Katy Perry encouraged her grandma to "go gangster" by dancing around her "bedazzled cane" at a party.

The singer is currently promoting her new movie Part of Me, a tour documentary that also follows Katy's split from her estranged husband Russell Brand. Her 91-year old grandma appears in the film and couldn't wait to hit the dance-floor at a bash to celebrate its release.

"At the after-party of the premiere, my friend Mia who's a DJ and in the movie, she put on Dre and my grandma just came out dancing," Katy giggled. "She has so much swagger. She has a bedazzled cane and I put it up in the air and we were both going gangster."

The 27-year-old singer loves her job but wants people to know there are some downsides to it. Asked what her single biggest regret was, Katy admitted she would love to complete her school studies.

"I regret not finishing high school," she told the Kyle and Jackie O show. "I would love to not have to depend on my spell-check or the fact I'm like, Do I use an apostrophe s' in this situation or an apostrophe re'?' How many versions of your' can there be?!"

Katy was thrown into the limelight with the release of her smash-hit single I Kissed A Girl in 2008. Many believed the song was written about movie star Scarlett Johansson, but the singer has set that rumour straight.

"It's about a girl from Santa Barbara that I used to be best friends with," she acknowledged. "I didn't kiss her, I kissed some other girls along the way but looking back she was always the girl who had the cooler things.