get_commodity_directives or get_commodity_map

I tried to use "compute_returns.py" as suggested by the official manual "Beancount - Calculating Portfolio Returns", but failed due to empty return of "get_commodity_directives" in "investments.py". I checked online docs but find only the "get_commodity_map" function which is suggested to appear in getters.py, but it is not the case in the getters.py in my beancount directory. Any suggestion?