pyROOT TH1::GetDefaultSumw2 throws error when called

[jemarq04]

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Description

When calling class method TH1::GetDefaultSumw2(), python3 throws the following error:

TypeError: static bool TH1::GetDefaultSumw2() =>
    TypeError: takes at most 0 arguments (1 given)

The method takes 0 arguments in the documentation. I assume the "1 given" here is the instance itself passed to the method of the class, but this doesn't happen with other methods of TH1 that I have noticed.

Reproducer

Fails when running in Python 3.9.14 (should be similar in any Python 3).

import ROOT
hist = ROOT.TH1F("h", "histogram", 100, -3, 3)
print(hist.GetDefaultSumw2()) #throws error here

Works find in C++ (ran with root temp.cc)

//in temp.cc
#include <iostream>                                                                      
#include "TH1.h"                                                                         
                                                                                         
void temp(){                                                                             
  auto *hist = new TH1F("h", "histogram", 100, -3, 3);                                   
  std::cout << hist->GetDefaultSumw2() << std::endl;                                     
}

ROOT version

ROOT Version: 6.36.04
Built for macosx64 on Aug 25 2025, 09:02:18
From tags/6-36-04@6-36-04

Installation method

homebrew package manager

Operating system

MacOS 15.5

Additional context

Bug also found in older version (6.32.13) found in new CMSSW virtual environments (e.g. CMSSW_15_0_14)

[ferdymercury]

Thanks for the report!

A workaround would be to call:

ROOT.TH1F.GetDefaultSumw2()

Since this is a static function.

@wlav or @guitargeek might know why it's failing if called from the class instance directly.

[guitargeek]

Thanks a lot for the report!

but this doesn't happen with other methods of TH1 that I have noticed.

If the behavior is inconsistent, this is indeed a problem. Can you extend your reproducer with some methods where it doesn't happen, to highlight the inconsistency? Thanks!

[ferdymercury]

(I think it only fails for some static methods)

>>> import ROOT
>>> hist = ROOT.TH1F("h", "histogram", 100, -3, 3)
>>> hist.AddDirectoryStatus()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: static bool TH1::AddDirectoryStatus() =>
    TypeError: takes at most 0 arguments (1 given)
>>> hist.Class()
<cppyy.gbl.TClass object at 0x5b2c8a787600>
>>> hist.Class_Name()
'TH1F'
>>> hist.Class_Version()
3
>>> hist.DeclFileLine()
902
>>> hist.DeclFileName()
'/github/home/ROOT-CI/build/include/TH1.h'
>>> hist.GetDefaultBufferSize()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: static int TH1::GetDefaultBufferSize() =>
    TypeError: takes at most 0 arguments (1 given)
>>> hist.StatOverflows()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: boolean value should be bool, or integer 1 or 0
>>> hist.StatOverflows(True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: static void TH1::StatOverflows(Bool_t flag = kTRUE) =>
    TypeError: takes at most 1 arguments (2 given)
>>> hist.SetDefaultSumw2()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: boolean value should be bool, or integer 1 or 0
>>> hist.SetDefaultSumw2(True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: static void TH1::SetDefaultSumw2(Bool_t sumw2 = kTRUE) =>
    TypeError: takes at most 1 arguments (2 given)
>>> hist.CanExtendAllAxes()
False

[wlav]

@wlav or @guitargeek might know why it's failing if called from the class instance directly.

The obvious diagnosis is that the call receives a self argument where it should not. Static/free functions are a bit funny since you can add methods to a Python class after the definition, in which case they should receive a self, but if they're part of the C++ class definition, then they should not.

As for "Python 3.9.14 (should be similar in any Python 3)" ... not really if you come from py3.7 or older: as of py3.8, there's limited support for vector calls, and full support as of py3.9. This changed the handling of self. If you come from py3.10 or newer, then yes.

[guitargeek]

Thanks for your help guys! I have a fix:

• [CPyCppyy] Correctly handle static method calls via derived instance #20496