Pandas SettingWithCopyWarning 解决方法求教

[youthfire]

想要修改其中一个序列

df['description'] = df['description'].str.upper().str[:30]

运行结果没问题，但总是提示

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

求教这个用.loc 的正确写法应该是什么呢？

[imn1]

用 str.slice()试试

[congeec]

df['description'] = df['description'].map(str.upper).map(lambda x: x[:30])

[lance6716]

df = df.copy() 然后再

[youthfire]

@lance6716 感谢，特地去了解了下 df.copy().

我的例子确实写的不太好，其实是我用另一个 cf 名 copy 了原来的 df，写的是 cf['description'] = cf['description'].str.upper().str[:30]，没了解 cf=df 不等于 df.copy，Stackflow 的解释更多些：

Because if you don't make a copy then the indices can still be manipulated elsewhere even if you assign the dataFrame to a different name.

For example:

df2 = df
func1(df2)
func2(df)
func1 can modify df by modifying df2, so to avoid that:

df2 = df.copy()
func1(df2)
func2(df)

也谢谢所有回答者。