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df.loc[:, 'col'] returning a view, but df.loc[df.index, 'col'] returning a copy

See original GitHub issue

Code Sample, a copy-pastable example if possible

x = pd.DataFrame(zip(range(4), range(4)), columns=['a', 'b'])
print(x)
   a  b
0  0  0
1  1  1
2  2  2
3  3  3

q = x.loc[:, 'a']
q += 2
print(x)
   a  b
0  2  0
1  3  1
2  4  2
3  5  3

x = pd.DataFrame(zip(range(4), range(4)), columns=['a', 'b'])
print(x)
   a  b
0  0  0
1  1  1
2  2  2
3  3  3

q = x.loc[x.index, 'a']
q += 2
print(x)
   a  b
0  0  0
1  1  1
2  2  2
3  3  3

Problem description

[df.loc[:, ‘col’] returning a view, but df.loc[df.index, ‘col’] returning a copy, intended? how can I make sure it is returning a copy?]

Expected Output

I thought .loc[] was always returning a copy

pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Windows OS-release: 7 machine: AMD64 processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: None pandas: 0.18.1 nose: 1.3.7 pip: 8.1.2 setuptools: 27.2.0 Cython: 0.24.1 numpy: 1.11.1 scipy: 0.18.1 statsmodels: 0.6.1 xarray: None IPython: 5.1.0 sphinx: 1.4.6 patsy: 0.4.1 dateutil: 2.5.3 pytz: 2016.6.1 blosc: None bottleneck: 1.1.0 tables: 3.2.2 numexpr: 2.6.1 matplotlib: 1.5.3 openpyxl: 2.3.2 xlrd: 1.0.0 xlwt: 1.1.2 xlsxwriter: 0.9.3 lxml: 3.6.4 bs4: 4.5.1 html5lib: None httplib2: None apiclient: None sqlalchemy: 1.0.13 pymysql: None psycopg2: None jinja2: 2.8 boto: 2.42.0 pandas_datareader: None

Issue Analytics

State:
Created 7 years ago
Comments:5 (4 by maintainers)

Top GitHub Comments

1reaction

jrebackcommented, Mar 9, 2017

this is as expected (see https://github.com/pandas-dev/pandas/issues/6149), but see below.

df.loc[:, columns] is treated as df[columns] which may return a view.

df.loc[indexer, columns] also may return return a view, but almost always does not in practice.

yes if indexer is df.index we could treat this as the former situation (IOW the indexer is exactly equals to the index of the frame). (its just a indexer.equals(df.index) type of comparison.

So i’ll mark this as a compat issue if you’d do a pull-request would be great. This is actually a very small change, see https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexing.py#L503

0reactions

jorisvandenbosschecommented, Oct 30, 2020

As noted in the PRs for 1.1.4 that reverted the fix for this, we should actually investigate what the expected behaviour is, because it’s not clear that the test that was added in https://github.com/pandas-dev/pandas/pull/34996 is actually correct (cc @jbrockmendel)

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