If you are coming to France for a long period of time and would like to have some data connection without roaming fees, and you're lucky enough to have an unsimlocked iPhone, your best option is probably Free Mobile.
It's a inexpensive subscription, but it would be very hard for someone from outside France to subscribe to it. You have 3 GB of data per month and free calls to the US, but you need a bank account in France, and so on.
If you have friends who would lend you a sim card, you can ask them.
If you're one of my follower/followed and would like one, I'd be happy to lend you one as I do have one. Just ask me!