BUG: SettingWithCopyWarning

[iliya-malecki]

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Reproducible Example

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 caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  available_data['loading'] = 1

Issue Description

Sorry but for the life of me i cant create a reproducible example. It is just a dataframe that was created using maindf.query('some_time > @todays_datetime')
available_data.values.base is maindf.values.base # False

Expected Behavior

not have the warning

Installed Versions

INSTALLED VERSIONS

commit : 5f648bf
python : 3.8.5.final.0
python-bits : 64
OS : Linux
OS-release : 5.4.0-1051-azure
Version : #53~18.04.1-Ubuntu SMP Fri Jun 18 22:32:58 UTC 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.3.2
numpy : 1.21.2
pytz : 2021.1
dateutil : 2.8.2
pip : 21.2.2
setuptools : 52.0.0.post20210125
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.1
IPython : 7.27.0
pandas_datareader: None
bs4 : None
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : 3.4.3
numexpr : None
odfpy : None
openpyxl : 3.0.9
pandas_gbq : None
pyarrow : 3.0.0
pyxlsb : None
s3fs : None
scipy : 1.7.1
sqlalchemy : 1.4.35
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
numba : None

[rhshadrach]

Here is what I believe is happening:

df = pd.DataFrame({'a': [1, 2, 3]})
df2 = df.query('a > 1')
df2['a'] = 5

The method .query may return a view or copy; it will certainly return a view when the condition always holds (e.g. replace a > 1 with a > 0 in the example above). When it makes a copy and then you are setting new values on that copy, pandas is generating this warning in case you meant to modify the original (df in the example above) instead. To silence the warning, you can copy after doing the query: df2 = df.query('a > 1').copy().

Does this resolve the issue?

[iliya-malecki]

@rhshadrach thank you, .copy() helped!
I have a couple of questions now:

1. Why is it a valid idea to return a view from .query()? What is the intention here? To me, it looks like a dangerous feature to have - conditionally ruining an original dataframe. I obviously dont understand what is happening under the hood, so i can only think about usage.
2. Is it a valid way to check whether a dataframe is a view or copy? available_data.values.base is maindf.values.base

[mroeschke]

1. query tries to follow numpy indexing semantics which can also return a view or copy depending on the indexing operation
2. You can use np.shares_memory to also see if the underlying data shares memory. https://numpy.org/doc/stable/reference/generated/numpy.shares_memory.html

Closing as this is not a bug rather a usage question