148 lines
5.4 KiB
ReStructuredText
148 lines
5.4 KiB
ReStructuredText
.. _mach_commands:
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=====================
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Implementing Commands
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=====================
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Mach commands are defined via Python decorators.
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All the relevant decorators are defined in the *mach.decorators* module.
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The important decorators are as follows:
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:py:func:`Command <mach.decorators.Command>`
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A function decorator that denotes that the function should be called when
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the specified command is requested. The decorator takes a command name
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as its first argument and a number of additional arguments to
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configure the behavior of the command. The decorated function must take a
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``command_context`` argument as its first.
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``command_context`` is a properly configured instance of a ``MozbuildObject``
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subclass, meaning it can be used for accessing things like the current config
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and running processes.
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:py:func:`CommandArgument <mach.decorators.CommandArgument>`
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A function decorator that defines an argument to the command. Its
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arguments are essentially proxied to ArgumentParser.add_argument()
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:py:func:`SubCommand <mach.decorators.SubCommand>`
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A function decorator that denotes that the function should be a
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sub-command to an existing ``@Command``. The decorator takes the
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parent command name as its first argument and the sub-command name
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as its second argument.
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``@CommandArgument`` can be used on ``@SubCommand`` instances just
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like they can on ``@Command`` instances.
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Here is a complete example:
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.. rstcheck: ignore-languages=python
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.. code-block:: python
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from mach.decorators import Command, CommandArgument
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@Command('doit', category='testing', description='Do ALL OF THE THINGS.')
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@CommandArgument('--force', '-f', action='store_true',
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help='Force doing it.')
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def doit(command_context, force=False):
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# Do stuff here.
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print("hello world")
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All paths are relative to the root source folder.
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Save your file somewhere e.g. ``testing/doit.py``.
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Add an entry for your command in ``python/mach/mach/command_util.py`` in
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the `MACH_COMMANDS` dictionary. Maintain the alphabetical order.
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e.g
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.. code-block:: python
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MACH_COMMANDS = {
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# ... previous entries here
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"doctor": MachCommandReference("python/mozbuild/mozbuild/mach_commands.py"),
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"doit": MachCommandReference("testing/doit.py"),
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"environment": MachCommandReference("python/mozbuild/mozbuild/mach_commands.py"),
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# ... rest of entries here
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}
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When the module is loaded, the decorators tell mach about all handlers.
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When mach runs, it takes the assembled metadata from these handlers and
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hooks it up to the command line driver. Under the hood, arguments passed
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to the decorators are being used to help mach parse command arguments,
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formulate arguments to the methods, etc. See the documentation in the
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:py:mod:`mach.base` module for more.
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The Python modules defining mach commands do not need to live inside the
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main mach source tree.
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Conditionally Filtering Commands
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================================
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Sometimes it might only make sense to run a command given a certain
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context. For example, running tests only makes sense if the product
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they are testing has been built, and said build is available. To make
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sure a command is only runnable from within a correct context, you can
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define a series of conditions on the
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:py:func:`Command <mach.decorators.Command>` decorator.
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A condition is simply a function that takes an instance of the
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:py:func:`mozbuild.base.MachCommandBase` class as an argument, and
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returns ``True`` or ``False``. If any of the conditions defined on a
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command return ``False``, the command will not be runnable. The
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docstring of a condition function is used in error messages, to explain
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why the command cannot currently be run.
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Here is an example:
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.. rstcheck: ignore-languages=python
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.. code-block:: python
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from mach.decorators import (
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Command,
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)
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def build_available(cls):
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"""The build needs to be available."""
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return cls.build_path is not None
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@Command('run_tests', category='testing', description='A description.' conditions=[build_available])
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def run_tests(command_context):
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# Do stuff here.
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By default all commands without any conditions applied will be runnable,
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but it is possible to change this behaviour by setting
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``require_conditions`` to ``True``:
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.. code-block:: python
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m = mach.main.Mach()
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m.require_conditions = True
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Minimizing Code in Commands
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===========================
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Mach command modules, classes, and methods work best when they are
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minimal dispatchers. The reason is import bloat. Currently, the mach
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core needs to import every Python file potentially containing mach
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commands for every command invocation. If you have dozens of commands or
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commands in modules that import a lot of Python code, these imports
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could slow mach down and waste memory.
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It is thus recommended that mach modules, classes, and methods do as
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little work as possible. Ideally the module should only import from
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the :py:mod:`mach` package. If you need external modules, you should
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import them from within the command method.
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To keep code size small, the body of a command method should be limited
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to:
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1. Obtaining user input (parsing arguments, prompting, etc)
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2. Calling into some other Python package
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3. Formatting output
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Of course, these recommendations can be ignored if you want to risk
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slower performance.
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In the future, the mach driver may cache the dispatching information or
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have it intelligently loaded to facilitate lazy loading.
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