熊猫-另一种SettingWithCopyWarning：我正在使用.loc替换特定值，但仍收到警告

Question

I thought I followed the best practice:

import pandas as pd
df=pd.DataFrame({'cond':['a', 'b', 'a'], 'value':[1.5, 2.5, 3.5]})

mask = df.cond == 'a'
df.loc[mask, 'value'] = df.loc[mask, 'value'] * (-1)

In essence, is condition is met, replace a value with it's negative. What is the better way to perform this operation so I do not trigger the warning?

Answer 1

The problem turns out not to be with the code as shown. That code example worked fine.

In the real world, df is a created as follows: df = df2[['cond', 'value']] . adding .copy() to this line, fixed the warning.

This solution was pointed our by @jezrael in the comments.