Stage and commit local file updates

updatefile.py

import os
import sys
import stat
import shutil
import tempfile
import logging as log

class UpdateFile(object):
    """Update a file atomically

    Iterate over this object to read lines from current file and then use
    .write() to make updates to a new copy of the file. These are not being
    done to the file directly but instead to a temporary file. To finalize
    the update to filename, use .commit() """

    def __init__(self, filename):
        self.filename = filename
        self.committed = False
        try:
            self.__fh = open(filename, 'rb')
            self.__tmp = tempfile.NamedTemporaryFile(delete=False,
                                                     dir=os.path.dirname(filename),
                                                     prefix='.%s-' % os.path.basename(filename))
            # Fix up permissions and ownership upfront
            shutil.copymode(self.filename, self.__tmp.name)
            s = os.stat(filename)
            os.chown(self.__tmp.name, s.st_uid, s.st_gid)
        except (IOError, OSError), e:
            log.error('Problem setting up tempfile contents for %s: %s',
                      filename, str(e))
            raise

    def __del__(self):
        self.__tmp.close()
        self.__fh.close()

    def __repr__(self):
        if self.committed:
            return '<UpdateFile %s (CLOSED) wrote %d bytes>' % \
                (self.filename, self.__tmp.tell())
        else:
            return '<UpdateFile %s bytes staged=%d>' % \
                (self.filename, self.__tmp.tell())

    def __iter__(self):
        # Makes this object act like other opened files
        return self.__fh

    def write(self, data):
        """Make updates to the temporary file before committing the final"""
        if not self.committed:
            self.__tmp.write(data)

    def commit(self):
        """Commit the data written to tempfile to original file"""
        if not self.committed:
            try:
                # sync and close file handles
                os.fdatasync(self.__tmp.file.fileno())
                self.__tmp.close()
                self.__fh.close()
                # Actual copy is a rename
                os.rename(self.__tmp.name, self.filename)
                self.committed = True
            except (IOError, OSError), e:
                log.error('Problem renaming tempfile into %s: %s',
                          self.filename, str(e))
                raise