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DataFrame。当读取API引用时，它说首先“计算第一组值”，但是当并排查看这两个输出时，我看不出有什么大的区别。

我是不是遗漏了什么？

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API

Answer 1

主要的区别是first()将跳过第一个非空值，而head(1)则不会。

如果我将np.nan放到您的示例中：

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

(而且，正如您所看到的，head()重置索引。)