Merge pull request #203 from mcara/add-custom-exception

align_wcs to raise custom exception when not enough inputs

CHANGELOG.rst

@@ -7,6 +7,13 @@ Release Notes
 .. 0.8.8 (unreleased)
    ==================
 
+0.8.8 (unreleased)
+==================
+
+- ``align_wcs`` now will raise a custom exception of type ``NotEnoughCatalogs``
+  when there are not enough input catalogs to perform alignment. [#203]
+
+
 0.8.7 (29-March-2024)
 =====================
 

tweakwcs/imalign.py

@@ -26,12 +26,20 @@
 
 __author__ = 'Mihai Cara'
 
-__all__ = ['fit_wcs', 'align_wcs']
+__all__ = ['fit_wcs', 'align_wcs', 'NotEnoughCatalogs']
 
 log = logging.getLogger(__name__)
 log.setLevel(logging.DEBUG)
 
 
+class NotEnoughCatalogs(ValueError):
+    """
+    An exception class used to notify that an alignment routine does not have
+    enough input catalogs to perform alignment.
+    """
+    pass
+
+
 @deprecated_renamed_argument('tpwcs', 'corrector', since='0.8.0')
 def fit_wcs(refcat, imcat, corrector, ref_tpwcs=None, fitgeom='general',
             nclip=3, sigma=(3.0, 'rmse'), clip_accum=False,
@@ -465,6 +473,14 @@ def align_wcs(wcscat, refcat=None, ref_tpwcs=None, enforce_user_order=True,
         ``refcat`` catalog, an expanded ``refcat`` with a combination of
         source positions from all input images.
 
+    Raises
+    ------
+    NotEnoughCatalogs
+        This exception is raised when there are not enough input catalogs
+        to perform alignment. For example, ``wcscat`` must be a list of at least
+        two correctors when ``refcat`` is `None`, or be a single corrector
+        when a refernce catalog is provided via ``refcat``.
+
     Notes
     -----
     **1. Weights:**
@@ -649,8 +665,10 @@ def align_wcs(wcscat, refcat=None, ref_tpwcs=None, enforce_user_order=True,
 
     # check that we have enough input images:
     if (refcat is None and len(wcs_gcat) < 2) or len(wcs_gcat) == 0:
-        raise ValueError("Too few input images (or groups of images) with "
-                         "non-empty catalogs.")
+        raise NotEnoughCatalogs(
+            "Too few input images (or groups of images) with "
+            "non-empty catalogs."
+        )
 
     # get the first image to be aligned and
     # create reference catalog if needed:

tweakwcs/tests/test_imalign.py

@@ -13,7 +13,12 @@
 from astropy.table import Table
 
 from tweakwcs.linearfit import build_fit_matrix
-from tweakwcs.imalign import fit_wcs, align_wcs, max_overlap_pair
+from tweakwcs.imalign import (
+    fit_wcs,
+    align_wcs,
+    max_overlap_pair,
+    NotEnoughCatalogs,
+)
 from tweakwcs import FITSWCSCorrector
 
 
@@ -26,7 +31,7 @@ def test_fit_wcs_empty_cat(empty_refcat, empty_imcat, mock_fits_wcs):
         meta={'catalog': Table([[], []], names=('x', 'y')), 'group_id': 1}
     )
 
-    with pytest.raises(ValueError) as e:
+    with pytest.raises(NotEnoughCatalogs) as e:
         align_wcs([tpwcs, tpwcs, tpwcs])
     assert e.value.args[0] == ("Too few input images (or groups of images) "
                                "with non-empty catalogs.")
@@ -472,7 +477,7 @@ def test_align_wcs_1im_no_ref(mock_fits_wcs):
     imcat = Table(xy, names=('x', 'y'))
     tpwcs = FITSWCSCorrector(mock_fits_wcs, meta={'catalog': imcat})
 
-    with pytest.raises(ValueError) as e:
+    with pytest.raises(NotEnoughCatalogs) as e:
         align_wcs(tpwcs, refcat=None, fitgeom='shift', match=None)
     assert e.value.args[0] == ("Too few input images (or groups of images) "
                                "with non-empty catalogs.")