LerpInterval is just the Python module that defines LerpPosInterval and similar intervals. LerpPosInterval itself is documented on the link referenced above.
nodePath.posInterval(blah blah blah)
is equivalent to:
LerpPosInterval(nodePath, blah blah blah)
This is just a tiny Python shorthand that is not documented anywhere specifically, but which we’re explaining to you now. It is true the manual should either explain this shorthand, or not use it in the first place.
What kind of duration do you expect an interval to have, if not a single number?
I don’t understand what you’re asking. What trick? The only thing going on here is that there is a method defined on NodePath called posInterval, which has the definition ynjh_jo quoted above–it immediately constructs and returns a LerpPosInterval by applying self as the first parameter, and all of the remaining parameters as the remaining parameters.
I’m still not sure what you’re asking about, I’m sorry. I’m just not getting your questions.
There is no magic going on here. There is no special handling of the first parameter as the “self” parameter. (Python can do this sort of thing, sort of, but it’s not normally the way you would call a method.)
What’s happening here is that Actor inherits from NodePath, and there is a method of NodePath called “posInterval” that simply constructs and returns a LerpPosInterval. So Actor inherits this method from NodePath.
Ostensibly, this method exists solely as a typing convenience, which is a little odd because it doesn’t save very much typing. But there you have it. Someone, at one point, thought it would be convenient to have such a method, and they defined it. But they defined it in such a way that the generated API docs can’t find it, so it doesn’t get documented. And we will probably remove it in the future.
So, yeah, the manual should probably avoid using this confusing little method in its example.
For the record, there is also a similar method called hprInterval that constructs and returns a LerpHprInterval, and so on.