Unify pytato reduction handling, support 'initial' arg

arraycontext/impl/pytato/fake_numpy.py

@@ -42,6 +42,10 @@ class PytatoFakeNumpyLinalgNamespace(BaseFakeNumpyLinalgNamespace):
     pass
 
 
+class _NoValue:
+    pass
+
+
 class PytatoFakeNumpyNamespace(BaseFakeNumpyNamespace):
     """
     A :mod:`numpy` mimic for :class:`PytatoPyOpenCLArrayContext`.
@@ -91,22 +95,50 @@ def minimum(self, x, y):
     def where(self, criterion, then, else_):
         return rec_multimap_array_container(pt.where, criterion, then, else_)
 
-    def sum(self, a, axis=None, dtype=None):
-        def _pt_sum(ary):
+    @staticmethod
+    def _reduce(container_binop, array_reduce,
+            ary, *,
+            axis, dtype, initial):
+        def container_reduce(ctr):
+            if initial is _NoValue:
+                try:
+                    return reduce(container_binop, ctr)
+                except TypeError as exc:
+                    assert "empty sequence" in str(exc)
+                    raise ValueError("zero-size reduction operation "
+                            "without supplied 'initial' value")
+            else:
+                return reduce(container_binop, ctr, initial)
+
+        def actual_array_reduce(ary):
             if dtype not in [ary.dtype, None]:
                 raise NotImplementedError
 
-            return pt.sum(ary, axis=axis)
-
-        return rec_map_reduce_array_container(sum, _pt_sum, a)
-
-    def min(self, a, axis=None):
-        return rec_map_reduce_array_container(
-                partial(reduce, pt.minimum), partial(pt.amin, axis=axis), a)
+            if initial is _NoValue:
+                return array_reduce(ary, axis=axis)
+            else:
+                return array_reduce(ary, axis=axis, initial=initial)
 
-    def max(self, a, axis=None):
         return rec_map_reduce_array_container(
-                partial(reduce, pt.maximum), partial(pt.amax, axis=axis), a)
+                container_reduce,
+                actual_array_reduce,
+                ary)
+
+    # * appears where positional signature starts diverging from numpy
+    def sum(self, a, axis=None, dtype=None, *, initial=0):
+        import operator
+        return self._reduce(operator.add, pt.sum, a,
+                axis=axis, dtype=dtype, initial=initial)
+
+    # * appears where positional signature starts diverging from numpy
+    def min(self, a, axis=None, *, initial=_NoValue):
+        return self._reduce(pt.minimum, pt.amin, a,
+                axis=axis, dtype=None, initial=initial)
+
+    # * appears where positional signature starts diverging from numpy
+    def max(self, a, axis=None, *, initial=_NoValue):
+        return self._reduce(pt.maximum, pt.amax, a,
+                axis=axis, dtype=None, initial=initial)
 
     def stack(self, arrays, axis=0):
         return rec_multimap_array_container(