gistfile1.py

import subprocess
from StringIO import StringIO

class XmpFile(object):

    def __init__(self, path, flags, *mode):
        # Store the path as an instance property for later
        self.path = "." + path
        self.file = os.fdopen(open(self.path, flags, *mode),
                              flag2mode(flags))
        self.fd = self.file.fileno()

    def _find_patch(self):
        # Tries to find a .rdiff file that exists next to self.file
        # Returns the filename if one exists, otherwise returns None
        if os.path.exists(self.path + ".rdiff"):
            return self.path + ".rdiff"
        return None

    def read(self, length, offset):
        patchfile = self._find_patch()
        fileobj = self.file

        # If a patchfile exists, apply it with rdiff and set the fileobj to the stdout
        if patchfile:
            stdout = subprocess.check_output(['rdiff', self.path, patchfile])
            fileobj = StringIO(stdout)

        fileobj.seek(offset)
        return fileobj.read(length)