I have a pandas dataframe: data. it has columns ["name", 'A', 'B']
What I want to do (and works) is:
d2 = data[data['name'] == 'fred'] #This gives me multiple rows d2['A'] = 0
This will set the column A on the fred rows to 0. I've also done:
indexes = d2.index data['A'][indexes] = 0
However, both give me the same warning:
/Users/brianp/work/cyan/venv/lib/python2.7/site-packages/pandas/core/indexing.py:128: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
How does pandas WANT me to do this?
To slice the columns, the syntax is df. loc[:,start:stop:step] ; where start is the name of the first column to take, stop is the name of the last column to take, and step as the number of indices to advance after each extraction; for example, you can select alternate columns.
This is a very common warning from pandas. It means you are writing in a copy slice, not the original data so it might not apply to the original columns due to confusing chained assignment. Please read this post. It has detailed discussion on this SettingWithCopyWarning
. In your case I think you can try
data.loc[data['name'] == 'fred', 'A'] = 0
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