Implementing Commands

Mach commands are defined via Python decorators.

All the relevant decorators are defined in the mach.decorators module. The important decorators are as follows:


A function decorator that denotes that the function should be called when the specified command is requested. The decorator takes a command name as its first argument and a number of additional arguments to configure the behavior of the command. The decorated function must take a command_context argument as its first. command_context is a properly configured instance of a MozbuildObject subclass, meaning it can be used for accessing things like the current config and running processes.


A function decorator that defines an argument to the command. Its arguments are essentially proxied to ArgumentParser.add_argument()


A function decorator that denotes that the function should be a sub-command to an existing @Command. The decorator takes the parent command name as its first argument and the sub-command name as its second argument.

@CommandArgument can be used on @SubCommand instances just like they can on @Command instances.

Here is a complete example:

from mach.decorators import (

 @Command('doit', help='Do ALL OF THE THINGS.')
 @CommandArgument('--force', '-f', action='store_true',
     help='Force doing it.')
 def doit(command_context, force=False):
     # Do stuff here.

When the module is loaded, the decorators tell mach about all handlers. When mach runs, it takes the assembled metadata from these handlers and hooks it up to the command line driver. Under the hood, arguments passed to the decorators are being used to help mach parse command arguments, formulate arguments to the methods, etc. See the documentation in the mach.base module for more.

The Python modules defining mach commands do not need to live inside the main mach source tree.

Conditionally Filtering Commands

Sometimes it might only make sense to run a command given a certain context. For example, running tests only makes sense if the product they are testing has been built, and said build is available. To make sure a command is only runnable from within a correct context, you can define a series of conditions on the Command decorator.

A condition is simply a function that takes an instance of the mozbuild.base.MachCommandBase() class as an argument, and returns True or False. If any of the conditions defined on a command return False, the command will not be runnable. The docstring of a condition function is used in error messages, to explain why the command cannot currently be run.

Here is an example:

from mach.decorators import (

def build_available(cls):
    """The build needs to be available."""
    return cls.build_path is not None

@Command('run_tests', conditions=[build_available])
def run_tests(command_context):
    # Do stuff here.

By default all commands without any conditions applied will be runnable, but it is possible to change this behaviour by setting require_conditions to True:

m = mach.main.Mach()
m.require_conditions = True

Minimizing Code in Commands

Mach command modules, classes, and methods work best when they are minimal dispatchers. The reason is import bloat. Currently, the mach core needs to import every Python file potentially containing mach commands for every command invocation. If you have dozens of commands or commands in modules that import a lot of Python code, these imports could slow mach down and waste memory.

It is thus recommended that mach modules, classes, and methods do as little work as possible. Ideally the module should only import from the mach package. If you need external modules, you should import them from within the command method.

To keep code size small, the body of a command method should be limited to:

  1. Obtaining user input (parsing arguments, prompting, etc)

  2. Calling into some other Python package

  3. Formatting output

Of course, these recommendations can be ignored if you want to risk slower performance.

In the future, the mach driver may cache the dispatching information or have it intelligently loaded to facilitate lazy loading.