Do you mean for walking around? Like any city, it depends on the neighborhood, but I've never had an issue ambling about many of the central neighborhoods circling the Seine, both as a solo female and as a 2+.
Below are two links on Paris safety. One is written by World Nomads editorial staff, and the other is link to a similar question in this forum:
http://answers.worldnomads.com/questions/france/3310/how-do-you-avoid-pickpockets-in-paris almost 8 years ago
Like any other city, just be alert. The craziest thing I ever encountered late night, was a young French guy asking me if I had a condom, he was on a date and the girl wouldn't play without protection. He was running up to everyone, it was funny, no-one had a condom. Just when he was about to give up, he asked this guy who must have been 80 years old. Ironically, he had a condom for the kid. Gotta love the French:) almost 8 years ago