Snippet to removeUnusedInfluences in Autodesk Maya using Python.

removeUnusedInfluences.py

import maya.cmds as cmds

'''
# EXAMPLE USAGES:
# Removes all unused influences from skinCluster1
remove_unused_influences('skinCluster1')

# Removes the two specified joints from Body_SkinCluster,
but only if they are not currently weighted to anything.
remove_unused_influences('Body_SkinCluster', ['leg_L0_5_jnt', 'leg_R0_5_jnt'])
'''

def remove_unused_influences(skinCls, targetInfluences=[]):
    '''
    Snippet to removeUnusedInfluences in Autodesk Maya using Python.
    The MEL version runs slowly, over every influence one at a time.
    "targetInfluences" allows you to directly specify which influences to remove.
    This will only remove targets which are not currently being used.
    '''
    allInfluences = cmds.skinCluster(skinCls, q=True, inf=True)
    weightedInfluences = cmds.skinCluster(skinCls, q=True, wi=True)
    unusedInfluences = [inf for inf in allInfluences if inf not in weightedInfluences]
    if targetInfluences:
        unusedInfluences = [
                inf for inf in allInfluences
                if inf in targetInfluences
                if inf not in weightedInfluences
                ]
    cmds.skinCluster(skinCls, e=True, removeInfluence=unusedInfluences)

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