def extract_n_element(df, column_name, n = 0):
  
  """
  Extract nth element in every row of a column-list
  
  df: pd.DataFrame
  column_name: name of the column-list
  n: int position of element to extract
  
  return: df with unnested-list column
  """
  
  dfcopy = df
  dfcopy[column_name] = [val[n] if val else None for val in dfcopy[column_name]]
  
  return dfcopy