Add method __astropy_table__

Allows for ``Table(cosmo)``.

Signed-off-by: Nathaniel Starkman (@nstarman) <nstarkman@protonmail.com>

astropy/cosmology/core.py

@@ -428,6 +428,26 @@ def __repr__(self):
 
         return namelead + ", ".join(fmtps) + ")"
 
+    def __astropy_table__(self, cls, copy, **kwargs):
+        """Return a `~astropy.table.Table` of type ``cls``.
+
+        Parameters
+        ----------
+        cls : type
+            Astropy ``Table`` class or subclass.
+        copy : bool
+            Ignored.
+        **kwargs : dict, optional
+            Additional keyword arguments. Passed to ``self.to_format()``.
+            See ``Cosmology.to_format.help("astropy.table")`` for allowed kwargs.
+
+        Returns
+        -------
+        `astropy.table.Table` or subclass instance
+            Instance of type ``cls``.
+        """
+        return self.to_format("astropy.table", cls=cls, **kwargs)
+
 
 class FlatCosmologyMixin(metaclass=abc.ABCMeta):
     """

astropy/cosmology/io/tests/test_table.py

@@ -110,8 +110,9 @@ def test_to_from_table_instance(self, cosmo, to_format, from_format):
         got = from_format(tbl, format="astropy.table")
         assert got == cosmo
 
-        # and it will also work if the cosmology is a string
-        tbl.meta["cosmology"] = _COSMOLOGY_CLASSES[tbl.meta["cosmology"]].__qualname__
+        # and it will also work if the cosmology is a class
+        # Note this is not the default output of ``to_format``.
+        tbl.meta["cosmology"] = _COSMOLOGY_CLASSES[tbl.meta["cosmology"]]
         got = from_format(tbl, format="astropy.table")
         assert got == cosmo
 
@@ -194,4 +195,4 @@ class TestToFromTable(ToFromFormatTestBase, ToFromTableTestMixin):
     """
 
     def setup_class(self):
-        self.io_functions = {"to": to_table, "from": from_table}
+        self.functions = {"to": to_table, "from": from_table}

astropy/cosmology/tests/test_core.py

@@ -306,6 +306,30 @@ def test_is_equivalent(self, cosmo):
         # different class
         assert not cosmo.is_equivalent(2)
 
+    # ------------------------------------------------
+
+    @pytest.mark.parametrize("in_meta", [True, False])
+    @pytest.mark.parametrize("table_cls", [Table, QTable])
+    def test_astropy_table(self, cosmo, table_cls, in_meta):
+        """Test ``astropy.table.Table(cosmology)``."""
+        tbl = table_cls(cosmo, cosmology_in_meta=in_meta)
+
+        assert isinstance(tbl, table_cls)
+        # the name & all parameters are columns
+        for n in ("name", *cosmo.__parameters__):
+            assert n in tbl.colnames
+            assert all(tbl[n] == getattr(cosmo, n))
+        # check if Cosmology is in metadata or a column
+        if in_meta:
+            assert tbl.meta["cosmology"] == cosmo.__class__.__qualname__
+            assert "cosmology" not in tbl.colnames
+        else:
+            assert "cosmology" not in tbl.meta
+            assert tbl["cosmology"][0] == cosmo.__class__.__qualname__
+        # the metadata is transferred
+        for k, v in cosmo.meta.items():
+            assert np.all(tbl.meta[k] == v)
+
 
 class CosmologySubclassTest(TestCosmology):
     """

docs/changes/cosmology/12213.feature.rst

@@ -1,2 +1,3 @@
 Register Astropy Table into Cosmology's ``to/from_format`` I/O, allowing
 a Cosmology instance to be parsed from or converted to a Table instance.
+Also adds the ``__astropy_table__`` method allowing ``Table(cosmology)``.

docs/cosmology/io.rst

@@ -142,8 +142,8 @@ to the ``Planck18`` cosmology from which it was created.
 
 .. EXAMPLE START: Planck18 to QTable and back
 
-    Another pre-registered format is "table", for converting a |Cosmology| to
-    and from a |QTable|.
+Another pre-registered format is "table", for converting a |Cosmology| to
+and from a |QTable|.
 
