In London, such a journey by Tube, with such a cargo, would be unthinkable. In Paris, where the underground trains come every two minutes and are rarely jammed, I reckoned it would be tolerable.
In fact, it was delightful. I have never had such a pleasant Metro journey: people smiled (a rare event in Paris); they held open doors for me; they offered seats; they asked questions about the gateau.
It was an American-made, Betty Crocker instant-mix cake, with special instructions for cooking at high altitudes. It had fancy whirls which I had got all wrong. Maybe, since we live on the fifth floor, we should have used the high-altitude instructions. I revealed none of this to my fellow passengers; I just said it was home-made. In other countries, it is pets (Britain) or children (America, Italy) which renderstrangers helplessly friendly: in France, it turns out, the secret is to carry a large, mis-shapen, home-made cake.
The battle for the soul of the European centre-left, real or presumed, between "Blairism" and "Jospinism" absorbs French politicians and newspapers much more than our own. Lionel Jospin, the Prime Minister, is constantly being teased by the French centre-right for not being Tony Blair.
The French intellectual Left, having originally welcomed Le Blairisme, was pleasantly outraged to find Britain echoing and reinforcing the belligerent American line during the recent Iraqi crisis.
In truth, there are many signs that Mr Jospin is managing the economy quite well and may be more than just a Socialist dinosaur who looks vaguely like a geography teacher.
The French centre-right refuses to accept this. One right-wing politician and former budget minister, Francois d'Aubert, described Mr Jospin this week as "inblairisable": a wonderful linguistic invention, which means, roughly speaking, "someone who could never be a Blair".
Or, perhaps, just someone who is "unblairable".
It would be difficult to make the case that Paris is a hardship post. Consider this, however. For two months, I've had a chronic dry cough and sore throat. Our infant daughter has had a series of minor respiratory problems, and one extremely serious one, since she was born four months ago.
When I left Paris last week to go into the country for three days, my cough and sore throat disappeared. When I returned to Paris it instantly returned.
Travelling on the Metro, with or without home-made patisserie, there is generally a low grumble from the throats of the passengers: the "Paris cough" a friend calls it. Air pollution is a serious problem in all large, French conurbations - an absurd state of affairs in such a large, empty country. A study published earlier this year reported that 42 per cent of all emergency doctor's calls to sick children in the Paris area were for respiratory complaints. The number of children under 14 admitted to hospital increases by half during periods of heavy atmospheric pollution. There are topographical reasons - the ring of hills surrounding Paris - why the dirty air tends to settle over the French capital.
Both the present, and last, governments have taken steps to control emissions from cars at periods of high pollution; private cars are restricted to those with odd or even numbers, a tougher measure than any officially contemplated in Britain.
New legislation is planned to encourage "green" cars, especially electric vehicles and those with super-efficient filters on their motors. Everyone knows, however, that a principal reason why French urban air is so poor is that France has a large proportion of diesel cars: a legacy of earlier government policy, based on the wrongful belief that they are cleaner than petrol-driven cars.
Diesel fuel is still taxed less heavily than petrol. The environment minister, and Green party leader, Dominique Voynet, has been campaigning to penalise, not encourage diesels.
But the lobby of French car manufacturers, which is still heavily committed to diesel models, remains too strong for her. She lost the battle in cabinet last year. When she raised the issue again recently, she was apparently met by ... polite but embarrassed coughs all around.