useful in some cases. This function should always return the parser.
  • argument_preprocessor: This function is optional, but allows your verb an opportunity to process the raw arguments before they are passed to argparse’s parse_args function. This can be useful when argparse is not capable of processing the options correctly.
  • main: This is the implementation of the verb, it gets called last and is passed the parsed arguments. The return type of this function is used for sys.exit, a return type of None is interpreted as 0.

Here is an invented example of main, prepare_arguments, and argument_preprocessor:

def prepare_arguments(parser):
    parser.add_argument('--some-argument', action='store_true', default=False)
    return parser

def argument_preprocessor(args):
    extras = {}

    if '-strange-argument' in args:
        extras['strange_argument'] = True

    return args, extras

def main(options):
    print('--some-argument:', options.some_argument)
    print('-strange-argument:', options.strange_argument)
    if options.strange_argument:
        return 1
    return 0

The above example is simply to illustrate the signature of these functions and how they might be used.

Verb Pattern API

API for implementing commands and verbs which used the verb pattern.

osrf_pycommon.cli_utils.verb_pattern.call_prepare_arguments(func, parser, sysargs=None)[source]

Call a prepare_arguments function with the correct number of parameters.

The prepare_arguments function of a verb can either take one parameter, parser, or two parameters parser and args, where args are the current arguments being processed.

  • func (Callabe) – Callable prepare_arguments function.
  • parser (argparse.ArgumentParser) – parser which is always passed to the function
  • sysargs (list) – arguments to optionally pass to the function, if needed

return value of function or the parser if the function returns None.

Return type:



ValueError if a function with the wrong number of parameters is given

osrf_pycommon.cli_utils.verb_pattern.create_subparsers(parser, cmd_name, verbs, group, sysargs, title=None)[source]

Creates argparse subparsers for each verb which can be discovered.

Using the verbs parameter, the availble verbs are iterated through. For each verb a subparser is created for it using the parser parameter. The cmd_name is used to fill the title and description of the add_subparsers function call. The group parameter is used with each verb to load the verb’s description, prepare_arguments function, and the verb’s argument_preprocessors if available. Each verb’s prepare_arguments function is called, allowing them to add arguments. Finally a list of argument_preprocessors functions and verb subparsers are returned, one for each verb.

  • parser (argparse.ArgumentParser) – parser for this command
  • cmd_name (str) – name of the command to which the verbs are being added
  • verbs (list) – list of verbs (by name as a string)
  • group (str) – name of the entry_point group for the verbs
  • sysargs (list) – list of system arguments
  • title (str) – optional custom title for the command

tuple of argument_preprocessors and verb subparsers

Return type:



Return unmodifed args and an empty dict for extras


List verbs available for a given entry_point group.

Parameters:group (str) – entry_point group name for the verbs to list
Returns:list of verb names for the given entry_point group
Return type:list of str
osrf_pycommon.cli_utils.verb_pattern.load_verb_description(verb_name, group)[source]

Load description of a verb in a given group by name.

  • verb_name (str) – name of the verb to load, as a string
  • group (str) – entry_point group name which the verb is in

verb description

Return type:



Split arguments by verb.

Given a list of arguments (list of strings), the verb, the pre verb arguments, and the post verb arugments are returned.

For example:

>>> args = ['--command-arg1', 'verb', '--verb-arg1', '--verb-arg2']
>>> split_arguments_by_verb(args)
('verb', ['--command-arg1'], ['--verb-arg1', '--verb-arg2'])
Parameters:arguments (list) – list of system arguments
Returns:the verb (str), pre verb args (list), and post verb args (list)
Return type:tuple