Python Pandas 警告：尝试在 DataFrame 切片的副本上设置值

Question

我有一个 Pandas DataFrame，我想用以下代码更改列的所有值：

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

我收到以下警告：

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)

正确的做法是什么并避免此类警告？

感谢您的帮助！

Answer 1

在你拥有 df 之前，它是其他一些数据帧的子集

df = alldf[cond].copy()

或者我们尝试assign

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