def leak_fix_shift(df, cols, state_col):
    # Fixes leaky data by shifting information back a year so the previous
    # year's data can be used for prediction.
    df = df.copy()
    states = df[state_col].value_counts().keys()
    for col in cols:
        for state in states:
            df.loc[df[state_col]==state, col] = df.loc[df[state_col]==state, col].shift(periods=1)
    return df