old_contents: """ Dataframe manipulation utilities.
  """
  import pandas as pd
  
  
  def unstack_and_build_index(df):
      """ Convert a dataframe with years along index and months along columns and
      build a correctly index timeseries, with monthly periods.
  
      Parameters
      ----------
      df : pandas.DataFrame
          DataFrame of the form
                Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
          1980    0   1   2   3   4   5   6   7   8   9  10  11
          1981   12  13  14  15  16  17  18  19  20  21  22  23
  
      Returns
      -------
      df : pandas.Series
          Series index with a monthly period of the form
  
  
      """
      series_out = df.transpose().unstack()
      series_out.name = "Values"
      series_out.index.names = ["Year", "Month"]
      df_out = series_out.reset_index()
      df_out.Year = df_out.Year.astype("str")
      # Temporarily set it to the first of the month since to_datetime fails for
      # Feb otherwise
      df_out["str_index"] = "01/" + df_out.Month + "/" + df_out.Year
      df_out.index = df_out["str_index"].apply(pd.to_datetime)
      df_out.index = df_out.index.to_period()
      df_out.index.name = "Date"
      return df_out["Values"]
  
new_contents: """ Dataframe manipulation utilities.
  """
  import pandas as pd
  
  
  def unstack_and_build_index(df):
      """ Convert a dataframe with years along index and months along columns and
      build a correctly index timeseries, with monthly periods.
  
      Parameters
      ----------
      df : pandas.DataFrame
          DataFrame of the form:
                Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
          1980    0   1   2   3   4   5   6   7   8   9  10  11
          1981   12  13  14  15  16  17  18  19  20  21  22  23
  
      Returns
      -------
      df : pandas.Series
          Series index with a monthly period of the form:
          1980-01 0
          1980-02 1
          ...
      """
      series_out = df.transpose().unstack()
      series_out.name = "Values"
      series_out.index.names = ["Year", "Month"]
      df_out = series_out.reset_index()
      df_out.Year = df_out.Year.astype("str")
      # Temporarily set it to the first of the month since to_datetime fails for
      # Feb otherwise
      df_out["str_index"] = "01/" + df_out.Month + "/" + df_out.Year
      df_out.index = df_out["str_index"].apply(pd.to_datetime)
      df_out.index = df_out.index.to_period()
      df_out.index.name = "Date"
      return df_out["Values"]
  
subject: Fix a docstring