Return the context for a given 1-offset line number.

def context(self, line):
        """
        Return the context for a given 1-offset line number.
        """
        # XXX due to a limitation in Visitor,
        # non-python code after the last python code
        # in a file is not added to self.lines, so we
        # have to guard against IndexErrors.
        idx = line - 1
        if idx >= len(self.lines):
            return self.prefix
        return self.lines[idx]