Directed by Dan Cutforth and Jane Lipsitz PG cert, general release, 96 min

OR YOU could mainline a carton of sherbet fountain while staring furiously at a stack of boiled sweets.

About a third of the way through this perfectly serviceable quasi-documentary on Katy Perry, the star ruminates on the philosophy behind her music. You know how these things go. You know how these things always go. The child of a preacher, Katy initially allowed record labels to squeeze her into various boxes before realising that she represented all those kids who are a “bit different” and who just want to “be themselves”.

Here they come – 100,000 girls have arrived at a stadium in São Paulo to assert their individuality by all dressing as similarly as possible to Ms Perry. Didn’t Monty Python’s The Life of Brian have something to say about all this?

None of which is to suggest that Katy Perry is necessarily a bad thing. Combining power-pop choruses with dance-floor beats, her music sits at the smart end of the candy colour spectrum. The relentless cheeriness is certainly a little exhausting. But Perry’s flip positivity is infinitely preferable to Lady Gaga’s pretentious posturing.

So we don’t mind Perry. What do we think of the film? Well, like that recent Jason Bieber documentary, Part of Me proves to be a surprisingly digestible entertainment. Interweaving energetic concert footage with interviews and archival material, the film tells the Perry story in lucid, engaging fashion. Her sister seems a good sort. Her parents, despite being hyper-religious, remain impressively philosophical about their daughter’s career path.

There is, however, a spectre at the feast. When Russell Brand, who separated from Perry late last year, first appeared backstage, there was, at the busy screening I attended, an audible murmur of disgust among the Perrystas.

The film plays this subplot very craftily indeed. We don’t get to hear the specific reasons for the break-up. But we do see Katy looking miserably at a text. Later, while in Brazil, she begins sobbing and causes her minders to worry that she might not make it on stage (as if). Eventually she powers forward and, later, we hear her discussing the break-up.

It’s a small masterpiece of manipulative editing: without causing the lawyers even the mildest flutter, the film-makers invite us to make a cad of Brand.

Whatever the truth of the story, that Brazil concert does confirm how difficult it must be to wear Katy Perry’s face every bleeding day. We have, by this stage, seen her rise through a trapdoor on at least four occasions. Each time, as the hatch opens, she slaps on a demented smile that shines brighter than an arc light and stretches wider than Oregon.

After the weeping fit, the Perry smile makes its appearance right on cue. It must be miserable having to be that happy all the time.