closest point

import maya.api.OpenMaya as om2
import maya.cmds as mc

def dagObjFromName(name):
    sel = om2.MSelectionList()
    sel.add(name)

    dag =sel.getDagPath(0)

    mobj = sel.getDependNode(0)

    return mobj, dag


def locatorToPoint(locator):
    pos = mc.xform(locator, q=1, rp=1, ws=1)
    return om2.MPoint(pos[0], pos[1], pos[2],1.0)

def createLocatorAtClosestPoint(mySphere, myLocator):
    obj,dag = dagObjFromName(mySphere)
    meshFn = om2.MFnMesh(dag)

    point = locatorToPoint(myLocator)

    resultPoint = meshFn.getClosestPoint(point,om2.MSpace.kWorld)[0]

    result = mc.spaceLocator()[0]

    mc.setAttr(result+'.t', resultPoint[0], resultPoint[1], resultPoint[2])


mySphere = mc.polySphere()[0]
myLocator = mc.spaceLocator()[0]
# now move locator arround and execute
createLocatorAtClosestPoint(mySphere, myLocator)