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  7. cros/

Chromite CLI Framework

This directory contains the core CLI system.

Adding a new command

New commands can be added by creating a new file in cros/ that follows the cros_{command_name}.py format where the command can live.

The command class's implementation must:

  • Be a subclass of cli.command.Command
  • Have a @command.CommandDecorator('command-name') decorator on the class
  • Define an EPILOG class constant
  • Implement the @classmethod AddParser(cls, parser)
  • Implement Run(self)

Rules of thumb:

  • The Run method should not be a full implementation of the command.
    • It should validate the raw arguments.
    • It should translate arguments to chromite constructs as applicable.
    • It should then call a reusable implementation (e.g. in service/).
    • Otherwise, it may be a wrapper around a command outside of chromite.

Argument Parser

Chromite has a subclass of argparse.ArgumentParser in the lib.commandline module. The ArgumentParser subclass adds some common functionality, and a set of standard arguments that all scripts can take advantage of.

Custom Argument Types

There are a number of custom types defined for the ArgumentParser, which can be used with type='custom_type_name'.

  • ab_url: Parse android build urls.
  • bool: Parse standard shell boolean values (y/yes, n/no, 1, 0, true, false).
  • build_target: Does a regex validation of the name and produces a BuildTarget instance.
  • date: Parse the argument as a Y-m-d formatted date.
  • path: Expands ~/ paths and then standardizes to the real path.
  • gs_path: Processes all known GS urls and provides the equivalent gs:// url.
  • local_or_gs_path: Processes the argument as a ‘path’ or a ‘gs_path’, as needed.
  • path_or_uri: Like local_or_gs_path, but also parse other uri formats.

Argument Deprecation Functionality

The ArgumentParser also provides support for the deprecated argument in the ArgumentParser.add_argument method. This argument allows marking an argument as deprecated by printing a warning message, but still processes the argument as it normally would. Using the --foo argument below would produce something like the following log message.

parser.add_argument('--foo', type=int, deprecated='Use --bar instead!')
Argument --foo is deprecated: Use --bar instead!

Defined Arguments

  • --log-level: The minimum logging level (default: notice).
  • --log-format: Change log line format.
  • -v/--verbose: Sets the verbose option to true and sets the log-level to info.
  • --debug: Sets verbose and debug options to true and sets the log-level to debug.
  • --nocolor: Disable log coloring.
  • --cache-dir: Override the cache directory when caching is enabled in the parser.

Argument Standards

There are only a few style rules on top of the standard rules imposed by the python ArgumentParser. Dashes should be preferred over underscores, e.g. --argument-name, not --argument_name. If there is historical context for --argument_name, it may be included, but --argument-name must also be included.

For negative flag arguments, use --no-foo instead of --nofoo, and store it to foo. e.g.

parser.add_argument('--no-foo', dest='foo', action='store_false', default=True)

Standard Arguments

Generally scripts are welcome to use any arguments they need. However, in the interest of standardizing the tools, some arguments have been defined as being reserved for specific usages. Not all the arguments will always apply, but when they are used, this is the form they must have.

-b--build-targetBuild Target
-n--dry-runDry Run

Build Target

This is the new version of the --board option that is in scripts pre-dating the standards. The build target option comes with parsing support in the form of the build_target type. The build target type will validate the name and construct a BuildTarget instance to assign to the variable rather than just storing the string.

parser.add_argument('-b', '--build-target', type='build_target')


The device argument is a long-standing type that just did not always use the same argument naming conventions. The device parser supports USB, File, SSH, and Servo. A specific device argument can accept any or all of the supported types. The example below supports USB and File, but not SSH or Servo.

    '-d', '--device',

Dry Run

A dry-run of a script should log information about steps it would be taking without executing operations that mutate anything. Implementing a dry-run for your script is very strongly recommended. A dry-run allows developers to more quickly experiment with and verify their usage and understanding of your script, and empowers them to make mistakes without worrying about consequences.

There are tentative plans to implement the dry-run argument as a default on our ArgumentParser subclass, making it available for all the commands. If done, any existing inclusions will be converted, so until then please feel free to add it to your script.



The force argument should only be used when there's an operation to “force” to complete that would otherwise abort, e.g. deleting an existing X, if it exists, to create a new X. If the operation would prompt the user, then --yes is the correct option.

parser.add_argument('-f', '--force', action='store_true', default=False)


This argument is used to specify maximum parallelism for concurrent tasks. The default should be unlimited when feasible, or the machine's CPU count when not. If unsure, lean towards unlimited and adjust if/when it is a problem. In particular, this helps avoid chained calls from unexpectedly being limited.

parser.add_argument('-j', '--jobs', type=int, default=0)


The package(s) argument should be a positional argument taking one or more packages as appropriate. The requirements for what must be included in the specified package(s) are dependent on the script itself.

parser.add_argument('packages', nargs='+')


The yes option is a pretty common argument to allow the user to skip manually confirming prompts and instead assume the user confirmed each case.

parser.add_argument('-y', '--yes', action='store_true', default=False)