Added command.py

Module is responsible for allowing the creation, registering and
initilisation of soypak commands.

Added class@RegisterCommand:
	- used by command classes as a metaclass. Will be auto-invoked upon
	  the class' declaration.
	- checks if a command is registered already with an existing name.
	- uses the class' `__doc__` attribute to register an entry on the
	  help display message with name and summary.

Added class@Command:
	- Inherited by command classes.
	- Keeps record of the registered commands and the instance of the
	  class which can then be invoked during `get_command()`.
	- Command@get_command (static method): find and return an
	  initialised instance of a command that has been registered.
	  Otherwise return `None`.
	- Command@run (virtual): to be implemented on a child class of
	  `Command`.

soypak/cli/command.py

@@ -0,0 +1,58 @@
+import argparse
+import sys
+import soypak.cli.parser as parser
+
+
+class RegisterCommand(type):
+    """Register a new command to soypak and add help information to the parser.
+
+    This class is to be used as metaclass for commands.
+    """
+    def __init__(cls, name, bases, _dict):
+        super().__init__(name, bases, _dict)
+        # get the name of the command
+        name = getattr(cls, "__cmd_name__", None)
+        if name is None:
+            raise Exception("Command has no registered name.")
+
+        # check if the command already exists (means we have duplicate registered name)
+        if name in Command.registered_cmds:
+            raise Exception("Duplicate command, already exists.")
+
+        # add the command to the registered list, with `name` and class object
+        Command.registered_cmds[name] = cls
+        # fetch the documentation for the command
+        _help = cls.__doc__
+        if not _help:
+            raise Exception("Command does not provide a __doc__ attribute for usage.")
+        # add the command to the argparse._action_groups list (will appear as a positional in help message)
+        parser.SoypakParser.add_action_command(cmd=name, help=_help.split('\n')[0])
+
+
+class Command:
+    """Class for creating a new type of command. Keep record of the registered commands for soypak."""
+    # store all registered commands. Those are commands which soypak implements
+    registered_cmds: dict[str, RegisterCommand] = {}
+
+    def __init__(self) -> None:
+        # get/create the parser instance
+        self.parser = parser.SoypakParser()
+
+
+    @staticmethod
+    def get_command(name, *, args: list[str]) -> RegisterCommand | None:
+        """Fetch and initialise a registered command, or return `None` is command cannot be found."""
+        return (
+            Command.registered_cmds[name](data=parser.SoypakParser().namespace, args=args) if name in
+            Command.registered_cmds else None
+        )
+
+
+    def die(self):
+        """System exit with status code 0"""
+        sys.exit(0)
+
+
+    def run(self):
+        """Virtual method which must be overridden by a new implementation in the child class."""
+        raise RuntimeError("Virtual method must be overridden by child class.")