(完全解决)Dataframe的赋值问题SettingWithCopyWarning: A value is trying to be set on a copy of a slice
情况
sample.loc[0][1]=-1
发出警告:
其实,这个根本不是警告,而是报错.因为我发现sample的值根本没有改变.
原因
不要使用切片的方式去赋值.上面其实可以分为两步,你这种索引方式pandas叫做chained indexing(链式索引),相当于
x=sample.loc[0]#得到series,速度慢
x[1]=-1
正确做法是:
sample.loc[0,1]=-1
这种做法不但pandas内部只需要执行一步索引,速度更加快了,而且还允许赋值.
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