Range Limited File Class

rangefile.py

import os
import sys

try:
  file
except NameError:
  import io
  file = io.FileIO

class RangeFile(file):
  _limitstart = None
  _limitend = None
  def __init__(self, *args, **kwargs):
    file.__init__(self, *args, **kwargs)
    if sys.platform == 'win32' and self.mode.find('b') == -1:
      raise Exception('RangeFile requires binary mode(mode=rb) for win32')

  def fileno(self): # trick the os.fstat?
    return -1

  def limitrange(self, startpos, endpos):
    if not (isinstance(startpos, int) and isinstance(endpos, int)):
      raise ValueError('Wrong Pos')
    if startpos > endpos:
      raise ValueError('Wrong Range')
    if startpos < 0:
      raise ValueError('Wrong Start Range')
    file.seek(self, 0, os.SEEK_END)
    filelen = file.tell(self)
    if endpos >= filelen:
      raise Exception('Wrong End Range')
    self._limitstart = startpos
    self._limitend = endpos
    file.seek(self, startpos)
    return self # make RangeFile('file').limitrange(0,100).read() possible :)

  def tell(self):
    if self._limitstart is None:
      return file.tell(self)
    return file.tell(self) - self._limitstart

  def read(self, size=-1):
    if self._limitstart is None:
      return file.read(self, size)
    currpos = file.tell(self)
    if size < 0:
      size = self._limitend - currpos + 1
    else:
      size = min(self._limitend - currpos + 1, size)
    return file.read(self, size)

  def seek(self, offset, whence=0):
    if self._limitstart is None:
      return file.seek(self, offset, whence)
    if whence == 0: # absolute, from beginning
      if offset < 0: raise ValueError('no negative allowed')
      if self._limitstart + offset <= self._limitend:
        return file.seek(self, self._limitstart + offset)
      else:
        raise Exception('Range Exceeded')
    if whence == 2: # relative to the end
      if offset > 0: raise ValueError('no positive allowed')
      if self._limitend + offset >= self._limitstart:
        return file.seek(self, self._limitend + offset)
      else:
        raise Exception('Range Exceeded')
    if whence != 1: raise Exception('relative to current expected')
    pos = file.tell(self)
    r = file.seek(self, offset, whence)
    currpos = file.tell(self)
    if currpos >= self._limitstart and currpos <= self._limitend:
      return r
    else:
      file.seek(self, pos)
      raise Exception('Range Exceeded')