With RDataFrame and python, how to fill an array in to a tree and save the tree

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Hi experts,
In brief, there is a root file containing many trees. I want to get a tree, slim it by some cuts, then add a new branch in the tree with a pdarray in ready, finally, write this tree into the file.
I chose Rdataframe, since it is convenient to add cuts. But I met problems when filling a branch with a pdfarray and save the tree to the rootfile. This is what I tried:

def array2tree(outArray, outName, outTree):
    outTree.Define(outName,"for(auto w : outArray) return w;")// wrong here!!!
    outTree.Define(outName,1)
    outTree.Write()//for rdataframe, no Write attribute.!!!

 As you can see above, outArray is a pdArray. outTree is a rdataframe. My doing to fill a branch is not proper...And I also don't know how to write the rdataframe into a file. 
If you have some experience, could you please share it with me? Thanks a lot!
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Hi @11111,
writing an RDF into a file is simple, you can use the Snapshot method.

The problem with the Define is that RDF has no notion of the outArray python variable. Declaring a python variable to RDF is possible but not super straightforward, see e.g. the discussion at Add new column to RDataFrame

Cheers,
Enrico