Unserializable histogram generated if computed without fill call

[yimuchen]

Originally posted here: scikit-hep/hist#586
And I found that this issue can be replicated entirely upstream using just dask-histogram

import pickle

import boost_histogram
import dask
import dask_histogram.boost
import numpy

h = dask_histogram.boost.Histogram(boost_histogram.axis.Regular(10, 0, 1))
# h.fill( # Toggle to include a fill call
#     dask.array.from_array(numpy.zeros(shape=(10))),
#     weight=dask.array.from_array(numpy.zeros(shape=(10))),
# )

o = dask.compute(h)
pickle.dump(o, open("hist_dask_test.pkl", "wb"))

Without a fill call, the resulting histogram will have dangling a self._dask field that breaks serialization.

[martindurant]

Hm, the computed object shouldn't have any ties to dask at all.

Note that cloudpickle (and dask's serialize) is able to serialise this object.

[martindurant]

The following fixes it, but I would like to figure out why the attribute is there in the first place

--- a/src/dask_histogram/boost.py
+++ b/src/dask_histogram/boost.py
@@ -137,7 +137,12 @@ class Histogram(bh.Histogram, DaskMethodsMixin, family=dask_histogram):
         return self.dask_name

     def __dask_postcompute__(self) -> Any:
-        return lambda x: self._in_memory_type(first(x)), ()
+        def f(x):
+            out = self._in_memory_type(first(x))
+            if hasattr(out, "_dask"):
+                del out._dask
+            return out
+        return f, ()

     def __dask_postpersist__(self) -> Any:

[martindurant]

I have not yet come up with any better solution, so if you would check what I put above, I would appreciate it.

[yimuchen]

I have not yet come up with any better solution, so if you would check what I put above, I would appreciate it.

Just confirmed that this indeed allows empty histograms to be pickled/unpickled as usual. Thanks!

[martindurant]

OK, let's make that PR for now, as certainly it doesn't seem to have a downside.