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_regular(self):
      has_aliases = isinstance(
          self.header, (tuple, list, np.ndarray, Index)
      )
      if has_aliases or self.header:
          coloffset = 0
  
          if self.index:
              coloffset = 1
              if isinstance(self.df.index, ABCMultiIndex):
                  coloffset = len(self.df.index[0])
  
          colnames = self.columns
          if has_aliases:
              if len(self.header) != len(self.columns):
                  raise ValueError(
                      "Writing {cols} cols but got {alias} "
                      "aliases".format(
                          cols=len(self.columns), alias=len(self.header)
                      )
                  )
              else:
                  colnames = self.header
  
          for colindex, colname in enumerate(colnames):
              yield ExcelCell(
                  self.rowcounter,
                  colindex + coloffset,
                  colname,
                  self.header_style,
              )
  
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 5 for calling the ExcelFormatter obj's underlying member method code with example usage: obj._format_header_regular() without return types