Swallowed creatures are considered Unseen Attackers. Yes or No?

Quote from Natrel >> 

I have to admit. While I am quite serious about holding my ground in this conversation, I can't help finding it hilarious to first being told by my objectors that the unseen rule doesn't apply to swallowed creatures because the situation doesn't feel sensible, just for them to justify their point of view by claiming that creatures basically have eyes inside their stomachs...

 Different people. I never specified anything about what feels sensible, just what the rules actually say.

Fair point. And yes, like you said, many swallowing creatures have Blindsight. The Tarrasque is a well-known example. Giant Frogs don't, though. But I don't think matters. Even if we don't consider the fact that swallow actions often explicitly give Total Cover to swallowed creatures, it can easily be inferred by the fact that there are several layers of fully encasing solid surfaces between a creature's own stomach and the outside of its body (I can't believe I even need to point that out)... And as we all know, Blindsight doesn't see through Total Cover.

Quote from Pantagruel666 >>

Quote from Natrel >> 

I have to admit. While I am quite serious about holding my ground in this conversation, I can't help finding it hilarious to first being told by my objectors that the unseen rule doesn't apply to swallowed creatures because the situation doesn't feel sensible, just for them to justify their point of view by claiming that creatures basically have eyes inside their stomachs...

 Different people. I never specified anything about what feels sensible, just what the rules actually say.

The rules actually say that an Unseen Attacker has advantage.

As for whether or not an attacker is unseen -- that is generally determined by the DM as you've pointed out.  The DM has to make a ruling about that based on the specific situation that is occurring in the game and there are plenty of edge cases that exist.  The DM will use various rules such as Line of Sight, Heavily Obscured areas, and so on for guidance and then ultimately the DM just has to make a ruling about whether or not a creature can be seen.  Whether or not there is an actual rule about whether or not a creature can see inside of its own stomach is beside the point because of course it cannot.  If a DM claimed to be making a RAW-based ruling and tried to then justify the ruling by saying that the creature can see inside of its own stomach, then I would strenuously object.  If a DM instead admitted that he would prefer to make a non-RAW ruling for this situation such that applying both advantage and disadvantage remains disadvantage instead of canceling out in this case and asked us to just go with it then of course that's fine.  But "how would I rule it" is a different conversation than "what is the RAW".  The OP was specifically looking for a RAW answer.

Quote from Natrel >>

And as we all know, Blindsight doesn't see through Total Cover. 

Yeah, but being swallowed doesn't actually provide total cover against the swallowing monster (I think I've only once used a swallowing monster, and I don't think the character who got swallowed ever tried to attack the worm, so...).

RAW all the modifiers cancel out even if it doesn’t make the best sense.

Quote from Natrel >>

Something that came up a while ago during a campaign with a particularly hostile DM I used to have: one player had been swallowed by another creature, which gave them Total Cover against the outside, as well as the Restrained and Blinded condition, which meant they had disadvantage on their attacks against the creature that had swallowed them.

I argued that because they were inside their stomach, the creature couldn't possibly see them

RAW: you are correct (discussed here).

RAI: if the writer's intended that they likely would've mentioned it (instead of just mentioning the two conditions that grant disadvantage). It seems like it didn't occur to them.

Either interpretation is fine.

Incidentally, and this is a house rule, I generally consider touch to provide the effect of sight, so you can cast spells that require sight if you're touching (outside of swallow, usually only applies to grapples). This does help a spellcaster who is trapped, as it means that you can, for example, magic missile the creature.