swapSelectedKeyPositions

import maya.cmds as cmds
import selectedAttributes

sel = cmds.ls(selection=True)

if cmds.scaleKey(attribute=True): # Is a key/curve selected?
    keys = cmds.keyframe(sel, query=True, selected=True) or []
    attributes = cmds.keyframe(sel, query=True, selected=True, name=True) or []    
    first_key = min(keys)
    last_key = max(keys)
    mid_pivot = (first_key + last_key) / 2
    for attribute in attributes:
        cmds.scaleKey(attribute,  time=(first_key, last_key), timeScale=-1, timePivot=mid_pivot )
else:
    channel_attributes = selectedAttributes.get()
    list_of_keys = []
    for attribute in channel_attributes:
        list_of_keys.extend(cmds.keyframe(attribute, query=True) or [])
        
    first_key = min(list_of_keys)
    last_key = max(list_of_keys)
    mid_pivot = (first_key + last_key) / 2
    for attribute in channel_attributes:
        cmds.scaleKey(attribute,  time=(first_key, last_key), timeScale=-1, timePivot=mid_pivot )