The French Constitutional Court's rejection of some details of President Francois Mitterrand's nationalization package is a reminder of how interesting an anomaly France's left-wing Socialist government is in today's Europe.
Its economic policies - nationalization, reflation through Keynesian type public spending and generous reforms of which labor will be the immediate beneficiary - run counter to the monetarist trend elsewhere in the Western industrialized world. Yet Mr. Mitterrand is more politically secure at the moment than are the heads of government in West Germany, Britain, Italy - or even in the United States. President Reagan's current term expires in 1985, Mr. Mitterrand's not till 1988.
Mr. Mitterrand and his prime minister, Pierre Mauroy, have no intention of letting the Constitutional Court divert them from the broad nationalization plan - 36 banks, five industrial groups, and two major investment companies.
A statement from the prime minister's office said implementation of the nationalization law will be delayed only as long as it takes to bring the details of it into line with the Constitution, to meet the Constitutional Court's objections.
Mr. Mitterrand won election as President of the republic last May after a tactically brilliant but painstaking campaign to make his Socialist Party the predominant party of the left - a role long monopolized by the Communists. At home, he needs to preserve among French workers and intellectuals his image as a man of the left, lest they return to the Communist fold. Hence his refusal to let up in his commitment to the domestic reforms he promised during the election , even though his finance minister, Jacques Delors, raised the question of a ''pause'' early in December.
While defending one of his flanks against the Communists, Mr. Mitterrand has four Communist ministers in his Cabinet. The aim is to keep the Communist Party as ''tame'' as possible - and keep an eye on it.
This is part of yet another Mitterrand paradox. Even though the President is a Socialist unashamed of the ''Marxist'' label, and with Communists in his Cabinet, his foreign policy in the key areas of the superpower struggle and nuclear weaponry has turned out to be more supportive of the US than was the approach of his right-of-center predecessor, Valery Giscard d'Estaing.
In today's France, there are fewer anti-US and antinuclear demonstrations than elsewhere in Western Europe. Mr. Mitterrand's response to events in Poland has been tougher than that of West German Chancellor Helmut Schmidt. And the French President openly supports the introduction of US cruise and Pershing II missiles in Europe (albeit not in France).
Admittedly, some of Mr. Mitterrand's third-world initiatives have irritated Washington - such as his sympathy with the Sandinista-dominated government in Nicaragua and his decision to sell arms to it. His joining with Mexico in calling for negotiations between right and left in El Salvador prior to next March's election there also jars the US. But, on balance, the Reagan administration is probably having less trouble with Mr. Mitterrand than it had expected when he was elected.
Yet if Mr. Mitterrand has gone a long way to reassure Washington since he replaced Mr. Giscard d'Estaing in the Elysee Palace, has he managed the same on the domestic front with French businessmen?
In a Dec. 9 television appearance, Mr. Mitterrand sought to meet the apparent nervousness of the business sector. He said unequivocally that after implementation of the nationalization program now being enacted, there would be no further nationalizations, ''no new wave of reform in that area,'' during the remaining term of the present National Assembly (expiring in 1986).
So far, the French economy has remained relatively steady despite the change of course ushered in with the Mitterrand presidency in May.
The inflation rate then was 14 percent. The overall rate for 1981 was 14.2 percent - which means only a minimal increase since the change of government. Mr. Mitterrand said on television he hopes it will not exceed 14.5 percent before government policies start bringing it down.
A devaluation of the franc last October should help exports and prevent any steep increase in the trade deficit. The rate of increase in unemployment fell off after the change of government in the summer: the percentage of jobless (7.8 ) is far lower than that in Britain (12.1).
But Mr. Mitterrand has been in office less than 250 days. The bills for his reforms have still not really started to come in. When they do, can France meet them?
The respected conservative Swiss newspaper, Neue Zurcher Zeitung - which has unusual international banking connections and had been skeptical about the Mitterrand economic policies from the outset - writes:
''The probability is great that France will again tangle itself in a net of interventionism which will result not in a liberation of expansive energies but in a further strangulation of its economy.
''Can France succeed where others have failed? This question will be definitively answered in no more than a year or two.''