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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_template_file(self):
 """ Retrieves Jinja2 template file path. """

        if os.path.exists(os.path.join(self.theme_dir, 'base.html')):
            return os.path.join(self.theme_dir, 'base.html')
        default_dir = os.path.join(THEMES_DIR, 'default')
        if not os.path.exists(os.path.join(default_dir, 'base.html')):
            raise IOError(u"Cannot find base.html in default theme")
        return os.path.join(default_dir, 'base.html')