DataFrame groupby().apply() 函数似乎被调用了两次 [英] DataFrame groupby().apply() function seems to be called twice

问题描述

这是一个简单的数据帧:

Here is a simple DataFrame:

import numpy as np
dd=pd.DataFrame(np.arange(35).reshape(7,5), columns=list('xyzwv'))
dd['w']=list('AABBBCC')

，即:

现在我试试下面的代码

def func(x):
    print(x)
    return x

dd.groupby('w').apply(func)

然后打印出来:

我认为出了问题，因为 正在打印两次.

I think something goes wrong because is being printed twice.

看起来好像为同一组调用了 func() 两次.我做错了什么?

It looks as if func() is being called twice for the same group. What mistake did I do?

推荐答案

apply 调用第一组两次以确定是否可以做一些优化，参见http://pandas.pydata.org/pandas-docs/stable/groupby.html#flexible-apply

apply calls the first group twice to work out whether it could do some optimizations, see http://pandas.pydata.org/pandas-docs/stable/groupby.html#flexible-apply

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