Avoid the magic number 6

with a module-level constant. Store that in the header FWIW.

Author: arunkannawadi

python/lsst/cell_coadds/_fits.py

@@ -119,6 +119,11 @@
 the form M.m, where M is the major version, m is the minor version.
 """
 
+MAX_POLYGON_VERTICES = 6
+"""Maximum number of vertices the overlap polygon region between a per-detector
+warp and the patch bounding box can have."""
+# 3 vertices from the detector, 3 vertices from the patch bounding box.
+
 logger = logging.getLogger(__name__)
 
 
@@ -328,7 +333,10 @@ def readAsMultipleCellCoadd(self) -> MultipleCellCoadd:
                             day_obs=visit_dict[visit].day_obs,
                             physical_filter=visit_dict[visit].physical_filter,
                         )
-                        num_vertices = row["num_vertices"] if written_version >= version.parse("0.7") else 6
+                        if written_version >= version.parse("0.7"):
+                            num_vertices = row["num_vertices"]
+                        else:
+                            num_vertices = None  # li[:None] returns the entire list.
                         visit_polygons[obs_id] = afwGeom.Polygon(
                             [Point2D(vertex) for vertex in row["polygon_vertices"][:num_vertices]]
                         )
@@ -673,22 +681,25 @@ def writeMultipleCellCoaddAsFits(
     number_of_vertices = []
     polygon_vertices_array = []
     for obs_id, poly in multiple_cell_coadd.common.visit_polygons.items():
-        if num_vertices := len(poly.getVertices()) > 6:
+        if num_vertices := len(poly.getVertices()) > MAX_POLYGON_VERTICES:
             logger.warning(
-                "Visit %d, detector %d has a polygon with %d vertices; "
-                "only the first 6 will be stored in the FITS file.",
+                "Visit %d, detector %d has a polygon with %d vertices. "
+                "This geometry should be impossible for two intersecting "
+                "convex quadrilaterals. Only the first %d will be stored in "
+                "the FITS file.",
                 obs_id.visit,
                 obs_id.detector,
                 num_vertices,
+                MAX_POLYGON_VERTICES,
             )
         number_of_vertices.append(num_vertices)
         vertices = poly.getVertices() + poly.getVertices()
-        vertices = vertices[:6]
+        vertices = vertices[:MAX_POLYGON_VERTICES]
         polygon_vertices_array.append(np.array(vertices))
     polygon_column = fits.Column(
         name="polygon_vertices",
-        format="12E",
-        dim="(2,6)",
+        format=f"{2 * MAX_POLYGON_VERTICES}E",
+        dim=f"(2,{MAX_POLYGON_VERTICES})",
         array=polygon_vertices_array,
     )
     number_of_vertices_column = fits.Column(
@@ -700,6 +711,7 @@ def writeMultipleCellCoaddAsFits(
         [visit_column, detector_column, number_of_vertices_column, polygon_column],
         name="VISIT_SUMMARY",
     )
+    visit_summary_hdu.header["POLYVERT"] = MAX_POLYGON_VERTICES
 
     visit_recarray = np.rec.fromrecords(
         recList=sorted(set(visit_records), key=lambda x: x[0]),  # Sort by visit.