 .. code-block::
 
@@ -156,6 +156,21 @@ to the ``Planck18`` cosmology from which it was created.
     -------- ------------ ------- ------- ------- ----------- -------
     Planck18        67.66 0.30966  2.7255   3.046 0.0 .. 0.06 0.04897
 
+Cosmology supports the astropy Table-like protocol (see :ref:`Table-like Objects`)
+to the same effect:
+
+.. code-block::
+
+    >>> from astropy.table import QTable
+    >>> ct = QTable(Planck18)
+    >>> ct
+    <QTable length=1>
+      name        H0        Om0    Tcmb0    Neff    m_nu [3]    Ob0
+             km / (Mpc s)            K                 eV
+      str8     float64    float64 float64 float64   float64   float64
+    -------- ------------ ------- ------- ------- ----------- -------
+    Planck18        67.66 0.30966  2.7255   3.046 0.0 .. 0.06 0.04897
+
 Now this :class:|QTable| can be used to load a new cosmological
 instance identical to the ``Planck18`` cosmology from which it was created.
 

docs/table/construct_table.rst

@@ -1184,6 +1184,8 @@ units, see the :ref:`columns_with_units` section.
 
 .. EXAMPLE END
 
+.. _Table-like Objects:
+
 Table-like Objects
 ==================
 

docs/whatsnew/5.0.rst

@@ -16,7 +16,7 @@ Support for reading, writing, and converting ``Cosmology``
 
 Four new methods -- ``read``, ``write``, ``to_format``, ``from_format`` -- have
 been added to the ``Cosmology`` class, enabling reading from and writing to
-files and converting between different python objects.
+files and converting between different Python objects.
 The methods use Astropy's Unified I/O registry so custom formats can be
 registered. Details are discussed in an addition to the docs.
 
@@ -36,14 +36,17 @@ Currently no file formats are registered, but the syntax is as follows:
 The transformations between ``Cosmology`` and `dict`/``QTable`` are
 pre-registered, e.g. enabling::
 
-    >>> from astropy.cosmology import Planck18
+    >>> from astropy.cosmology import Cosmology, Planck18
     >>> cm = Planck18.to_format("mapping")
     >>> cm
     {'cosmology': <class 'astropy.cosmology.flrw.FlatLambdaCDM'>,
      'name': 'Planck18',
      'H0': <Quantity 67.66 km / (Mpc s)>,
      'Om0': 0.30966,
      ...
+
+    or to a Table with:
+
     >>> ct = Planck18.to_format("astropy.table")
     >>> ct
     <QTable length=1>
@@ -53,11 +56,29 @@ pre-registered, e.g. enabling::
     -------- ------------ ------- ------- ------- ----------- -------
     Planck18        67.66 0.30966  2.7255   3.046 0.0 .. 0.06 0.04897
 
-    >>> from astropy.cosmology import Cosmology
+    Even more succinctly:
+
+    >>> from astropy.table import QTable
+    >>> QTable(Planck18)
+    <QTable length=1>
+      name        H0        Om0    Tcmb0    Neff    m_nu [3]    Ob0
+             km / (Mpc s)                              eV
+      str8     float64    float64 float64 float64   float64   float64
+    -------- ------------ ------- ------- ------- ----------- -------
+    Planck18        67.66 0.30966  2.7255   3.046 0.0 .. 0.06 0.04897
+
+    All the format conversions can be reversed:
+
     >>> cosmo = Cosmology.from_format(cm, format="mapping")
+    >>> cosmo
+    FlatLambdaCDM(name="Planck18", H0=67.7 km / (Mpc s), Om0=0.31,
+                  Tcmb0=2.725 K, Neff=3.05, m_nu=[0. 0. 0.06] eV, Ob0=0.049)
+
+    >>> cosmo = Cosmology.from_format(ct, format="astropy.table")
     >>> cosmo == Planck18
     True
 
+
 .. _whatsnew-5.0-cosmology-units:
 
 ``Cosmology`` units module