Catching TH2::Rebin error in pyRoot

[BPH]

Hi, I have pyroot script where I take a 2D histogram and rebin this until the number of effective entries is sufficiently large. i.e. I do something like this:

while int(my_th1d.GetEffectiveEntries() < 10):
  my_th2d.RebinY()
  my_th1d = my_th2d.ProjectionX( "a", iBin, iBin, "ed")

The problem is that I get an error for a particular value of iBin (an integer taking values corresponding to the y-axis bins):

Error in <TH2D::Rebin2D>: Illegal value of nygroup=2

I want to be able to catch this error in pyRoot, maybe using try and except so that I can then change nygroup to a larger value or exit the while loop some other way. Is there a way to do this in pyRoot? How can I keep track of the fact that there is an ROOT error. I have seen the rootpy logger (7. Logging — rootpy 1.0.0.dev0 documentation) module, but I’m not sure how to use this for my needs.

Many thanks,
Ben

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ROOT Version: 6.20/06
Platform: SLC7
Compiler: linuxx8664gcc

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[jalopezg]

Hi @BPH,

In ROOT C++ you can set a custom error handler via ::SetErrorHandler() (declared in TError.h); setting up a custom error handler would allow you to catch those errors and act accordingly. However, I am not sure if this can be used in PyROOT or whether there is another approach. Perhaps, @etejedor can help here.

Cheers,
J.

[BPH]

Thank you jalopezg! I have a hacky workaround where I just check if the number of effective entries stays the same 3 times in a row and then exits the loop if it does. This works, so not the end of the world if a nice solution cannot be found. Woulds still be nice to do this in a non-hacky way though!

Appreciate the help
Ben

[jalopezg]

Hi @BPH,

Ignore my previous reply ;-). As it seems, the error comes from the my_th2d.RebinY() call, where the default value for the ngroup parameter is 2. In this case, you can check that the number of bins is at least 2 before trying to call RebinY() by using GetNbinsY().

Cheers,
J.

[BPH]

Hi @jalopezg ,

Ah, of course. The rebinning is failing because the number of bins after rebinning is not compatible with the ngroup parameter. Thanks a lot! That’s a good solution

Cheers,
Ben