import bmesh
import bpy


def stretchAxis(mesh, idx, tgt_min, tgt_max):
    """ Squish/stretch the vertex coordinates of a mesh so that their .co[idx] ranges from tgt_min to tgt_max. """
    bm = bmesh.new()
    bm.from_mesh(mesh)
    bm.verts.ensure_lookup_table()
    
    min = bm.verts[0].co[idx]
    max = min
    
    for v in bm.verts:
        val = v.co[idx]
        if (val<min):
            min = val
        if (val > max):
            max = val
            
    # remap co[idx] from min..max to tgt_min..tgt_max
    for v in bm.verts:
        old = v.co[idx]
        v.co[idx] = (old-min)*(tgt_max-tgt_min) / (max-min) +tgt_min
    bm.to_mesh(mesh)
    bm.free()

#
# squish or stretch all the selected meshes to a specific thickness along a
# specific axis.
#

idx = 2 # the z coordinate
for obj in bpy.context.scene.objects:
    if (obj.select):
        stretchAxis(obj.data, idx, 0, 1)
        obj.location[idx] = 1
        obj.scale[idx] = 1