Filtering within pyopensky query

For a project in Python. I am trying to filter within the first query to the impala database using pyopensky.

The filter I want to apply (beyond time interval and bounding box) is the filter to only recognize aircraft and exclude the on ground vehicles such as cars and the like which are not aircraft.
What is the best option to do such filtering?

Things I have noticed is onground vehicles mostly do not have squawk codes, Their callsign are not of the same length as aircraft. But as far as I am aware there is no option within the query to filter with something along the lines of squawk != NaN or len(callsign) <= 4

I look forward to any help

Currently, the groud filter is not implemented in pyopensky. I think it is possible that. Please file an issue at the following link. I will take a look at possible solutions.
github.com/junzis/pyopensky/issues

BTW, some ground vehicles do have squawk codes... so we have to approach it differently.