fix __str__ for wcs with a None transform (#489)

CHANGES.rst

@@ -5,6 +5,8 @@
 
 - Add a minimum version requirement for ``asdf-wcs-schemas``. [#488]
 
+- Fix ``WCS.__str__`` for instances without transforms. [#489]
+
 0.20.0 (2023-11-29)
 -------------------
 

gwcs/tests/test_wcs.py

@@ -1316,3 +1316,8 @@ def test_spatial_spectral_stokes():
     assert_allclose(gw_sky.data.lat, aw_sky.data.lat)
     assert_allclose(gw_spec.value, aw_spec.value)
     assert_allclose(gw_stokes.value, aw_stokes.value)
+
+
+def test_wcs_str():
+    w = wcs.WCS(output_frame="icrs")
+    assert 'icrs' in str(w)

gwcs/wcs.py

@@ -1338,13 +1338,13 @@ def _get_axes_indices(self):
     def __str__(self):
         from astropy.table import Table
 
-        #col1 = [item[0] for item in self._pipeline]
         col1 = [step.frame for step in self._pipeline]
         col2 = []
         for item in self._pipeline[: -1]:
-            #model = item[1]
             model = item.transform
-            if model.name is not None:
+            if model is None:
+                col2.append(None)
+            elif model.name is not None:
                 col2.append(model.name)
             else:
                 col2.append(model.__class__.__name__)