new PyROOT wants square brackes rather than round brackets. If you saw round branches somewhere in our docs let us know.
This should work (works for me with master, but v6.26 should behave exactly the same way):
In [1]: import ROOT
ROOT
In [2]: df = ROOT.RDataFrame(10).Define("x", "42")
d.f
In [3]: r = df.Take['int']("x")
In [4]: r.GetValue()
Out[4]: <cppyy.gbl.std.vector<int> object at 0x561a06a46d10>
In [5]: r.GetValue().size()
Out[5]: 10
I saw the round brackets in the solution linked above, not the docs, though I’m not sure where the square bracket behavior is documented, anyway. (I don’t have to do a lot of templating in pyROOT).