计算每个pandas.DataFrame列的numpy.std？

我想得到我numpy.std的每一栏pandas.DataFrame.

这是我的代码:

import pandas as pd
import numpy as np

prices = pd.DataFrame([[-0.33333333, -0.25343423, -0.1666666667],
                       [+0.23432323, +0.14285714, -0.0769230769],
                       [+0.42857143, +0.07692308, +0.1818181818]])

print(pd.DataFrame(prices.std(axis=0)))

这是我的代码输出:

pd.DataFrame([[ 0.39590933],
              [ 0.21234018],
              [ 0.1809432 ]])

这是正确的输出(如果计算np.std)

pd.DataFrame([[ 0.32325862],
              [ 0.17337503],
              [ 0.1477395 ]])

为什么我有这样的差异？我该如何解决这个问题？

注意:我试过这样做:

print(np.std(prices, axis=0))

但我有以下错误:

Traceback (most recent call last):
  File "C:\Users\*****\Documents\******\******\****.py", line 10, in 
    print(np.std(prices, axis=0))
  File "C:\Python33\lib\site-packages\numpy\core\fromnumeric.py", line 2812, in std
    return std(axis=axis, dtype=dtype, out=out, ddof=ddof)
TypeError: std() got an unexpected keyword argument 'dtype'

谢谢!