Ensure tables with masked and unmasked columns roundtrip properly

astropy/io/fits/convenience.py

@@ -541,12 +541,13 @@
             # Binary FITS tables support TNULL *only* for integer data columns
             # TODO: Determine a schema for handling non-integer masked columns
             # with non-default fill values in FITS (if at all possible).
+            # Be careful that we do not set null for columns that were not masked!
             int_formats = ("B", "I", "J", "K")
-            if not (col.format in int_formats or col.format.p_format in int_formats):
-                continue
-
-            fill_value = tarray[col.name].fill_value
-            col.null = fill_value.astype(int)
+            if (
+                col.format in int_formats or col.format.p_format in int_formats
+            ) and hasattr(table[col.name], "mask"):
+                fill_value = tarray[col.name].fill_value
+                col.null = fill_value.astype(int)
     else:
         table_hdu = BinTableHDU.from_columns(
             tarray, header=hdr, character_as_bytes=character_as_bytes

astropy/io/fits/tests/test_connect.py

@@ -21,7 +21,7 @@
     python_to_tdisp,
 )
 from astropy.io.tests.mixin_columns import compare_attrs, mixin_cols, serialized_names
-from astropy.table import Column, QTable, Table
+from astropy.table import Column, MaskedColumn, QTable, Table
 from astropy.table.table_helpers import simple_table
 from astropy.time import Time
 from astropy.units import allclose as quantity_allclose
@@ -1064,3 +1064,15 @@ def test_meta_not_modified(tmp_path):
 def test_is_fits_gh_14305():
     """Regression test for https://github.com/astropy/astropy/issues/14305"""
     assert not connect.is_fits("", "foo.bar", None)
+
+
+def test_keep_masked_state_integer_columns(tmp_path):
+    """Regression test for https://github.com/astropy/astropy/issues/15417"""
+    filename = tmp_path / "test_masked.fits"
+    t = Table([[1, 2], [1.5, 2.5]], names=["a", "b"])
+    t["c"] = MaskedColumn([1, 2], mask=[True, False])
+    t.write(filename)
+    tr = Table.read(filename)
+    assert not isinstance(tr["a"], MaskedColumn)
+    assert not isinstance(tr["b"], MaskedColumn)
+    assert isinstance(tr["c"], MaskedColumn)

docs/changes/io.fits/15473.bugfix.rst

@@ -0,0 +1,3 @@
+Ensure that tables written to FITS with both masked and unmasked columns
+roundtrip properly (previously, all integer columns would become masked
+if any column was masked).