Setting values on a copy of a slice from a DataFrame

Question

I have a small dataframe  say this one        Mass32      Mass44   12  0 576703    0 496159 13  0 576658    0 495832 14  0 576703    0 495398     15  0 576587    0 494786 16  0 576616    0 494473       I would like to have a rolling mean of column Mass32  so I do this   x  Mass32s     pandas rolling mean x Mass32  5  shift -2    It works as in I have a new column named Mass32s which contains what I expect it to contain but I also get the warning message      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 the caveats in the documentation    http   pandas pydata org pandas-docs stable indexing html indexing-view-versus-copy   I m wondering if there s a better way to do it  notably to avoid getting this warning message

User · Accepted Answer

This warning comes because your dataframe x is a copy of a slice. This is not easy to know why, but it has something to do with how you have come to the current state of it.

You can either create a proper dataframe out of x by doing

x = x.copy()

This will remove the warning, but it is not the proper way

You should be using the DataFrame.loc method, as the warning suggests, like this:

x.loc[:,'Mass32s'] = pandas.rolling_mean(x.Mass32, 5).shift(-2)

[python] Setting values on a copy of a slice from a DataFrame

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