Python adding a list to a slice of another list

I have this code that's not working:

    self.lib_tree.item(song)['values'][select_values] = adj_list
    self.lib_tree.item(album)['values'][select_values] += adj_list
    self.lib_tree.item(artist)['values'][select_values] += adj_list

The full code is this:

def toggle_select(self, song, album, artist):

    # 'values' 0=Access, 1=Size, 2=Selected Size, 3=StatTime, 4=StatSize,
    #          5=Count, 6=Seconds, 7=SelSize, 8=SelCount, 9=SelSeconds
    # Set slice to StatSize, Count, Seconds
    total_values = slice(4, 7)       # start at index, stop before index
    select_values = slice(7, 10)     # start at index, stop before index

    tags = self.lib_tree.item(song)['tags']
    if "songsel" in tags:
        # We will toggle off and subtract from selected parent totals
        tags.remove("songsel")
        self.lib_tree.item(song, tags=(tags))
        # Get StatSize, Count and Seconds
        adj_list = [element * -1 for element in \
                    self.lib_tree.item(song)['values'][total_values]]
    else:    
        tags.append("songsel")
        self.lib_tree.item(song, tags=(tags))
        # Get StatSize, Count and Seconds
        adj_list = self.lib_tree.item(song)['values'][total_values]  # 1 past

    self.lib_tree.item(song)['values'][select_values] = adj_list
    self.lib_tree.item(album)['values'][select_values] += adj_list
    self.lib_tree.item(artist)['values'][select_values] += adj_list
    if self.debug_toggle < 10:
        self.debug_toggle += 1
        print('artist,album,song:',self.lib_tree.item(artist, 'text'), \
                                   self.lib_tree.item(album, 'text'), \
                                   self.lib_tree.item(song, 'text'))
        print('adj_list:',adj_list)

The adj_list has the correct values showing up in debug.

How do I add a list of values to the slice of a list?

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