In you are headed to Hungary, be aware that in mid-March that country imposed a law requiring photographers (presumably that includes tourists) from taking anyone’s photo, even in public, without permission.
You shouldn’t face that problem elsewhere in Europe, but that doesn’t mean you can or should poke your lens in someone else’s face or place.
For example, don’t assume that it’s okay to take photos in an art museum just because you see someone else doing so. When you enter the museum, ask about its policy on the taking of photographs. Sometimes all photography is banned, sometimes only flash photography. At other times, photographs of the museum’s general collection can be taken, but not those of special exhibitions.
European cathedrals and churches often have eye-catching architecture and it should be permissible to photograph their exteriors. But by mindful when shooting inside that these are places of worship where people often come to pray on their own when no formal services are being conducted, and they probably don’t want you to photograph their divine supplications.
Even out on the street, people don’t necessary want their comings and goings noted “on film.” And even “in public” people often enjoy “private moments” that you probably should not photograph.
So if someone who sees you raise your camera gives you a disapproving look, forego the shot and wait until you can take one that won’t offend the sensibilities of your subject.
But sometimes a photo shot from behind a person will turn out to be a more compelling image than one that you might have taken face-on.