The victim, a 31-year-old man, was attacked after getting into an argument with two men and a woman.
He was taken to hospital for treatment to a serious head injury, but has since been discharged.
It happened outside a nightclub in Silver Street during the early hours of Sunday, September 10.
West Yorkshire Police today released details of the assault as they try to trace a member of the public who came to the victim's aid, before taking the victim and his friend home.
A spokesman said: "Officers are now appealing directly to the member of public who helped the injured man to contact police, as it is believed they may have information valuable to the investigation
"The victim was wearing a distinctive green puffa jacket and his friend was wearing a navy ‘stadium’ jacket with a white logo on the left lapel.
"Police are also appealing to the wider public and for anyone who witnessed the assault outside the nightclub to get in contact with officers."
Anyone with information is asked to contact police on 101, quoting crime reference 13170422674.
Information can also be passed to independent charity Crimestoppers anonymously on 0800 555 111.