Police can demand identification at any moment and for whatever reason, though it's unlikely to happen. Always carry photo ID or a copy of your passport, and – most importantly – always be courteous and cooperative.
Drugs and most other substances that are illegal in the US and many European countries are also illegal here, though marijuana has been somewhat decriminalized in Argentina (and is legal in Uruguay).
If arrested, you have the constitutional right to a lawyer, to a telephone call and to remain silent (beyond giving your name, nationality, age and passport number). Don’t sign anything until you speak to a lawyer. If you don’t speak Spanish, a translator should be provided for you.