Add NDCube method to convert to NDData instance.

changelog/887.feature.rst

@@ -0,0 +1 @@
+Add new method, `ndcube.NDCube.to_nddata`, which allows easy conversion of an `~ndcube.NDCube` to a subclass of `~astropy.nddata.NDData`. Users have the option of changing attribute values when doing so by designated the new values via kwargs.

ndcube/ndcube.py

@@ -1461,6 +1461,66 @@ def fill_masked(self, fill_value, uncertainty_fill_value=None, unmask=False, fil
             self.mask = False
         return None
 
+    def to_nddata(self, *, nddata_type=NDData, **kwargs):
+        """
+        Constructs new type instance with the same attribute values as this `~ndcube.NDCube`.
+
+        Any attributes not supported by the new class (``nddata_type``), will be discarded.
+        Values of supported attributes can be altered on the output object by setting a
+        kwarg with the new value, e.g. ``data=new_data``.
+
+        Parameters
+        ----------
+        nddata_type:
+            The type of the returned object. Must be a subclass of `~astropy.nddata.NDData`
+            or a class that behaves like one.  Default=`~astropy.nddata.NDData`.
+
+        kwargs:
+            Inputs to the ``nddata_type`` constructor that should differ from the values
+            stored in attributes of this instance. For example, to set different data values
+            on the returned object, set a kwarg ``data=new_data``, where ``new_data`` is an
+            an array of compatible shape and dtype.  Note that kwargs given by the user and
+            attributes on this instance that are not supported by the ``nddata_type``
+            constructor are ignored.
+
+        Returns
+        -------
+        new_nddata: object
+            An object of class given by ``nddata_type`` with the same attribute values as
+            this `~ndcube.NDCube` instance, except for any alterations specified by the
+            kwargs.
+
+        Examples
+        --------
+        To create an `~astropy.nddata.NDData` instance which is a copy of an `~ndcube.NDCube`
+        (called ``cube``) without a WCS, do:
+
+        >>> nddata_without_coords = cube.to_nddata(wcs=None) # doctest: +SKIP
+        """
+        # Build dictionary of new attribute values from this NDCube instance
+        # and update with user-defined kwargs.
+        new_kwargs = {key.strip("_"): value for key, value in self.__dict__.items()}
+        new_kwargs.update(kwargs)
+        extra_coords, global_coords = None, None
+        if nddata_type is NDCube:
+            extra_coords = new_kwargs.pop("extra_coords")
+            global_coords = new_kwargs.pop("global_coords")
+        # Inspect call signature of new_nddata class and
+        # remove unsupported items from new_kwargs.
+        nddata_sig = inspect.signature(nddata_type).parameters.keys()
+        new_kwarg_keys = list(new_kwargs.keys())
+        for key in new_kwarg_keys:
+            if key not in nddata_sig:
+                del new_kwargs[key]
+        # Construct and return new instance.
+        new_nddata = nddata_type(**new_kwargs)
+        if extra_coords:
+            extra_coords._ndcube = new_nddata
+            new_nddata._extra_coords = extra_coords
+        if global_coords:
+            new_nddata._global_coords = global_coords
+        return new_nddata
+
 
 def _create_masked_array_for_rebinning(data, mask, operation_ignores_mask):
     m = None if (mask is None or mask is False or operation_ignores_mask) else mask

ndcube/tests/helpers.py

@@ -26,6 +26,7 @@
     'assert_cubes_equal',
     'assert_cubesequences_equal',
     'assert_extra_coords_equal',
+    'assert_global_coords_equal',
     'assert_metas_equal',
     'assert_wcs_are_equal',
     'figure_test',
@@ -93,6 +94,12 @@ def assert_extra_coords_equal(test_input, extra_coords):
         assert_wcs_are_equal(test_input._wcs, extra_coords._wcs)
 
 
+
+def assert_global_coords_equal(test_input, global_coords):
+    assert test_input.items() == global_coords.items()
+    assert test_input.physical_types == global_coords.physical_types
+
+
 def assert_metas_equal(test_input, expected_output):
     if type(test_input) is not type(expected_output):
         raise AssertionError(

ndcube/tests/test_ndcube.py

@@ -4,6 +4,7 @@
 import numpy as np
 import pytest
 
+import astropy.nddata
 import astropy.units as u
 import astropy.wcs
 from astropy.wcs.wcsapi import BaseHighLevelWCS
@@ -238,3 +239,23 @@ def test_fill_masked_ndc_uncertainty_none(ndc, fill_value, uncertainty_fill_valu
             uncertainty_fill_value=uncertainty_fill_value,
             fill_in_place=True
         )
+
+
+def test_to_nddata(ndcube_2d_ln_lt):
+    ndc = ndcube_2d_ln_lt
+    new_data = ndc.data * 2
+    output = ndc.to_nddata(data=new_data, wcs=None)
+    assert type(output) is astropy.nddata.NDData
+    assert output.wcs is None
+    assert (output.data == new_data).all()
+
+
+def test_to_nddata_type_ndcube(ndcube_2d_ln_lt_uncert_ec):
+    ndc = ndcube_2d_ln_lt_uncert_ec
+    ndc.global_coords.add("wavelength", "em.wl", 100*u.nm)
+    new_data = ndc.data * 2
+    output = ndc.to_nddata(data=new_data, nddata_type=NDCube)
+    assert type(output) is NDCube
+    assert (output.data == new_data).all()
+    helpers.assert_extra_coords_equal(output.extra_coords, ndc.extra_coords)
+    helpers.assert_global_coords_equal(output.global_coords, ndc.global_coords)