def assignment_id(df):
    """Resets and drops the current dataframe's
    index and sets it to the assignment_hash

    assignment_id is treated as a string identifier to account
    for hits in details with no assignment or assignment_id
    """
    df = df.reset_index(drop=True)
    df.index.name = 'assignment_id'
    df.index = df.index.astype(str)
    return df
