"The important thing is we are going to be in front of 15,000 people all listening to every word we say, unlike our home life which is lonely and sad," he told The Associated Press.
"That is the only reason we're doing the show, because we've all become so depressed over the years that our wives don't find us interesting, our children don't even laugh at us anymore."
The five delighted reporters with gags and jokes, sitting behind the wrong name cards and answering questions addressed to another member. They also made plenty of references to their advanced age, saying medical attendants will be present during the show, which has the working title "One down, five to go."
That's in reference to the sixth member, Graham Chapman, who died of cancer in 1989.
The Pythons said the London show will include animation, material that had never been performed live before, and some of their best-loved sketches including the "dead parrot" and the "crunchy frog" skits from their heyday.
But Cleese said he wouldn't reprise his "Ministry of Silly Walks" performance, because he now has an artificial knee and hip.
The group had its first big success with the Monty Python's Flying Circus TV show, which ran from 1969 until 1974, winning fans around the world with its bizarre sketches.
It branched out into movies including "Life of Brian" in 1979 and "Monty Python and the Holy Grail" in 1975 both often cited among the funniest movies of all time and backed theatrical shows such as "Monty Python's Spamalot."
Idle said the London show was planned as a single event, but that further engagements are possible. The show will be filmed for possible distribution.
The Pythons last performed together in Los Angeles in 1980, and last appeared on a U.K. stage 40 years ago.