Python Pandas 警告：試圖在 DataFrame 的切片副本上設置值

Question

我有一個 Pandas DataFrame，我想使用以下代碼更改列的所有值：

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)

這樣做並避免此類警告的正確方法是什么？

感謝您的幫助！

Answer 1

在您擁有 df 之前，它是其他一些數據幀的子集

df = alldf[cond].copy()

或者我們嘗試assign

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