Paris — Shifts in French politics are most discernible at lunchtime. For a Frenchman, sharing a meal is the ultimate sign of amity and reconciliation. So, when former President Valery Giscard d'Estaing recently lunched with his rightist archrival, Paris Mayor Jacques Chirac, political commentators had a feast day.
Equally newsworthy was Mr. Giscard d'Estaing's rejection this week of a midday invitation from his opponent on the left, President Francois Mitterrand.
The political interpretation was easy: Giscard d'Estaing had patched up his quarrel with Mr. Chirac, but no meeting of the minds was possible with Mr. Mitterrand.
With local elections coming in March, patching up the feud between Giscard d'Estaing and Chirac was important. A split vote among conservatives could have been disastrous.
So lunch was arranged.
The menu was front-page news. And observers said the opposition's election chances had significantly improved. Another lunch, in December at Giscard d'Estaing's Paris residence, was seen as sealing the detente.
While making up with Chirac, however, Giscard decided to keep his options open. So, in an interview, he envisaged the possibility of ''cohabitation'' with the Socialists if he won a majority in the 1986 legislative elections.
Mitterrand took up the offer and, not surprisingly, invited Giscard to lunch.
This much ''cohabitation,'' however, was not what Giscard had in mind.
''I do not wish to participate in these social events,'' he said. ''But if the President of the Republic would one day desire to talk to me about questions facing the country, I am at his disposal.''
But not, of course, for lunch.