The quick answer to your question is, if you buy a recent model you have a very good chance it'll work in France. Now for more detail.
France uses predominantly the GSM network while in Canada usage is still mixed, so if you're buying a phone from an operator, make sure you choose a GSM compatible operator such as Rogers or Fido and avoid. Bell or Telus which use aTelus both operate GSM networks but still have older, different networknetworks, so if you're buying from them make sure the phone is not CDMA only.
Even if you buy a GSM phone, European frequencies are slightly different so make sure you buy what is called a multiband phone (supporting 850/1900 and 900/1800MHz for 2G and 900/2100 for 3G frequencies). You can find these in the phone manual or you can just ask the seller. As for LTE, major Canadian operators operate on band 7 used in France so you should be ok in dense urban areas, but might be less helpful in rural areas.
Only applies if you're buying from an operator, make sure you unlock the phone before returning to France. I've done it many times and is very simple, you pay for a code the operator sends you, install a package and restart your phone.
Most recent smartphones, including Apple and Samsung ones, sold in North America support multiple frequencies and you can buy them unlocked if you wish. Unless you're buying an older or limited model phone, it's unlikely you'll have to worry much about it.