在pandas库中使用.apply并获得“np不可调用”错误

我试图应用我的色散函数:

def disp_calc(returns, p, wi):    #apply(disp_calc, rows = ...)
    wi = wi/np.sum(wi)
    ri_p = returns_stock.subtract(p,axis=0)    
    rp = np.nansum(wi*((ri_p)**2))
    return np.sqrt(rp)

for i in sectors: 
    returns_sector = returns[sectordict[i]]                  
    pr = returns[sectordict[i]].apply(np.mean,axis=1)         
    w = newmerge.Weight[newmerge.SectorSymbol == i]        
    disp_df[date] = returns_sector.apply(disp_calc(returns_sector,pr,w),axis=1) 

我一直得到 error

TypeError: ("'numpy.float64' object is not callable", u'occurred at index 2017-01-03 00:00:00')

returns_sector df .