Paris is statistically one of the safest metropolitan areas in Europe. Violent crime is low, though other crimes like pickpocketing is common. Be alert, keeping your essentials close on your person, especially in crowded areas such as trains, metro stations, and popular tourist areas. Consider wearing a money belt and keep bags in front of you.
In case technology fails you and your phone dies with no adapter to charge it with, have your vital information written down on a piece of paper — such as hotel address and a taxi service phone number.