def _get_lines_from_file(self, filename, lineno, context_lines, loader=None, module_name=None):
        source = None
        if loader is not None and hasattr(loader, "get_source"):
            source = loader.get_source(module_name)
            if source is not None:
                source = source.splitlines()
        if source is None:
            try:
                with open(filename, 'rb') as fp:
                    source = fp.readlines()
            except (OSError, IOError):
                pass
        if source is None:
            return None, [], None, []

        # If we just read the source from a file, or if the loader did not
        # apply tokenize.detect_encoding to decode the source into a Unicode
        # string, then we should do that ourselves.
        if isinstance(source[0], six.binary_type):
            encoding = 'ascii'
            for line in source[:2]:
                # File coding may be specified. Match pattern from PEP-263
                # (http://www.python.org/dev/peps/pep-0263/)
                match = re.search(br'coding[:=]\s*([-\w.]+)', line)
                if match:
                    encoding = match.group(1).decode('ascii')
                    break
            source = [six.text_type(sline, encoding, 'replace') for sline in source]

        lower_bound = max(0, lineno - context_lines)
        upper_bound = lineno + context_lines

        pre_context = [line.strip('\n') for line in source[lower_bound:lineno]]
        context_line = source[lineno].strip('\n')
        post_context = [line.strip('\n') for line in source[lineno+1:upper_bound]]

        return lower_bound, pre_context, context_line, post_context
Returns context_lines before and after lineno from file.
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/views/debug.py/ExceptionReporter._get_lines_from_file