Is it safe to practice karate when i still have a cast on my hand?

Depends: Upon what the injury is . A non displaced fracture towards the end of healing or a recently operated upon ligament repair with pins in? The former may allow it but not the ligament repair! and by practice do you mean breaking cinder blocks or doing flexibility exercises. So there can be quite a range. Best talk to your doctor about specifics relevant to your injury.

Yes: As long as you are not doing striking and practicing flexibility and forms, you should be fine. However, its probably best to discuss this with the doctor that put the cast on since they are most familiar with your specific diagnosis.