The idea of Communists in the French government may sound strange and controversial so I will set out some context for you and round it off in the end with a point about the reaction to it. The reason this context is important is knowing the extent of the agreement between the parties reflects on the actual power dynamic between the two.
During the 1970s the PCF (Parti Communiste Français) and the PS (Parti Socialiste) were in what was called the L'Union de la gauche (Union of the left). Francois Mitterrand wished to make the PS into a political force. The two parties ended up making common electoral strategies and deciding that in the second ballot (of the voting process), one candidate would go through to represent the left. The truth is, L'Union de la gauche was designed not just against the right but to basically dismantle Communist support. It can be described as “keep your friends close, but your enemies even closer”. The PCF were damaged by both the arrangement that Mitterrand had made as well as the Cold War/French politics of the time.
The PCF was not as popular as it once was. During the post-WW2 period it was extremely popular due to its part in the French Resistance. It was even so popular that the Americans did worry about its influence. But subsequent events diminished this. During the May 1968 riots they couldn't decide whether or not to support the student action, initially there was support but eventually they were against revolutionary action. The reactionary force against the riots gave the Gaullists a landslide. To many of these students who were Communist and of the left this was a betrayal by the PCF. The subsequent Soviet & Warsaw Pact Invasion of Czechoslovakia also damaged their position. Like many Western Communist parties, they ended up distancing themselves, some for the first time, from the Soviet Union. The publication of The Gulag Archipelago was so damaging that the PCF had to get rid of its policy of “dictatorship of the proletariat” in its 1976 congress to try and soften its image.
The L'Union de la gauche in turn had an effect of creating a strong anti-Communist opposition against them. This made a clear bi-polarisation of French politics. The centre and the right combined together and won the 1973 election. While the PCF felt it had the upper-hand over the PS, eventually they realised that the grouping was benefiting the PS more than themselves. This weakening of the PCF allowed Mitterrand to build support from the left inside the L'Union. 'Our fundamental objective is to rebuild a great Socialist party on the ground occupied by the Communists in order to demonstrate that out of five million Communist voters, three million can vote Socialist.' This was what he said in the Socialist International of 1972. Eventually the PCF leader, Georges Marchais, tried to take a more radical economic stance but this was derided by the PS. The Union collapsed in 1977.
In 1978 the PS was beating the PCF in voting share. The PS had managed to rough out more Marxist elements in the party and become seen as more centre-left. Mitterrand managed to make his personality seem like a radical republican, to guarantee the people's liberty, democracy and economic reform (as opposed to opposition to capitalism). In the 1981 French presidential elections Mitterrand managed to get through to the second round and eventually win. It is said that Marchais, while publicly supporting Mitterrand, apparently did not behind the scenes. During much of the campaign Marchais had attacked Mitterrand and therefore diminished his own support in the first round. The PS had become a dominant force in French politics and the PCF collapsed to around 16% of the vote. The PS allowed 4 PCF politicians to become ministers. Effectively, this was part of the same strategy as before. Mitterrand had effectively neutered them. They had so little influence and were unable to do much from this position. It was also to allow the PS to partly obtain the PCF's diminished support for future elections.
Both America, West Germany and even the Soviet Union initially worried about the PCF inclusion in the government. For the Americans & West Germans it was a threat to security and to the Soviet's it marked a change in the predictable centre-right French governments it had dealt with before. Ultimately though, because of the PCF's diminished support and Mitterrand's strategy it could not be seen as a threat to the government. Mitterrand's foreign policy would essentially destroy most fears. Mitterrand had a hard line stance against dialogue, unlike his predecessor, due to the Soviet occupation of Afghanistan. Mitterrand supported NATO weapons installations in Europe. He supported Reagan's hard-line approach to the USSR during Reagan's first term. It wasn't until later on that Mitterrand cooled to a more traditional dialogue to the USSR.
Sources:
R. Gildea, France since 1945 (2002).
S. Hazareesingh, Political Traditions in Modern France (1994).
T. Judt, Marxism and the French Left (1986).
M. MacLean, ed, The Mitterrand Years: Legacy and Evaluation (1998).
M. Mandelbaum, ed. Western Approaches to the Soviet Union (1988)
Edit: Slight clarification.