Resolving variables in a text string in Python

resolve-defines.py

    self.defines = {'HOME': self.spfe_dirs['home'],
                        'SPFEOT_HOME': self.spfe_dirs['spfe_ot_home'],
                        'CONTENT_SET_BUILD_DIR': self.content_set_build_dir,
                        'CONTENT_SET_OUTPUT_DIR': self.content_set_output_dir,
                        'CONTENT_SET_BUILD_ROOT_DIR': self.content_set_build_root_dir
        }

    def _resolve_defines(self, string):
        defines_pattern = re.compile('\$\{([^}]*)\}')
        resolved = re.sub(defines_pattern, self._replace_defines, string)
        return resolved

    def _replace_defines(self, match):
        try:
            return self.defines[match.group(1)]
        except KeyError:
            Exception("Invalid define ${" + match.group(1) + "}")