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df.columns = ['f_' + i if ix < 15 else i for ix, i in enumerate(df.columns)]
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df.columns = ['f_' + i if ix < 15 else i for ix, i in enumerate(df.columns)]
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df.columns = ['f_' + i if ix < 15 else i for ix, i in enumerate(df.columns)]
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s.add_prefix('item_')
item_0 1
item_1 2
item_2 3
item_3 4
dtype: int64