Accessing Python from ROOT

ROOT
1

Hi,

I would need to create either a TArrayF and/or a TH2F in python and would need to have access to it within ROOT.
I tried to apply to TArrayF and TH2F the example in the User’s Guide, namely:

// create a TBrowser on the Python side, and transfer it back and forth
root[] TBrowser* b = (void*)Python::Eval(“ROOT.TBrowser()”);
(class TObject*)0x8d1daa0
root[] TPython::Bind(b,“b”);

Now, if I want to do the same thing it doesn’t work for TArrayF - or at least there are some things that I don’t understand or I can’t handle.
Here is what I have for TH2F:

root [0] gSystem->Load(“libPyROOT”)
(int)0
root [1] TH2F * h = (void*)TPython::Eval(“ROOT.TH2F()”);
root [2] TPython::Bind(h, “h”);

now, going further along the lines of the User’s Guide :

root [3] TPython::Exec(“h.SetName(‘histo’)”);
root [4] TPython::Exec(“print h.GetName()”);
histo
root [5] TPython::Exec(“h.SetBins(50, 0.0, 25.0, 50, 0.0, 25.0)”);
root [6] TPython::Exec(“h.SetBinContent(25, 25, 12.34)”);
root [7] TPython::Exec(“print h.GetBinContent(25, 25)”);
12.3400001526

Now, lets see wether we have it in ROOT:

root [8] h->GetBinContent(25,25)
(const Double_t)1.23400001525878910e+001

Great. Now comes the challange: can we do the same for a TArrayF?

root [0] gSystem->Load(“libPyROOT”)
(int)0
root [1] TArrayF * a = (void*)TPython::Eval(“ROOT.TArrayF()”);
root [2] TPython::Bind(a, “a”);
Error: Function Bind(a,“a”) is not defined in current scope (tmpfile)(1)
*** Interpreter error recovered ***

No. Why doesn’t it work for TArrayF? Maybe I missed the point…

Another question: which one do you excpect to be faster: working with TArray (and TMatrix) or TH2F? E.g. loading and playing with 2D arrays.
(note: I need PyROOT just for using the Python Image Library to load multiimage TIF files.)

Thanks!

Balint

2

Balint,

with two lines like this:root [1] TH2F * h = (void*)TPython::Eval("ROOT.TH2F()"); root [2] TPython::Bind(h, "h");
you are risking that the TH2F on the python side will already be destroyed by the python garbage collector before you bind it to h, and I think that is happening for your TArrayF example. The two statements should rather be something like: root [0] TPython::Exec("h = ROOT.TH2F()"); root [1] TH2F* h = (void*)TPython::Eval("h");
Same for the TArrayF.

As for:[quote]Another question: which one do you excpect to be faster: working with TArray (and TMatrix) or TH2F? E.g. loading and playing with 2D arrays.[/quote]
The histogram uses its TArray base class internally for the data representation, so I’d say that the only major difference is the binning, and which is faster depends strongly on your usage.

Cheers,
Wim

3

Hi,

Thanx for helping!

Cheers,
Balint