运行Pandas代码触发SettingWithCopyWarning：此为何属于链式索引？

运行以下Pandas代码时，触发了SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame警告：

import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randn(5, 3),
                  columns=['A', 'B', 'C'])
df2 = df[(df.A > 0) | (df.B > 0)]
df2.at[-1, 'C'] = df['C'].iloc[-1]

请问这段代码里的链式索引体现在哪里？

问题更新 #

后来发现是误用了-1索引，它并没有按预期返回最后一行。将代码替换为df2.at[df2.index[-1], 'C'] = df['C'].iloc[-1]后，警告就消失了。

内容的提问来源于stack exchange，提问作者Viktor