Is a Schengen visa for Spain valid for France?

Oi, good news - Schengen visas normally entitle you to entry to any member-states listed in the Schengen Area, and France is a member-state. Thus, you don't need to get a separate visa. That said, you might be asked to present documents that prove reasons for your stay, where you're staying and how you're leaving. Basically, print out any hostel or hotel bookings you have, proof of flights back to your home country, or bank statements showing you have the funds to purchase either of these things. Depending on the mood of the passport control official, you may or may not be asked to present these things, but better safe than sorry :)