Creating new CLI Modules

Modules are the interface to the MusicDB API for the command line tool musicdb. The following sections explain how to build a module.

Loading of Modules

When musicdb gets called, all python modules in the module directory gets loaded and the MDBM_CreateArgumentParser method gets called. So, the command line argument parser can handle the parameter given to MusicDB.py. After some further initialization the module gets called. This is done by creating an instance of the class and calling the MDBM_Main method. The __init__ method gets the configuration and database loaded by musicdb.

The process can be seen as the following (simplified):

# Load Module
modulename = "testmodul.py"
modulspath = "./mods"
modfp, modpath, moddesc = imp.find_module(modulename, [modulespath])
module     = imp.load_module(modulename, modfp, modpath, moddesc)
modclass   = getattr(module, modulename)

# Extend MusicDB argument parser set
parserset  = argparser.add_subparsers(title="Modules", metavar="module", help="module help")
modclass.MDBM_CreateArgumentParser(parserset, modulename)

# Initialize MusicDB
args     = argparser.parse_args()
config   = MusicDBConfig(args.configpath)
database = MusicDatabase(args.databasepath)

# Execute module
modclass = getattr(modules[modulename], modulename)
modobj   = modclass(config, database)
exitcode = modobj.MDBM_Main(args)
exit(exitcode)

Creation of new Modules

Modules are a class with the same man as the file. The class is derived from the MDBModule class as shown in the following class diagram. Modules are regular python files stored in the MusicDB module directory (/mod) Each module class must have a static method MDBM_CreateArgumentParser(parserset, modulename) that returns a sub-argument-parser.

The argument parser module argparse from Python is used for this. Furthermore a main method called MDBM_Main(self, args) is needed. This method implements the main function of the CLI mod and gets called if the command is given to musicdb Last, the __init__ method gets a MusicDB Configuration object and a MusicDB Database object.

The following example shows a template of a module:

import argparse
from musicdb.lib.modapi         import MDBModule
from musicdb.lib.cfg.musicdb    import MusicDBConfig
from musicdb.lib.db.musicdb     import MusicDatabase

class module(MDBModule):
    def __init__(self, config, database):
        MDBModule.__init__(self)


    @staticmethod
    def MDBM_CreateArgumentParser(parserset, modulename):
        parser = parserset.add_parser(modulename, help="This is an example module")
        parser.set_defaults(module=modulename)


    # return exit-code
    def MDBM_Main(self, args):

        # execute command
        try:
            print("Hello from MusicDB")
            return 0
        except:
            return 1

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Its base class is as simple as the following:

import argparse

class MDBModule(object):

    @staticmethod
    def MDBM_CreateArgumentParser(parserset, modulename):
        parser = parserset.add_parser(modulename, help="FATAL: MISSING IMPLEMENTATION MDBM_CreateArgumentParser(parserset, modulename)")
        parser.set_defaults(module=modulename)

    # return exit-code
    def MDBM_Main(self, args):
        print("FATAL: MISSING IMPLEMENTATION MDBM_Main(self, args)")
        return 1

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