code: def padBitmapData(self, data):
      # Make sure that the data isn't bigger than the fixed size.
      assert len(data) <= self.imageSize, (
          "Data in indexSubTable format %d must be less than the fixed size."
          % self.indexFormat
      )
      # Pad the data so that it matches the fixed size.
      pad = (self.imageSize - len(data)) * b"\0"
      return data + pad
  
imports: from fontTools.misc import sstruct
  from .None import DefaultTable
  from fontTools.misc.textTools import bytesjoin, safeEval
  from .BitmapGlyphMetrics import BigGlyphMetrics, bigGlyphMetricsFormat, SmallGlyphMetrics, smallGlyphMetricsFormat
  import struct
  import itertools
  from collections import deque
  import logging
label_desc: Use image node_id 3 for calling the FixedSizeIndexSubTableMixin obj's underlying member method code with example usage: obj.padBitmapData(data) and returns: unknown