PRELIMINARY TITLE
On penal guarantees and on the application of the Criminal Law
Article 2
1. No felony or misdemeanour shall be punishable by a punishment that is not foreseen by Law prior to it being committed. Likewise, the Laws establishing security measures shall not have retroactive effect.
2. Notwithstanding this, criminal laws that favour the person found guilty shall have retroactive effect, even though final judgement may have been handed down and the convict is serving a prison sentence at the time of them coming into force. When doubt arises in determining the most favourable law, the prisoner shall be heard. However, offences committed while a temporary Law is in force shall be judged pursuant thereto, except for express provisions to the contrary.
Non-retroactivity
Nulla poena sine lege
EDIT.