"Distance" (or angular magnitude) between two quaternions?

Find the difference quaternion qd = inverse(q1)*q2).

Than find the angle between q1 and q2 by angle = 2 * atan2(qd.vec().length(), qd.w()) // NOTE: signed

The "angle" here, is the angle of rotation from q1 to q2 by shortest arc.

Also you can use this lib function from pyquaternion. Quaternion.absolute_distance(q0, q1)