Merge pull request #804 from lsst/tickets/DM-51672

DM-51672: Improve failure modes for MultibandExposure PSF generation

Author: fred3m

python/lsst/afw/image/_exposure/_multiband.py

@@ -52,8 +52,11 @@ class IncompleteDataError(Exception):
         The full list of bands in the `MultibandExposure` generating
         the PSF.
     """
-    def __init__(self, bands, position, partialPsf):
-        missingBands = [band for band in bands if band not in partialPsf.bands]
+    def __init__(self, bands, position, partialPsf=None):
+        if partialPsf is None:
+            missingBands = bands
+        else:
+            missingBands = [band for band in bands if band not in partialPsf.bands]
 
         self.missingBands = missingBands
         self.position = position
@@ -100,6 +103,9 @@ def computePsfImage(psfModels, position, useKernelImage=True):
         except InvalidParameterError:
             incomplete = True
 
+    if len(psfs) == 0:
+        raise IncompleteDataError(list(psfModels.keys()), position, None)
+
     left = np.min([psf.getBBox().getMinX() for psf in psfs.values()])
     bottom = np.min([psf.getBBox().getMinY() for psf in psfs.values()])
     right = np.max([psf.getBBox().getMaxX() for psf in psfs.values()])