传递什么Pandas数据类型以转换或应用于groupby

在尝试调试groupby函数应用程序时,有人建议我使用虚函数"查看正在传递的内容"到每个组的函数中.当然,我是游戏:

import numpy as np
import pandas as pd

np.random.seed(0) # so we can all play along at home

categories = list('abc')
categories = categories * 4
data_1 = np.random.randn(len(categories))
data_2 = np.random.randn(len(categories))

df = pd.DataFrame({'category': categories, 'data_1': data_1, 'data_2': data_2})

def f(x):
    print type(x)
    return x

print 'single column transform'
df.groupby(['category'])['data_1'].transform(f)
print '\n'

print 'single column (nested) transform'
df.groupby(['category'])[['data_1']].transform(f)
print '\n'

print 'multiple column transform'
df.groupby(['category'])[['data_1', 'data_2']].transform(f)

print '\n'
print '\n'

print 'single column apply'
df.groupby(['category'])['data_1'].apply(f)
print '\n'

print 'single column (nested) apply'
df.groupby(['category'])[['data_1']].apply(f)
print '\n'

print 'multiple column apply'
df.groupby(['category'])[['data_1', 'data_2']].apply(f)

这将以下内容放入我的标准输出中:

single column transform





single column (nested) transform







multiple column transform










single column apply





single column (nested) apply






multiple column apply

所以看起来像:

转变

单列:3 Series

单列(嵌套):2 Series和3DataFrame

多列:3 Series和3DataFrame

应用

单列:3 Series

单列(嵌套):4 DataFrame

多列:4 DataFrame

这里发生了什么？任何人都可以解释为什么这6个调用中的每一个都导致上面描述的一系列对象被传递给指定的函数？