Monkey patching the python cfb library to support overwrite of data.

cfb_monkey_patch.py

from cfb.directory.entry import Entry
from cfb.constants import ENDOFCHAIN


# Monkey patching Entry
def overwrite(self, data):
    """
    Writes `data` to the current directory entry at the current position. If
    `data` is bigger than the available space in the entry, then the rest of
    `data` is ignored. It returns the number of bytes written.
    """
    self.source.seek(self._source_position)

    cursor = 0
    size = min(len(data), self.size - self.tell())
    while cursor < size:
        if self.tell() > self.size:
            break
        if self._sector_number == ENDOFCHAIN:
            break

        to_write = size - cursor
        to_end = self.sector_size - self._position_in_sector
        to_do = min(to_write, to_end)
        self.stream.write(data[cursor:cursor+to_do])

        cursor += to_do
        self._position += to_do
        self._source_position = self.source.tell()

        if to_write >= to_end:
            self._position_in_sector = 0

            self._sector_number = self.next_sector(self._sector_number)
            position = (self._sector_number + int(not self._is_mini)) \
                << self.sector_shift
            self.stream.seek(position)
        else:
            self._position_in_sector += to_do

    return cursor

Entry.write = overwrite