SettingWithCopyWarning for string manipulation in dataframes with a non-monotonic index

[egrublyte]

Hi,

I get a SettingWithCopyWarning for string manipulation in dataframes with a non-monotonic index.

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

This only happens if the integer index skips a value. The code to reproduce:

from pandas import DataFrame
# create example dataframe:
df = DataFrame ({'column1':['a', 'a', 'a'], 'column2': [4,8,9] })
df
# assign string to 'column1':
df['column1'] = df['column1'] + 'b'
df
# it works just fine - no warnings
#now remove one line from dataframe df:
df = df [df['column2']!=8]
df
# adding string to 'column1' gives warning:
df['column1'] = df['column1'] + 'c'
df

NB: this only happens after a fresh login into python; if you run it again, it will not appear any more (!)

System:

• Pandas 0.13
• Python 2.7.2
• Ubuntu 12.04LTS 64bit

Is this is an actual issue or just a nonsensical warning?

Kind regards,
Egle

[jreback]

this is a correct warning (though the message in this case is a bit inapplicable).

You are in fact setting a copy.

You prob don't care; it is mainly to address situations like:

df['foo'][0] = 123... which sets the copy (and thus is not visible to the user)

This operation, make the df now point to a copy of the original

df = df [df['column2']!=8]

If you don't care about the 'original' frame, then its ok

If you are expecting that the

df['column1'] = df['columns'] + 'c'

would actually set the original frame (they are both called 'df' here which is confusing)
then you would be suprised.

You can completly turn off this warning by setting pd.set_option('chained_assignement',None)
(this warning is mainly for new users to avoid setting the copy)

or you could do:

df.is_copy = False (after the indexing operation)

Note that when you have the same index as the original frame, this DOES NOT create a copy, so no warning is generated, it is only because of the indexing which ultimately calls take which causes a copy.

Copies don't normally matter except when you are then trying to set them in a chained manner.

[egrublyte]

Thank you for the detailed explanation!