In Paris, say, if you want to do a picnic and see sites and museums do you buy your meal early and carry it into the museum or can you check a bag with it? Also, Can you drink wine in the open say during one of the evenings on the Seine? Same at Versailles, do you buy in the morning and then take with you?
The last time we visited Paris, we visited the Musee d'Orsay in the morning, then went over to the Eiffel Tower and had lunch in the park near there. We sat on a bench, and discretely enjoyed our cheese, foie gras pate, bread and wine. We were concerned about taking my husband's pocket knife into the museum, so we opened the bottle beforehand, left the pocketknife at the hotel, and cut our cheese with a plastic knife. No problems. And it was beautiful and fun.
I always carry drinks and often carry food in my bag. I've never had a problem in Paris (or elsewhere). Either I can carry the bag with me through the site or I can check it, with the edibles inside. I think there may be a prohibition against drinking wine in the open, but I have seen it done a lot. As long as you are discreet - not drinking right out of the bottle as you stroll along the Champs Elysees, I think you will be fine.
I do seem to have read somewhere that while drinking in public is now prohibited,, its enforced with discretion. In other words, having a glass of wine while enjoying a nice picnic is not going to raise an eyebrow,, but getting drunk and calling attention to yourself will be an issue, and will get you a ticket. Fair enough. I have been able to put a sandwich ( usually a lovely ham and cheese baguette ) in my purse( which is messenger bag sized) and carry it with me whereever, at Versailles I have purchased a sandwich outside the grounds, carried it in, and then just purchased a drink from the stands in the gardens. In Paris my picnics are usually purchased and consumed soon after( pop into a bakery and or deli, then just find a place to sit and eat), so I am not a serious picnic packer. If one packs a large complete picnic you may have to put it in a seperate bag and check that bag at museums etc.
I've never lugged stuff through museums, so no answer there. But I can't guess why anybody would see it as a threat to the artifacts. Outside, you can drink anything anywhere. If they ever made a law against that , they'd have to rebuild the Bastille to house all the offenders - - and my wife would be in Cell #1. My sophisticated self has even been seen sitting on a curb sucking a grocery store beer since I'm too cheap to pay the tariff in a cafe. EDIT: Based on what Pat says, and my subsequent snooping around, apparently there are either new (or recently being enforced) bans on public drinking but enforcement seems spotty and mostly at large gatherings, at specified times, and tied to raucous behavior.
I usually just map out a grocery store ahead of time to pick up my picnic stuff when I'm ready to eat, that way I don't have to worry about refrigeration. If I couldn't find a store, I'd pack something that doesn't need refrigeration (fruit, chocolate, bread, wine) - I really doubt a museum will have a problem with a normal looking couple/family carrying picnic stuff.