Closest position on mesh to point...

position_on_mesh.py

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

mpoint_to_point = lambda mpoint: [float(p) for p in mpoint][:3]
loc_at_mpoint = lambda point: mc.xform(mc.spaceLocator(), t=point, ws=True)
dag_iterator = om.MItDag(om.MItDag.kDepthFirst, om.MFn.kInvalid)

def get_dag_path(xform):
    selectionList = om.MSelectionList()
    try:
        selectionList.add(xform)
    except:
        return None
    return selectionList.getDagPath(0)
    
# if we're taking a cube that was just created at origin
dag_path = get_dag_path('pCubeShape1')

shape_mfnMesh = om.MFnMesh(dag_path)
test_point = om.MPoint(2.0, 0.0, 0.0)
uvs = list(shape_mfnMesh.getUVAtPoint(test_point, space=om.MSpace.kWorld, uvSet='map1'))
face_id = uvs.pop()
loc_at_mpoint(mpoint_to_point(test_point))

dag_path.fullPathName()
mesh_polygon = om.MItMeshPolygon(dag_path)

while not mesh_polygon.isDone():
    if mesh_polygon.index() == face_id:
        point = mesh_polygon.getPointAtUV(uvs, om.MSpace.kWorld)
        loc_at_mpoint(mpoint_to_point(point))
    mesh_polygon.next(None)