def get_line_map(self, filename):
        """The line map for `filename`.

        A line map is a list of character offsets, indicating where each line
        in the text begins.  For example, a line map like this::

            [13, 19, 30]

        means that line 2 starts at character 13, line 3 starts at 19, etc.
        Line 1 always starts at character 0.

        """
        if filename not in self.source_map:
            template_source = read_template_source(filename)
            if 0:   # change to see the template text
                for i in range(0, len(template_source), 10):
                    print("%3d: %r" % (i, template_source[i:i+10]))
            self.source_map[filename] = make_line_map(template_source)
        return self.source_map[filename]
The line map for `filename`.

        A line map is a list of character offsets, indicating where each line
        in the text begins.  For example, a line map like this::

            [13, 19, 30]

        means that line 2 starts at character 13, line 3 starts at 19, etc.
        Line 1 always starts at character 0.