From http://tech-artists.org/forum/showthread.php?4254-openMaya-contexts&highlight=MPxcontext Open Maya custom context sample code

OM_custom_context.py

# WORKS ONLY IN LEGACY VIEWPORT

import maya.OpenMaya as om
import maya.OpenMayaMPx as mpx
import sys


kPluginCtxName = 'myContext'
kPluginCmdName = 'myContextCmd'


class context(mpx.MPxContext):
    def __init__(self):
        mpx.MPxContext.__init__(self)

    def doPress(self, event):
        sys.stderr.write('It works I hope!!!!!')


class contextCommand(mpx.MPxContextCommand):
    def __init__(self):
        mpx.MPxContextCommand.__init__(self)

    def makeObj(self):
        return mpx.asMPxPtr(context())

    @classmethod
    def creator(cls):
        return mpx.asMPxPtr(cls())


def initializePlugin(mobject):
    mplugin = mpx.MFnPlugin(mobject, "Passerby", "1.0", "Any")
    try:
        mplugin.registerContextCommand(kPluginCtxName, contextCommand.creator)
    except:
        sys.stderr.write("Failed to register context command: %s\n" % kPluginCtxName)
        raise


# Uninitialize the script plug-in
def uninitializePlugin(mobject):
    mplugin = mpx.MFnPlugin(mobject)
    try:
        mplugin.deregisterContextCommand(kPluginCtxName, kPluginCmdName)
    except:
        sys.stderr.write("Failed to deregister context command: %s\n" % kPluginCtxName)
        raise