Type of empty array

[fleble]

Dear experts,

I am writing ROOT files using uproot. One problem I face is when a branch is empty, thus having unknown type, which cannot be written down with uproot.

>>> ak.__version__
1.5.1
>>> uproot.__version__
4.3.7

This script

ak_array = ak.Array([[], [], []])
ak_array = ak.values_astype(ak_array, np.float64)

tree = {"branch": ak_array}

with uproot.recreate("test.root") as file:
    file["test"] = tree

terminates with the following error:

TypeError: cannot write Awkward Array type to ROOT file:

   var * unknown

And:

>>> ak_array = ak.Array([[], [], []])
>>> ak.values_astype(ak_array, np.float64)
<Array [[], [], []] type='3 * var * unknown'> 

Is it possible to give a type to empty to empty awkward array?

[fleble]

After some attempts, I found a trick by concatenating an ak array of the desired type, and filtering out the part that was concatenated.

[jpivarski]

Converted into an issue: scikit-hep/uproot5#822.

And also #2188.