Correct place for SettingWithCopyWarning?

[michaelaye]

I am really happy about the SettingWithCopyWarning but is this situation the right place to pop up? Because what I want to do seems to work, maybe I don't see the danger of a possible case?

In [19]: df.convert_objects().dtypes
Out[19]: 
hour      float64
minute    float64
second    float64
tb        float64
dtype: object

In [20]: df['hour'] = df.hour.astype('int')
-c:1: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_index,col_indexer] = value instead

In [21]: df.dtypes
Out[21]: 
hour        int64
minute    float64
second    float64
tb        float64
dtype: object

using master.

[jreback]

can you post a full transcript?

df.convert_objects() returns a copy (which is correct), but you seem to be using the original frame (and am not sure what/where that came from). It could be that one is a copy.

[jtratner]

but convert_objects() shouldn't have is_copy set, because you actually
want to use the final one.

[jreback]

it doesn't as it's a new object

[michaelaye]

So, yes, I created this dataframe as a slice from a bigger one, sps is a list with 3 dataframes:

df = sps[0]['hour minute second tb'.split()]

and I confirmed, that if I just to the type conversion in the big one, I don't get this warning.

But what if all I want to work with is this new dataframe, and I do lot's of other stuff with it, and then eventually I set some values in my copy, isn't that legitimate, if all I want is to work with this smaller copy?
Or is there a better way and I'm doing it wrong?

[jtratner]

Just use ix there and you're good.

I do think we're going to need some kind of method (eg mark_not_copy())
to remove the copy in the case where you want to work with the selected
DataFrame

[jreback]

there is

df._setitem_copy(False) or df.copy() which will do that

I could make .setitem_nocopy() a public method?

@michaelaye

the warning is exactly for the case above, e.g. when you do df[col][row] which implicity creates a copy
which are then trying to assign

[jtratner]

Better idea: just make is_copy a public property and let people handle it
themselves.

[jreback]

ok....#5658 should fix it up