def get_train_na_percentages(train):
    """
  Return a Series with the percentage of Na values per columns in train dataframe.
  Must be called just after impute_train_missing_data().

  Keyword arguments:
  train -- the train dataframe
  """
    na_cols_pctg_train = train[train.columns[train.isna().sum() > 0]].isna().sum() / train.shape[0]
    return na_cols_pctg_train