在Python dataframe中计算每个客户的趋势

我使用下面的函数来计算每个客户的流出量的趋势(斜率)。然而，对于整个数据集，我得到了相同的数字。是否有一种方法可以迭代数据集中的每个客户id并获得每个客户的个别趋势?提前谢谢!

import numpy as np

def trendline(data, order=1):
    coeffs = np.polyfit(data.index.values, list(data), order)
    slope = coeffs[-2]
    return float(slope)
outflow = payment['AMOUNT'] 
month = payment['MONTH']

df_ = pd.DataFrame({'month': month, 'outflow': outflow})
slope_outflow = trendline(df_['outflow'])
slope_outflow

问题来源StackOverflow 地址：/questions/59382063/calculating-trend-per-customer-in-python-dataframe