BUG: Creating Index name using `names` names argument, doesn't set index name

[jsnowacki]

Code Sample, a copy-pastable example if possible

# This is true
i = pd.Index([], name='a')
assert i.name == 'a'
assert i.names == ('a',)

# This is false (fails assertion)
i = pd.Index([], names=('a',))
assert i.name == 'a'
assert i.names == ('a',)

Problem description

Setting Index name using name parameter works correctly, but using names parameter doesn;t do anything, i.e. i.name is None. While the documentation doesn't list names as a parameter, since it is an attribute, it gets listed as a parameter, e.g. in IDE.

Expected Output

# This is true
i = pd.Index([], name='a')
assert i.name == 'a'
assert i.names == ('a',)

# This is also true
i = pd.Index([], names=('a',))
assert i.name == 'a'
assert i.names == ('a',)

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.3.final.0 python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: None.None

pandas: 0.22.0
pytest: 3.3.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 0.19.1
pyarrow: 0.7.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: 0.1.3
pandas_gbq: 0.2.1
pandas_datareader: 0.5.0

[TomAugspurger]

I think this is accommodate the fact that Index(...) may return a MultiIndex

In [9]: pd.Index([('a', 'b')], names=['a', 'b'])
Out[9]:
MultiIndex(levels=[['a'], ['b']],
           labels=[[0], [0]],
           names=['a', 'b'])

It'd be good to document that, and perhaps add it to the signature? That may be more trouble than it's worth if it breaks other things.

[spacesphere]

As I can see, names is used only when trying to create MultiIndex, otherwise it's just ignored.
There's the set_names method that can be used instead of plain assignment to the name attribute when creating new Index object to avoid described behaviour. It doesn't seem to break other things.

[TomAugspurger]

@PoppyBagel, thanks. Interested in making a PR adding and documenting names?

[spacesphere]

@TomAugspurger, if you don't mind I could work on this issue.
But there's still a question: should names be added as an explicit parameter to the Index constructor or it may be left in optional kwargs? I think it's too specific case to change the signature (if I got your previous comment right).

[TomAugspurger]

If possible, it'd be nice to have it be a regular keyword just like `name`. It will just go at the end of the current arguments, and we'll document that it's only relevant for `data` that result in a MultiIndex.

…

On Mon, Jan 8, 2018 at 12:38 PM, Kate ***@***.***> wrote: @TomAugspurger <https://github.com/tomaugspurger>, if you don't mind I could work on this issue. But there's still a question: should names be added as an explicit parameter to the Index constructor or it may be left in optional kwargs? I think it's too specific case to change the signature (if I got your previous comment right). — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#19082 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABQHIjcoZnodlmFAUm6U7hjeQLB_ohoQks5tImClgaJpZM4RTgJg> .