I used the default value of numpy.histogram. They might have use these value as it is really fast and the result is very small. I do agree that this value almost never work for my cases. I need at least 100 bins. The only problem with using an automatic one is the time it can take on big data set. 'khune' and 'blocks' are  too slow. We might use 'scotts' or 'freedman' as default.