False negative on SettingWithCopyWarning

[adamklein]

This is pandas 0.15.2

In [1]: from pandas import *

In [2]: df = DataFrame({"A" : [1,2,3,4,5], "B": [3.1, 4, 5, 6, 7]})

In [3]: df[["A", "B"]]["A"] = 5  # no warning - bug?!

In [4]: df
Out[4]:
A    B
0  1  3.1
1  2  4.0
2  3  5.0
3  4  6.0
4  5  7.0

In [5]: df[["A"]]["A"] = 5
C:\Anaconda\Scripts\ipython-script.py:1: SettingWithCopyWarning:
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
if __name__ == '__main__':

[jorisvandenbossche]

Confirmed with master

[jreback]

hmm, at least False Positives are over.

[OmerJog]

This is still an issue in master

[erfannariman]

What is the expected behaviour here?

[bersbersbers]

Here's another one:

import pandas as pd

index = pd.MultiIndex.from_product([["vars"], ["fruit", "color"]])
df = pd.DataFrame([["apple", "green"]], columns=index)

# No warning!
df.vars.color = "red"

# Still green!
print(df)

# No effect, but warning: A value is trying to be set on a copy of a slice ...
df.vars.color[0] = "red"

After df.vars.color = "red", color is not set to "red", and there's no warning.