BUG: reset_index can reset index name

[p-himik]

import pandas as pd
assert (pd.DataFrame(index=pd.Index([], name='x'))
        .reset_index(level=[]).index.name) is None

The root of the problem is in pandas.core.common._default_index:

def _default_index(n):
    from pandas.core.index import RangeIndex
    return RangeIndex(0, n, name=None)

Probably, a new optional argument name can be introduced to _default_index and used in reset_index.

[gfyoung]

@p-himik : Thanks for the issue! Indeed this is a bug, for the documentation says the following:

"For a standard index, the index name will be used (if set), otherwise a default ‘index’ or ‘level_0’ (if ‘index’ is already taken) will be used."

Here's a similar bug that should also be pointed out:

s = pd.Series([1, 2, 3])
s.index.name = "foo"

s.index += 1
assert s.index.name == "foo"

The index name magically disappears like in your example (probably linked to the code you pointed out). PR is welcome to patch these issues!

[hurcy]

I would like to working on this issue.

[gfyoung]

@hurcy : By all means! You can read the documentation here about contributing. Let us know if you have any questions about the code or contributing.

[jreback]

In [16]: pd.DataFrame(index=pd.Index([], name='x')).reset_index(level=[]).index.name is None
Out[16]: True

In [17]: pd.__version__
Out[17]: '0.21.0.dev+371.g0ee1675'

The original assertion is already true in master. I think the issue is that level=[] is actually an invalid specification.

@chris-b1 @jorisvandenbossche

[hurcy]

Then should I remove the test case? 2017. 8. 18. 오전 9:51 Jeff Reback <notifications@github.com> 작성:

…

In [16]: pd.DataFrame(index=pd.Index([], name='x')).reset_index(level=[]).index.name is None Out[16]: True In [17]: pd.__version__ Out[17]: '0.21.0.dev+371.g0ee1675' The original assertion is already true in master. I think the issue is that level=[] is actually an invalid specification. @chris-b1 @jorisvandenbossche — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub, or mute the thread.

[minggli]

Hello, is this issue still open for a fix? IMHO, reset_index without an index name is not necessarily a bad thing. Do we actually want to keep index name during reset_index()?