Almost all packages depend on the GIL (at least at first).
They assume they can "take the GIL", and then go and look at the various Python datastructures you passed them without worrying about them changing as they are being read.
The later, when writing out their answer (which might involve editing something they were given), they can assume they are not changing. For example, you could write code which extends the length of a list to 100, then fill in all the members, not worrying half way through your loop another thread shrinks the list back down.
