pandas 左对齐熊猫滚动对象

Left-align a pandas rolling object

提问by Alex

Using pandas 0.18.1, I'd like to take the rolling average of a one-column dataframe. Since version 0.18.0, this is done with rolling() objects. The default for these rolling objects is to be right-justified. There is a boolean argument you can pass, center=True, to align the rolling object to the center value, but there doesn't seem to be a way to left-align it. Here's an example:

使用 pandas 0.18.1，我想取一列数据框的滚动平均值。从 0.18.0 版本开始，这是通过滚动（）对象完成的。这些滚动对象的默认设置是右对齐。您可以传递一个布尔参数，center=True，将滚动对象与中心值对齐，但似乎没有办法将其左对齐。下面是一个例子：

df = pandas.DataFrame({'A': [2,3,6,8,20, 27]})
df
    A
0   2
1   3
2   6
3   8
4  20
5  27

The standard method automatically aligns to the right, so there's no value at the first two indecies with a window of size three:

标准方法会自动向右对齐，因此在窗口大小为 3 的前两个 indecies 处没有值：

 df.rolling(window=3).mean()
           A
0        NaN
1        NaN
2   3.666667
3   5.666667
4  11.333333
5  18.333333

We can center-align the operation like this:

我们可以像这样居中对齐操作：

df.rolling(window=3).mean(center=True)
           A
0        NaN
1   3.666667
2   5.666667
3  11.333333
4  18.333333
5        NaN

But what I'm looking for is this:

但我要找的是这个：

df.rolling(3).mean()
            A
 0   3.666667
 1   5.666667
 2  11.333333
 3  18.333333
 4        NaN
 5        NaN

I can accomplish this by doing it with the default right alignment, and then re-indexing it, or by reversing the order of the rows and then doing it "right-aligned" but these are work-arounds for what should be a straight-forward operation.

我可以通过使用默认的右对齐来实现这一点，然后重新索引它，或者通过反转行的顺序然后“右对齐”来完成，但这些是应该是直线的变通方法 -前向操作。

回答by jezrael

I think you can use shift:

我认为你可以使用shift：

a = df.rolling(window=3).mean().shift(-2)
print (a)
           A
0   3.666667
1   5.666667
2  11.333333
3  18.333333
4        NaN
5        NaN