Custom command inside maya. Similarly custom nodes can be created. Part of ADN.

custom_command.py

import maya.OpenMayaMPx as ommpx
import sys


class Sample(ommpx.MPxCommand):
    kPluginCmdName = "foo"

    def __init__(self):
        ommpx.MPxCommand.__init__(self)

    @classmethod
    def cmdCreator(cls):
        return ommpx.asMPxPtr(cls())

    def doIt(self, argList):
        print("Hello World")


def initializePlugin(mobject):
    mplugin = ommpx.MFnPlugin(mobject)
    try:
        mplugin.registerCommand(Sample.kPluginCmdName, Sample.cmdCreator)
    except:
        sys.stderr.write("Failed to register command: {}".format(Sample.kPluginCmdName))
        raise


def uninitializePlugin(mobject):
    mplugin = ommpx.MFnPlugin(mobject)
    try:
        mplugin.deregisterCommand(Sample.kPluginCmdName)
    except:
        sys.stderr.write(
            "Failed to unregister command: {}".format(Sample.kPluginCmdName)
        )
        raise