class_docstr: Class for formatting a DataFrame to a list of ExcelCells,
  
  Parameters
  ----------
  df : DataFrame or Styler
  na_rep: na representation
  float_format : string, default None
          Format string for floating point numbers
  cols : sequence, optional
      Columns to write
  header : boolean or list of string, default True
      Write out column names. If a list of string is given it is
      assumed to be aliases for the column names
  index : boolean, default True
      output row names (index)
  index_label : string or sequence, default None
          Column label for index column(s) if desired. If None is given, and
          `header` and `index` are True, then the index names are used. A
          sequence should be given if the DataFrame uses MultiIndex.
  merge_cells : boolean, default False
          Format MultiIndex and Hierarchical Rows as merged cells.
  inf_rep : string, default `'inf'`
      representation for np.inf values (which aren't representable in Excel)
      A `'-'` sign will be added in front of -inf.
  style_converter : callable, optional
      This translates Styler styles (CSS) into ExcelWriter styles.
      Defaults to ``CSSToExcelConverter()``.
      It should have signature css_declarations string -> excel style.
      This is only called for body cells.
code: def _format_header(self):
      if isinstance(self.columns, ABCMultiIndex):
          gen = self._format_header_mi()
      else:
          gen = self._format_header_regular()
  
      gen2 = ()
      if self.df.index.names:
          row = [
              x if x is not None else "" for x in self.df.index.names
          ] + [""] * len(self.columns)
          if reduce(lambda x, y: x and y, map(lambda x: x != "", row)):
              gen2 = (
                  ExcelCell(
                      self.rowcounter, colindex, val, self.header_style
                  )
                  for colindex, val in enumerate(row)
              )
              self.rowcounter += 1
      return itertools.chain(gen, gen2)
  
imports: import itertools
  import re
  import warnings
  import numpy
  from pandas.compat import reduce
  from pandas.core.dtypes import missing
  from pandas.core.dtypes.common import is_float, is_scalar
  from pandas.core.dtypes.generic import ABCMultiIndex, ABCPeriodIndex
  from pandas import Index
  import pandas.core.common
  from pandas.io.formats.css import CSSResolver, CSSWarning
  from pandas.io.formats.format import get_level_lengths
  from pandas.io.formats.printing import pprint_thing
label_desc: Use image node_id 6 for calling the ExcelFormatter obj's underlying member method code with example usage: obj._format_header() and returns: itertools