BUG: Index does not inherit existing Index or DatatetimeIndex object …

[IamGianluca]

This PR is meant to fix issue #11193
The issue prevent, when creating a new Index object based on a passed Index or DatetimeIndex object, to inherit the existing Index name if a new name is not provided.

I've also created two unit test which test the new behaviour on both cases (passing an Index or DatetimeIndex object) and added a comment in the whatsnew documentation.

[max-sixty]

Great!

• This could test other classes (PeriodIndex, etc), both when they're called with their own constructor (pd.PeriodIndex(period_index), and with different constructors (pd.Index(period_index)).
• This could also update all the fields in _attributes, rather than just name (although it'd have to protect against differently typed Indexes having different attributes. In many cases it would be identical; a standard Index only has name in _attributes.)

I wonder if you could just call _infer_new from this (unmerged) PR: #11497, and that would take care of all this.

[IamGianluca]

I'll rebase on master and squash my commits. I should update my PR shortly.

[jreback]

just say creation, don't use inheriting, which has a different meaning

[jreback]

use double backticks around Index and DatetimeIndex

[jreback]

minor correction, pls rebase as I just merged #11497 which might impact this. ping when green.

[IamGianluca]

I've just rebased @jreback I'll add a second comment when the Travis CI build is completed.

[jreback]

just construct the expected index and compare

expected = self.create_index()
expected.name = 'foo'
tm.assert_index_equal(result, expected)

also check with a new passed name

[IamGianluca]

I can work on this. Please ignore the last PR.

[IamGianluca]

@jreback What about the case when the object passed to Index is a MultiIndex? I think this is a corner case and should be avoided in principle. As a workaround, I would write the unit test as follows, even though is not very elegant:

    expected = self.create_index()
    if not isinstance(expected, MultiIndex):
        expected.name = 'foo'
        result = pd.Index(expected)
        tm.assert_index_equal(result, expected)

        result = pd.Index(expected, name='bar')
        expected.name = 'bar'
        tm.assert_index_equal(result, expected)

Should Index raise an error when we try to pass a MultiIndex object?

[IamGianluca]

@jreback let me know if this needs further changes.

[jreback]

need a doc-note, and you are changing things which are not relevant. e.g. pls rebase on master first.

[jreback]

@IamGianluca can you update

[IamGianluca]

I've updated the unit test @jreback

[jreback]

merged via fc40dcc

thanks!