BUG/API: preserve dtype in Index append()

[jorisvandenbossche]

If you combine two Index objects using append, if they are both object dtype to start with, it seems we still infer the dtype for the result instead of preserving object dtype:

idx1 = pd.Index(["b", "a"], dtype=object)
idx2 = pd.Index(["c", "d"], dtype=object)

>>> idx1.append(idx2)
Index(['b', 'a', 'c', 'd'], dtype='object')

>>> pd.options.future.infer_string = True
>>> idx1.append(idx2)
Index(['b', 'a', 'c', 'd'], dtype='str')    # <-- upcast to string

I would expect that to preserve the dtype of the calling / passed Index (or at least its "common" dtype, which in this case clearly is object dtype)

[rhshadrach]

Hopefully a rare case, but what about the behavior of:

idx1 = pd.Index([1, 2], dtype=object)
idx2 = pd.Index([3, 4])
print(idx1.append(idx2))

I would think this should also be object, as otherwise we get values-dependent behavior.

[Swati-Sneha]

take

[jorisvandenbossche]

I would think this should also be object, as otherwise we get values-dependent behavior.

Indeed, I would also expect object dtype in that case