def write_file (filename, content, backup=False, callback=None):
    """Overwrite a possibly existing file with new content. Do this
    in a manner that does not leave truncated or broken files behind.
    @param filename: name of file to write
    @type filename: string
    @param content: file content to write
    @type content: string
    @param backup: if backup file should be left
    @type backup: bool
    @param callback: non-default storage function
    @type callback: None or function taking two parameters (fileobj, content)
    """
    # first write in a temp file
    f = file(filename+".tmp", 'wb')
    if callback is None:
        f.write(content)
    else:
        callback(f, content)
    f.close()
    # move orig file to backup
    if os.path.exists(filename):
        os.rename(filename, filename+".bak")
    # move temp file to orig
    os.rename(filename+".tmp", filename)
    # remove backup
    if not backup and os.path.exists(filename+".bak"):
        os.remove(filename+".bak")
Overwrite a possibly existing file with new content. Do this
    in a manner that does not leave truncated or broken files behind.
    @param filename: name of file to write
    @type filename: string
    @param content: file content to write
    @type content: string
    @param backup: if backup file should be left
    @type backup: bool
    @param callback: non-default storage function
    @type callback: None or function taking two parameters (fileobj, content)