Serialization for number of histogram entries

[alexander-held]

When creating a histogram with ROOT and serializing it with uproot, the GetEntries() (number of times it has been filled) count does not seem to get serialized correctly:

import ROOT
import uproot

h = ROOT.TH1D('h', 'h', 10, 0, 1)
h.Sumw2()
h.Fill(0.2, 0.1)
h.Fill(0.4, 0.5)
assert h.GetEntries() == 2.0

# save with uproot, load with ROOT
with uproot.recreate("tmp.root") as f:
    f["h"] = h

g = ROOT.TFile.Open("tmp.root")
print(g["h"].GetEntries())  # this is 0.6 now, but should be 2

This was tested with uproot 5.3.10.

I originally ran into this when testing the serialization of hist histograms instead:

import hist

h = hist.Hist.new.Reg(10, 0, 1).Weight()
h.fill([0.2, 0.4], weight=[0.1, 0.5])

For this setup, the GetEntries() (after saving with uproot and loading with ROOT) similarly ends up being 0.6 but I am not sure if boost-histogram has this kind of property in the first place.