How to extract values from TSpline?

[yixianz]

Hi! I'm new to uproot so maybe I'm missing something obvious.
I was trying to figure out how I can extract values from TSpline3. In ROOT there are functions like Eval(), FindX(), etc; what's the equivalent way of doing this in uproot?

[jpivarski]

The short answer is that any TSpline3 objects read in with Uproot don't have methods, only data members. In Python, we have the ROOT file and therefore all of the data, but not the C++ code. So there's no way to do this now.

The longer answer is that methods like this can be added to the Model_TSpline3_vN class by adding a "behavior" in https://github.com/scikit-hep/uproot5/tree/main/src/uproot/behaviors . It involves implementing these methods in Python, operating on the data that's already there.

The first step, whether you're thinking about contributing these functions or just getting something to work for yourself, is to look at the all_members and access individual members by member("fMemberName") to see the data that are there, and then think about what you need to compute.

[yixianz]

The output is really long; I'm just copying relevant pieces here:

TSpline3 (v2): TSpline (v2)
    fPoly: TSplinePoly3* (TStreamerLoop)
    fValBeg: double (TStreamerBasicType)
    fValEnd: double (TStreamerBasicType)
    fBegCond: int (TStreamerBasicType)
    fEndCond: int (TStreamerBasicType)

TSpline (v2): TNamed (v1), TAttLine (v2), TAttFill (v2), TAttMarker (v2)
    fDelta: double (TStreamerBasicType)
    fXmin: double (TStreamerBasicType)
    fXmax: double (TStreamerBasicType)
    fNp: int (TStreamerBasicType)
    fKstep: bool (TStreamerBasicType)
    fHistogram: TH1F* (TStreamerObjectPointer)
    fGraph: TGraph* (TStreamerObjectPointer)
    fNpx: int (TStreamerBasicType)

... 

TSplinePoly3 (v1): TSplinePoly (v2)
    fB: double (TStreamerBasicType)
    fC: double (TStreamerBasicType)
    fD: double (TStreamerBasicType)

TSplinePoly (v2): TObject (v1)
    fX: double (TStreamerBasicType)
    fY: double (TStreamerBasicType)

[jpivarski]

It's probably the TStreamerLoop, which is a more obscure TStreamer type and may not be implemented correctly. (But then, I wonder why the object didn't fail deserialization with a message about having the wrong number of bytes: misinterpreting 13 TSplinePoly3 as 1 TSplinePoly3 would set the number of bytes read incorrectly.)

I can take a look at this if you supply a small file that can be accessed publicly (i.e. it shouldn't contain any private data). I can use it to fix the TStreamerLoop implementation, if that's what the problem is, and then include it in our automated testing for the future.

[yixianz]

That would be great! Thank you for your help.
I made a simple TSpline3 with 3 knots (x = [1,2,3], y = x^2), which should be accessible from this link:
https://drive.google.com/file/d/1ZcLWr1_udVZLNZne4GfWtDlqazUERsFT/view?usp=drive_link.

[jpivarski]

This should be fixed in #1277. Let me know if it works for you on all of your files, and if so, I'll merge it in and it will make its way into the next release.

[yixianz]

Yes, it works! Thank you!