[FEAT] Scaling a slice of a histogram

[alexander-held]

Hi, I ran into a scenario where I wanted to scale a part of a multi-dimensional histogram:

import hist

h = hist.Hist.new.Reg(3, 0, 3).StrCat(["a", "b"], name="cat").Double()
h.fill([1,1,2], cat="a")
h.fill([0], cat="b")

h[:, "a"] *= 2

This is not currently supported:

Traceback (most recent call last):
  File "[...]/test.py", line 7, in <module>
    h[:, "a"] *= 2
  File "[...]/lib/python3.9/site-packages/hist/basehist.py", line 327, in __setitem__
    return super().__setitem__(self._index_transform(index), value)
  File "[...]/lib/python3.9/site-packages/boost_histogram/_internal/hist.py", line 924, in __setitem__
    raise TypeError("Not supported yet")
TypeError: Not supported yet

@agoose77 pointed out an alternative that works on Gitter:

h[:, "a"] = h[:, "a"].values() * 2

I opened this issue as a feature request for the syntactic sugar of the first approach. Sorry if this is already being tracked elsewhere, I was not really sure what to search for to find potentially related issues!