Packing on an object-dtyped column may fail if items are not comparable

[hombit]

Bug report

Today @mi-dai and myself were working on packing SALT3 data into nested pandas. When we tried to run nf.add_nested(src, on='SNID', name='lc') it failed, because it happened that SNID column had object dtype and consisted of heterogeneous data: numbers and strings. A reproducible example:

from nested_pandas import NestedFrame

nf = NestedFrame({"SNID": [1, 'abc']})
nf.add_nested(NestedFrame({"SNID": [1, 1, 'abc', 'abc'], "x": [1, 2, 3, 4]}), name='lc', on='SNID')

It failed in our code, basically because this doesn't work in pandas:

nd.set_index('SNID').sort_index()

I'm not sure if it is our problem or not. We potentially may fix this use-case with catching the TypeError and trying to cast on column values to str. Or we can do nothing, because we wouldn't like to change user's data and fix pandas behavior.

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