groupby.first() 和 groupby.head(1) 有什么区别？

Question

两者都返回每个组的第一行的数据框架。在阅读API参考时，它首先说"计算第一组值"，但是当两个输出并排查看时，我看不出主要区别。

我想念什么吗？

df = pd.DataFrame({'id' : [1,1,1,2,2,3,3,3,3,4,4,5,6,6,6,7,7],
                    'value'  : ["first","second","second","first",
                                "second","first","third","fourth",
                                "fifth","second","fifth","first",
                                "first","second","third","fourth","fifth"]})

第一个API

Answer 1

主要区别是 first()将跳至第一个非null值，而 head(1)不会。

如果我将np.nan放入您的示例：

df = pd.DataFrame({'id' : [1,1,1,2,2,3,3,3,3,4,4,5,6,6,6,7,7],
                   'value'  : [np.nan,"second","second","first",
                               "second","first","third","fourth",
                               "fifth","second","fifth","first",
                               "first","second","third","fourth","fifth"]})

然后我们有：

>>> df.groupby('id').head(1)
    id   value
0    1     NaN      # NaN is included
3    2   first
5    3   first
9    4  second
11   5   first
12   6   first
15   7  fourth
>>> df.groupby('id').first()
     value
id        
1   second          # NaN is skipped
2    first
3    first
4   second
5    first
6    first
7   fourth

（如您所见，head()重置索引。）