Astropy Table Drops Mask When Adding Structured Array

You are working in the astropy/astropy repository at commit 6ed769d58d89380ebaa1ef52b300691eefda8928.

Problem

When you add a masked structured numpy array as a column to an astropy.table.Table, the mask is silently discarded. The column ends up stored as an NdarrayMixin — a plain ndarray wrapper — instead of a MaskedColumn, so all mask information is lost.

Reproduction

import numpy as np
import numpy.ma as ma
from astropy.table import Table

# Create a structured masked array
data = ma.MaskedArray(
    data=np.array([(1, 2.0), (3, 4.0)], dtype=[("x", int), ("y", float)]),
    mask=np.array([(True, False), (False, True)], dtype=[("x", bool), ("y", bool)]),
)

t = Table()
t["col"] = data

# Bug: the mask is gone
print(type(t["col"]))         # <class 'astropy.table.NdarrayMixin'>  (wrong)
print(t["col"].data.mask)     # AttributeError or all-False            (wrong)

After the fix the column should be stored as a MaskedColumn and the mask should be preserved faithfully.

Expected Behavior

import numpy as np
import numpy.ma as ma
from astropy.table import Table

data = ma.MaskedArray(
    data=np.array([(1, 2.0), (3, 4.0)], dtype=[("x", int), ("y", float)]),
    mask=np.array([(True, False), (False, True)], dtype=[("x", bool), ("y", bool)]),
)

t = Table()
t["col"] = data

assert isinstance(t["col"], MaskedColumn)
assert t["col"].data.mask[0]["x"] == True
assert t["col"].data.mask[1]["y"] == True

For non-masked structured arrays the current behavior (automatic wrapping as NdarrayMixin) must be preserved — that is the case covered by test_ndarray_mixin[True] and it must keep passing.

Requirements / Interface

The fix must be contained entirely in the astropy/table/ subdirectory, specifically in one or both of:

• •astropy/table/column.py
• •astropy/table/table.py

The relevant code path is the block that auto-converts a structured numpy ndarray to NdarrayMixin. You need to add a conditional branch so that when the incoming structured array has a mask (i.e. isinstance(col, np.ma.MaskedArray)), it is stored as a MaskedColumn instead.

No changes to test files are required; the hidden grader will inject its own test patch.

Examples

Non-masked structured array — behavior unchanged (NdarrayMixin)

import numpy as np
from astropy.table import Table

plain = np.array([(1, 2.0), (3, 4.0)], dtype=[("x", int), ("y", float)])
t = Table()
t["plain"] = plain
# still NdarrayMixin — do NOT change this
from astropy.table import NdarrayMixin
assert isinstance(t["plain"], NdarrayMixin)

Masked structured array — NEW behavior (MaskedColumn)

import numpy as np
import numpy.ma as ma
from astropy.table import Table, MaskedColumn

data = ma.MaskedArray(
    data=np.array([(10, 20), (30, 40)], dtype=[("a", int), ("b", int)]),
    mask=np.array([(False, True), (True, False)], dtype=[("a", bool), ("b", bool)]),
)
t = Table()
t["s"] = data
assert isinstance(t["s"], MaskedColumn)
# mask survives round-trip
assert bool(t["s"].data.mask[0]["b"]) is True
assert bool(t["s"].data.mask[1]["a"]) is True

Constraints

• •Your patch must not change the behavior for non-masked structured arrays.
• •Do not modify astropy/table/tests/ — the grader injects its own test file.
• •All existing tests in astropy/table/tests/test_mixin.py and astropy/table/tests/test_table.py must remain green.
• •Standard library and numpy/astropy internals only; no new dependencies.
• •Python 3.8+ compatible.

Scoring

Your submission is accepted if:

1. astropy/table/tests/test_mixin.py::test_ndarray_mixin[False] transitions from FAIL to PASS.
2. astropy/table/tests/test_table.py::test_structured_masked_column transitions from FAIL to PASS.
3. All listed pass-to-pass regression tests remain green.

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