Petri was born in Rome on 29 January 1929. He was expelled for political reasons from San Giuseppe di Merode, a school run by a priest on the Piazza di Spagna, and joined the youth organization of the Italian Communist Party (PCI). He wrote for "L'Unità" and for "Gioventù nuova" as well as for "Città aperta". He left the party in 1956 after the Hungarian uprising.