Graphlab SFrame构造函数将NumPy S1 dtype转换为int

Question

以下是一个例子：

import graphlab as gl
import numpy.random as npr    
npr.choice(['a', 'b', 'c', 'd'], size = 100, replace=True).dtype
df_so = gl.SFrame({"col1": npr.randint(low = 1, high = 3, size = 100), 
               "col2": npr.randint(low = 1, high = 10, size = 100), 
               "col3": npr.choice(['a', 'b', 'c', 'd'], size = 100, replace=True)})
df_so.column_types

我想了解：

• 为什么？
• 如果有办法防止这种情况发生。