1行コードを分解するサンプルたくさん。

class Acla:
    
    def unite_selection(self):
        #selected = [x for x in cmds.ls(sl=True) if cmds.listRelatives(x, type="mesh")]
        selected=[]
        for x in cmds.ls(sl=True):
            xx= cmds.listRelatives(x, type="mesh")
            if xx:
                selected.append(xx)
        
        
        if not selected:
            print("not selected")
            return
        #invalid_meshes = [x for x in cmds.listRelatives(selected, type="mesh", f=True) if not cmds.listConnections(x)]
        
        invalid_meshes = []
        for x in cmds.listRelatives(selected, type="mesh", f=True):
            xx= cmds.listConnections(x)
            if not xx:
                invalid_meshes.append(xx)
        
        if invalid_meshes:
            print("invalid", invalid_meshes)
            return
        res = cmds.polyUnite(*cmds.ls(sl=True), ch=0, op=True, muv=1)
        print(res)

ClassIns=Acla()
ClassIns.unite_selection()