What is the point of views in pandas if it is undefined whether an indexing operation returns a view or a copy?

Question

I have switched from R to pandas. I routinely get SettingWithCopyWarnings, when I do something like

df_a = pd.DataFram         


        

Answer 1

I agree this is a bit funny. My current practice is to look for a "functional" method for whatever I want to do (in my experience these almost always exist with the exception of renaming columns and series). Sometimes it makes the code more elegant, sometimes it makes it worse (I don't like assign with lambda), but at least I don't have to worry about mutability.

So for indexing, instead of using the slice notation, you can use query which will return a copy by default:

In [5]: df_a.query('col1 > 1')
Out[5]:
   col1
1     2
2     3
3     4

I expand on it a little in this blog post.

Edit: As raised in the comments, it looks like I'm wrong about query returning a copy by default, however if you use the assign style, then assign will make a copy before returning your result, and you're all good:

df_b = (df_a.query('col1 > 1')
            .assign(newcol = 2*df_a['col1']))