Factor common code from source count metrics.

These tasks could probably be factored further, since the only
difference is in the criteria used for filtering sources, but handling
columns in a general way would be nontrivial and should probably await
the addition of more varied metrics.

python/lsst/pipe/tasks/metrics.py

@@ -107,11 +107,7 @@ def run(self, sources):
                 deblended = ((sources["parent"] == 0)           # top-level source
                              & (sources["deblend_nChild"] > 0)  # deblended
                              )
-                if "sky_source" in sources.schema:
-                    # E712 is not applicable, because
-                    # afw.table.SourceRecord.ColumnView is not a bool.
-                    deblended = deblended & (sources["sky_source"] == False)  # noqa: E712
-                nDeblended = np.count_nonzero(deblended)
+                nDeblended = np.count_nonzero(_filterSkySources(sources, deblended))
             except LookupError as e:
                 # Probably "parent"; all other columns already checked
                 raise MetricComputationError("Invalid input catalog") from e
@@ -192,14 +188,39 @@ def run(self, sources):
                 children = ((sources["parent"] != 0)            # deblend child
                             & (sources["deblend_nChild"] == 0)  # not deblended
                             )
-                if "sky_source" in sources.schema:
-                    # E712 is not applicable, because
-                    # afw.table.SourceRecord.ColumnView is not a bool.
-                    children = children & (sources["sky_source"] == False)  # noqa: E712
-                nChildren = np.count_nonzero(children)
+                nChildren = np.count_nonzero(_filterSkySources(sources, children))
             except LookupError as e:
                 # Probably "parent"; all other columns already checked
                 raise MetricComputationError("Invalid input catalog") from e
             meas = Measurement(self.config.metricName, nChildren * u.dimensionless_unscaled)
 
         return Struct(measurement=meas)
+
+
+def _filterSkySources(catalog, matches):
+    """Filter out any sky sources from a vector of matches.
+
+    If no sky source information is available, all sources are assumed to
+    be non-sky.
+
+    Parameters
+    ----------
+    catalog : `lsst.afw.table.SourceCatalog`
+        The catalog to filter.
+    matches : `numpy.ndarray` [`bool`], (N,)
+        A vector of existing source matches, of the same length as ``catalog``,
+        where matched sources are marked `True`.
+
+    Returns
+    -------
+    filtered : `numpy.ndarray` [`bool`], (N,)
+        A version of ``matches`` with any sky sources filtered out
+        (set to `False`). May be the same vector as ``matches`` if
+        no changes were made.
+    """
+    if "sky_source" in catalog.schema:
+        # E712 is not applicable, because afw.table.SourceRecord.ColumnView
+        # is not a bool.
+        return matches & (catalog["sky_source"] == False)  # noqa: E712
+    else:
+        return matches