The seasons of a love affair are played out beguilingly in this wonderfully sweet, sad, smart new movie from Damien Chazelle – the director of Whiplash – and the Venice film festival could not have wished for a bigger sugar rush to start the proceedings. It’s an unapologetically romantic homage to classic movie musicals, splashing its poster-paint energy and dream-chasing optimism on the screen. With no little audacity, La La Land seeks its own place somewhere on a continuum between Singin’ in the Rain and Woody Allen’s Everyone Says I Love You, with a hint of Alan Parker’s Fame for the opening sequence, in which a bunch of young kids with big dreams, symbolically stuck in a traffic jam on the freeway leading to Los Angeles, get out of their cars and stage a big dance number.

To be honest, this is where an audience might find its tolerance for this picture’s unironic bounce tested, coming as it does right at the top of the show. It takes a little while to get acclimatised, and for the first five minutes, the showtune feel to the musical score might make you feel you’re watching a Broadway adaptation. But very soon I was utterly absorbed by this movie’s simple storytelling verve and the terrific lead performances from Ryan Gosling and Emma Stone who are both excellent – particularly Stone, who has never been better, her huge doe eyes radiating wit and intelligence when they’re not filling with tears. Gosling, for his part, has a nice line in sardonic dismissal to conceal how hurt he is or how in love he is.

The two of them get a meet cute in the traffic jam. Stone is Mia, a wannabe movie star like pretty much everyone else, and while waiting, she is distractedly going through her pages for an audition she has later in the day. Chazelle, incidentally, creates a mischievous reveal in which we are later struck by the dull listless way she runs the lines to herself, and the passionate way she sells them later to the producer. I wonder if the director was influenced by Naomi Watts’s actress in that other La-La-Land extravaganza: Mulholland Drive, by David Lynch.

But she doesn’t notice the cars ahead starting, and holds up the driver behind her: a disagreeable guy in a macho convertible, who pulls belligerently round to overtake, scowling at Mia and receiving the finger in return. This is Seb, played by Gosling, a pianist and jazz evangelist who is living a scuzzy apartment in the city.

A little like Mr Fletcher, the terrifying jazz teacher played by JK Simmons in Chazelle’s Whiplash, Seb is a purist and an uncompromiser, a difficult guy to get to know or like. He is lonely and unhappy, claiming to his exasperated sister (Rosemarie DeWitt) that he is just playing rope-a-dope with life and fate, waiting for them to wear themselves out beating him, after which he will come storming back. Seb is fired from a restaurant, where the manager (a cameo for Simmons) is enraged by his tendency to favour brilliant free-jazz improvisations instead of the tinkling background music he gets paid for. But it is here, again, that Seb meets Mia, and then again at a party, where Seb has humiliatingly got a gig playing synth in an 80s-style band. It is fate.

Winter turns to spring and then to summer, and their affair begins to take off: Mia encourages Seb to find a way to open the jazz club he dreams of, but to prove to her he’s not a flake, he takes a regular gig playing the piano in a jazz-rock band led by an old frenemy of his. Suppressing his fears that he is selling out, Seb in his turn encourages Mia to write her one-woman show – that toe curling staple of the needy actress. But there is trouble in store: having been careless in what they wished for, Mia and Seb find that success and careers are to come between them. There is a brilliant scene in which a surprise supper Seb has cooked for Mia descends into a painful row as they quarrel about how their lives are panning out.

Chazelle creates musical numbers for the pair of them, and Gosling and Stone carry these off with delicacy and charm, despite or because of the fact that they are not real singers. The director must surely have considered the possibility of casting, say, Anna Kendrick in the role of Mia, who would undoubtedly have given the musical aspect some real punch. But Stone fits the part beautifully: something in the hesitancy and even frailty of her singing voice is just right. Both actors are also very accomplished dancers within a shrewdly limited range.

La La Land is such a happy, sweet-natured movie – something to give you a vitamin-D boost of sunshine.