Generate a diff between self.local_settings and the example file.

def gendiff(self, force=False):
        """Generate a diff between self.local_settings and the example file.

        """

        with DirContext(self.local_settings_dir) as dircontext:
            if not os.path.exists(self.local_settings_diff) or force:
                with open(self.local_settings_example, 'r') as fp:
                    example_lines = fp.readlines()
                with open(self.local_settings_file, 'r') as fp:
                    local_settings_lines = fp.readlines()
                local_settings_example_mtime = time.strftime(
                    self.time_fmt,
                    time.localtime(
                        os.stat(self.local_settings_example).st_mtime)
                )

                local_settings_mtime = time.strftime(
                    self.time_fmt,
                    time.localtime(os.stat(self.local_settings_file).st_mtime)
                )

                print('generating "%s"...' % os.path.join(
                    dircontext.curdir,
                    self.local_settings_diff)
                )
                with open(self.local_settings_diff, 'w') as fp:
                    for line in difflib.unified_diff(
                        example_lines, local_settings_lines,
                        fromfile=self.local_settings_example,
                        tofile=self.local_settings_file,
                        fromfiledate=local_settings_example_mtime,
                        tofiledate=local_settings_mtime
                    ):
                        fp.write(line)
                print('\tDONE.')
                sys.exit(0)
            else:
                sys.exit(
                    '"%s" already exists.' %
                    os.path.join(dircontext.curdir,
                                 self.local_settings_diff)
                )