I have a dataset where the classes are unbalanced. The classes are either '1' or '0' where the ratio of class '1':'0' is 5:1. How do you calculate the prediction error for each class and the rebalance weights accordingly in sklearn with Random Forest, kind of like in the following link: http://www.stat.berkeley.edu/~breiman/RandomForests/cc_home.htm#balance
For your problem, suppose if 1 class is represented 5 times, as 0 class is, and you balance classes distributions, then simply use:
sample_weight = np.array([5 if i == 0 else 1 for i in y])
It will assign a weight of 5 to all 0 instances and weight of 1 to all 1 instances.
You can learn more about Random Forest and its working parameters from this blog. This blog explains the code with scikit learn to help easy understanding.
Hope this answer helps.