Python Pandas警告：试图在DataFrame的切片副本上设置值

Question

我有一个Pandas DataFrame，我想使用以下代码更改一列中的所有值：

df["Population"] = round(df["Population"]/1000000,1)

我收到了以下警告：

A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  return super().rename(
<ipython-input-6-59bf041bb022>:2: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  df["Population"] = round(df["Population"]/1000000,1)

如何正确完成并避免此类警告？

感谢您的帮助！

- idejuan

Answer 1

在你获得 df 之前，它是另一个数据框的子集

df = alldf[cond].copy()

或者我们尝试assign

df = df.assign(Population = round(df["Population"]/1000000,1))

- BENY

---

谢谢。你建议的第二个解决方案已经起作用了。不过，我已经找到了为什么一开始会出现错误的原因。我在一个副本数据框上工作，因为在前一行中我将df分配给了一个副本。我通过这篇文章 https://dev59.com/GFsX5IYBdhLWcg3wf_v3 找到了解决方案。 - idejuan

这是一个有价值的资源，可以帮助理解Pandas中的SettingWithCopyWarning问题：https://towardsdatascience.com/explaining-the-settingwithcopywarning-in-pandas-ebc19d799d25 - idejuan