import maya.cmds as cmds
import maya.mel as mm

#get mesh objects.
meshes = maya.cmds.ls(type="mesh")
for mesh in meshes:
    cmds.select(mesh)


    #check if attribute exists, if not, create.
    if not mm.eval( 'attributeExists "test" "%s"' % mesh):
        cmds.addAttr( shortName='tst', longName='test', dataType="string")