get _ensure_systematics daskified

Author: lgray

Diff for: src/coffea/nanoevents/methods/base.py

@@ -18,9 +18,12 @@
 class _ClassMethodFn:
     def __init__(self, attr: str, **kwargs: Any) -> None:
         self.attr = attr
+        self.kwargs = kwargs
 
     def __call__(self, coll: awkward.Array, *args: Any, **kwargs: Any) -> awkward.Array:
-        return getattr(coll, self.attr)(*args, **kwargs)
+        allkwargs = self.kwargs
+        allkwargs.update(kwargs)
+        return getattr(coll, self.attr)(*args, **allkwargs)
 
 
 @awkward.mixin_class(behavior)
@@ -36,12 +39,35 @@ def add_kind(cls, kind: str):
         """
         cls._systematic_kinds.add(kind)
 
-    def _ensure_systematics(self):
+    def _ensure_systematics(self, _dask_array_=None):
         """
         Make sure that the parent object always has a field called '__systematics__'.
         """
         if "__systematics__" not in awkward.fields(self):
-            self["__systematics__"] = {}
+            if _dask_array_ is not None:
+                x = awkward.Array(
+                    awkward.Array([{}]).layout.to_typetracer(forget_length=True)
+                )
+                _dask_array_._meta["__systematics__"] = x
+
+                def add_systematics_hack(array):
+                    if awkward.backend(array) == "typetracer":
+                        array["__systematics__"] = x
+                        return array
+                    array["__systematics__"] = {}
+                    return array
+
+                temp = dask_awkward.map_partitions(
+                    add_systematics_hack,
+                    _dask_array_,
+                    label="ensure-systematics",
+                    meta=_dask_array_._meta,
+                )
+                _dask_array_._meta = temp._meta
+                _dask_array_._dask = temp._dask
+                _dask_array_._name = temp._name
+            else:
+                self["__systematics__"] = {}
 
     @property
     def systematics(self):
@@ -109,11 +135,13 @@ def add_systematic(
             print("vf  ", varying_function)
             print("da  ", _dask_array_, type(_dask_array_))
             _dask_array_.map_partitions(
-                _ClassMethodFn("add_systematic"),
-                name,
-                kind,
-                what,
-                varying_function,
+                _ClassMethodFn(
+                    "add_systematic",
+                    name=name,
+                    kind=kind,
+                    varying_function=varying_function,
+                ),
+                what=what,
             )
 
         self._ensure_systematics()