def train_test_split(df, test_start='2015-04'):
    """Splits dataset approx 75% train 25% test by making everything issued on
    or after 2015-04 a test set row.
    """
    is_in_test_set = df['issue_d'] >= test_start
    df_train, df_test = df[~is_in_test_set].copy(), df[is_in_test_set].copy()
    return df_train, df_test