BUG: `MultiIndex.append` now drops names if not the same

[attack68]

Pandas version checks

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Reproducible Example

m1 = pd.MultiIndex.from_tuples([("a", 1)], names=["str", "int"])
m2 = pd.MultiIndex.from_tuples([("b", 2)])

m1.append(m2)

Issue Description

On previous pandas versions the names arg from m1 was maintained when m2 was appended that had no names.

On 2.0.3 the names are dropped from the return. Caught this in tests of a package upgrade.

Is this a bug or was this intentional - I can see arguments for both cases?

Expected Behavior

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Installed Versions

Replace this line with the output of pd.show_versions()

[lukemanley]

I think this was introduced in #53697. This wasn't intentional, but it is now consistent with a regular index:

In [1]: import pandas as pd

In [2]: idx1 = pd.Index([1], name="foo")

In [3]: idx2 = pd.Index([2])

In [4]: idx1, idx2
Out[4]: (Index([1], dtype='int64', name='foo'), Index([2], dtype='int64'))

In [5]: idx1.append(idx2)
Out[5]: Index([1, 2], dtype='int64')

I'd be in favor of keeping Index and MultiIndex consistent and would probably lean towards the new behavior (but can see an argument for keeping the names)

[phofl]

Agree with @lukemanley

[attack68]

I agree both Index types should be consistent.

Is it the right behaviour that when an index which has no meta data is appended to an index with some meta data that that meta data gets destroyed?

In general, I consider an append as an operation which usually adds a smaller, possibly singular item, to a larger, more significant object in memory.

Im typing on a phone now and cant see what happens if we append an index with meta data to one without, or the outcome when both have meta data that conflict with other.

Personally I would prefer that meta data is preserved where possible and would raise on conflict.

[phofl]

append was the odd one out, all other methods already behaved like append now (union, intersection, ...).

Names are generally only propagated if they match.

Having None as names is not equivalent to no meta information, users can set names to None. This could be implemented properly through using no_default as the default, but I am personally not interested enough.

[attack68]

Having None as names is not equivalent to no meta information, users can set names to None. This could be implemented properly through using no_default as the default, but I am personally not interested enough.

Fair enough, I can see this as being a bit of nuisance to implement and maintain.

Prefer to leave this issue open if anyone else likes to add user perspectives.