By all means bring a money belt or similar item to protect yourself from pick pockets and thieves. I use a zippered 'pouch' that my belt loops through and it slips down my side under my pants. When I want some cash or a CC, I just pull it up. Money belts that require one to dig down towards into your groin area can be embarrassing.
I estimate how much cash I need for the day and keep than in a pants pocket. Therefore I can never lose enough money at once to ruin the trip.
On a trip to Florence, a woman in our group hung her purse from the top of the chair where it swung about behind her. While we were eating an Italian coupole approached our server and asked for directions, in a minute they somehow got into a huge argument with the server. When the show was over and the rude Italian couple left, the woman's purse was gone. It was all a clever diversion so they could steal her purse. She had everything in the purse, money, credit cards, ATM cards, and her passport. Quite a haul for the crooks.
The restaurant owner came out and was very embarrassed that this happened at his place and gave the lady a free dessert at the end of the meal and free espresso for the rest of us.
We made it through Nice and Barcelona, and had no trouble. We were also careful in Aix-en-Provence because someone we met in Avignon had her wallet stolen out of her purse there, even though it is not well known as a pick-pocket haven. The local police, however, told her otherwise.
We didn't suffer crowd jostling in and out of buses, trams or metro. We didn't get crowded or pushed at escalators or metro turnstiles. We kept an eye out, but saw no suspicious people following us or watching other people. We had to deal with some annoying and persistent walking beggars in trains stations and on streets - always individuals alone, not carrying anything like a clipboard or jacket draped over arms used to disguise theft, just held out hands. We never saw teams of young people or groups that could be working in concert.
We were well prepared - all items tethered to us on under clothing (including tethers for our phones). I got a PacSafe purse for overbody wearing that had several security features. although most valuables were underclothing or inside and tethered to front pockets in our clothing. One of the PacSafe purse features was the ability to clip the strap through a chair back or table/chair leg, but I just left it on and in front when we ate - or put my leg through it.
When we got to the most notorious areas we would only have the day's cash and one credit and one debit card in outside pockets (in tethered coin purses and wallets - I usually had a few bills in one tethered wallet in my hip pants pockets and the cards with a bit more cash tethered in coin purse in my breast pocket) and backups under clothing, and any excess cash, alternate cards, plus passports locked in luggage in the hotel room. My purse was actually a decoy as I didn't have cash in it, although I did often keep my iPhone (with tether) inside it. Many people were outside walking holding phones - including the locals. My phone case had a wrist strap that I could clip inside my bag or keep around my wrist if holding. I kept it out of sight on in city transportation, however. My watch could display our route (although I needed my glasses to read it!!!)
Anyway - we had quite a pleasant trip, with no unpleasantness or apparent predators (other than beggars). I'm sure being prepared helped. We were expecting to see something - especially at train stations on on the metro - but it was uncrowded and we didn't see any action.