BUG: indexing a QTable with a Quantity column doesn't work as expected

[neutrinoceros]

import astropy.units as u
from astropy.table import QTable

t = QTable({"a": u.Quantity([0], unit="m")})
t.add_index(["a"])
t.loc[t["a"][0]]

fails with

Traceback (most recent call last):
  File "/Users/clm/dev/astropy-project/coordinated/astropy/t.py", line 6, in <module>
    t.loc[t["a"][0]]
    ~~~~~^^^^^^^^^^^
  File "/Users/clm/dev/astropy-project/coordinated/astropy/astropy/table/index.py", line 888, in __getitem__
    rows = self._get_rows(item)
           ^^^^^^^^^^^^^^^^^^^^
  File "/Users/clm/dev/astropy-project/coordinated/astropy/astropy/table/index.py", line 867, in _get_rows
    for key in item:
  File "/Users/clm/dev/astropy-project/coordinated/astropy/astropy/units/quantity.py", line 1283, in __iter__
    raise TypeError(
TypeError: 'Quantity' object with a scalar value is not iterable

In contrast, the following works

import astropy.units as u
from astropy.table import QTable

t = QTable({"a": u.Quantity([0], unit="m")})
t.add_index(["a"])
t.loc[t["a"][0].value]  # passing `.value` instead of the full quantity

Using a Table instead of a QTable also works as expected

import astropy.units as u
from astropy.table import Table

t = Table({"a": u.Quantity([0], unit="m")})
t.add_index(["a"])
t.loc[t["a"][0]]

I'm not 100% sure the current behaviour is actually buggy: this is a matter of intent.
This was first discussed in #15826 where @taldcroft found that multi-indexing wouldn't work seamlessly for Quantity indices, as he assumed.

This report is adapted from my comment #15826 (comment)_

[mhvk]

Note that for non-scalar input, the error is different,

t = QTable({"a": u.Quantity([20., 21., 22.], unit="m")})
t.add_index(["a"])
t.loc[t["a"][1:]]
# KeyError: 'No matches found for key 21.0 m'

[mhvk]

Actually, the cause is easy (though maybe a nice fix less so): the index uses a regular Table to store the index columns, which means that Quantity get converted to Column instances, and therefore the look-up doesn't work: see

astropy/astropy/table/index.py

Lines 93 to 96 in caf9a1a

	elif len(columns) == 1:
	col = columns[0]
	row_index = Column(col.argsort(kind="stable"))
	data = Table([col[row_index]])

If I change Table to QTable there, the example with a slice works fine (note there is another explicit Table on l.113)

The scalar still does not work, but can also be made to work by changing

astropy/astropy/table/index.py

Line 852 in caf9a1a

if not isinstance(item, (list, np.ndarray)): # single element

to

            if not isiterable(item):  # single element

(with a from astropy.utils import isiterable on top).

For an implementation, possibly easiest is not to pass in columns to Index but rather a Table/QTable with the desired columns (since the columns are going to be added to a table anyway).

[neutrinoceros]

For an implementation, possibly easiest is not to pass in columns to Index but rather a Table/QTable with the desired columns (since the columns are going to be added to a table anyway).

I think it's feasible but it isn't as easy as I first thought. I'm game to try it out again if no objection is raised !

[mhvk]

Sure!