Correct way to set value on a slice in pandas

Question

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?

Answer 1

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