Is the tab key hard-wired?

I have a script that needs to run some special update code every time I enter or leave edit mode. To this end I created a custom editmode toggle button, which I tied to the spacebar. Works as expected: every time I hit spacebar, Blender toggles edit mode and writes a message in the terminal.

However, I would really like to overwrite the behavior of the tab button. I would expect that all that is needed is to replace ‘SPACE’ by ‘TAB’ in the script below, but it does not work. When I hit tab, the built-in editmode toggle is run, and not my custom script. Is this a but, or the intended behaviour?

import bpy

class MH_OT_editmode_toggle(bpy.types.Operator):
    bl_idname = "mh.editmode_toggle"
    bl_label = "Editmode toggle"

    def execute(self, context):
        ob = context.object
        if not (ob and ob.type == 'MESH'):
            print("No mesh")
        elif ob.mode == 'EDIT':
            print("Object mode set")
            print("Edit mode set")

def register():
    km =['3D View']"mh.editmode_toggle", 'SPACE', 'PRESS')

def unregister():

if __name__ == "__main__":