Python interface: PyROOT

ROOT can be used from Python thanks to its Python-C++ bindings, called PyROOT. PyROOT is HEP’s entrance to all C++ from Python, e.g. for frameworks and their steering code. The PyROOT bindings are automatic and dynamic: no pre-generation of Python wrappers is necessary.

With PyROOT, all the ROOT functionality can be accessed from Python while, at the same time, benefiting from the performance of the ROOT C++ libraries.

PyROOT is compatible with both Python2 (>= 2.7) and Python3.

Using PyROOT requires working knowledge of Python. Detailed information about the Python language can be found in the Python Language Reference.

Together with ROOT 6.22, a major revision of PyROOT has been released. This new PyROOT has extensive support for modern C++ (it operates on top of cppyy), is more pythonic and is able to interoperate with widely-used Python data-science libraries (e.g. NumPy, pandas, Numba). Therefore, we strongly recommend to move to the new PyROOT!

Building and installing

PyROOT is enabled by default when building and installing ROOT. Please refer to Building ROOT from source for specific information on how PyROOT can be built, and to Installing ROOT to learn about the ways to install ROOT.

Getting started

Once ROOT has been installed, PyROOT can be used both from the Python prompt and from a Python script. The entry point to PyROOT is the ROOT module, which needs to be imported first:

import ROOT

After that, all the ROOT C++ functionality (classes, functions, etc.) can be accessed via the ROOT module. In the example below, we show how to create a histogram (an instance of the TH1F class) and randomly fill it with a gaussian distribution.

h = ROOT.TH1F("myHist", "myTitle", 64, -4, 4)


There are a number of tutorials that show how to use the various ROOT features from Python. They can be found in the link below.

→ PyROOT tutorials

Interactive graphics

Just like from C++, it is also possible to create interactive ROOT graphics from Python thanks to PyROOT. As an example, let’s consider the following code, which creates a one-dimensional function and draws it:

>>> import ROOT
>>> f = ROOT.TF1("f1", "sin(x)/x", 0., 10.)
>>> f.Draw()

As a result of running the code above from the Python prompt, a new canvas will open up to show the drawn function:

Figure: Example of graphics generated with PyROOT.

Note that the code above can also be written in a script file and executed with Python. In that case, the script will run to completion and the Python process will terminate, making the created canvas disappear. If you want to keep the Python process alive and thus be able to inspect your canvas, run the script with python -i

User interface

Besides being the entry point to all ROOT functionality, the ROOT Python module also offers an interface to configure PyROOT and to manipulate PyROOT objects at a lower level. The next subsections describe in more detail the characteristics of the PyROOT user interface.

Configuration options

After importing the ROOT module, one can access the PyROOT configuration object as ROOT.PyConfig. Such object has a set of properties that can be modified to steer the behaviour of PyROOT. For the configuration to be taken into account, it needs to be applied right after ROOT is imported. For example:

import ROOT
ROOT.PyConfig.DisableRootLogon = True
ROOT.PyConfig.IgnoreCommandLineOptions = False

The available configuration options are described next:

  • DisableRootLogon (default False): just like it happens in C++, PyROOT also supports the rootlogon functionality. When PyROOT starts, it will look for a file called in the user’s home directory and, if such file exists, PyROOT will import it. This can be used to make some settings every time PyROOT is launched, for example defining some style for your plots:
# Content of
import ROOT
myStyle = ROOT.TStyle('MyStyle','My graphics style')
myStyle.SetCanvasColor(ROOT.kBlue) # My canvases will have blue color!

If PyROOT cannot find in the user’s home directory, it will look for the equivalent in C++ (.rootlogon.C), first in ROOT’s etc directory, then in the user’s home directory and finally in the current working directory. Note that it is also possible to use both the Python and the C++ rootlogons, since the latter can be loaded from the former, for instance with ROOT.gROOT.LoadMacro('.rootlogon.C').

If the user would like to completely disable the rootlogon functionality, they can do so by setting PyConfig.DisableRootLogon to True.

  • IgnoreCommandLineOptions (default True): if a PyROOT script is executed with some command line arguments, they will be ignored by default by ROOT, so the user is free to process them as they wish. However, by setting PyConfig.IgnoreCommandLineOptions to False, those arguments will be forwarded to ROOT for parsing, e.g. to enable the batch mode from the command line. A complete list of the arguments accepted by ROOT can be found here.

  • ShutDown (default True): when PyROOT is terminating, during its cleanup phase, the ROOT C++ interpreter is shut down. If PyROOT is executed as part of a longer-running application that needs the C++ interpreter, PyConfig.ShutDown can be set to False to prevent that shutdown.

  • StartGUIThread (default True): except when executed from IPython, a Jupyter notebook or in interactive mode, PyROOT starts a thread that periodically polls for ROOT events (e.g. GUI events) to process them. If a given PyROOT application does not need this event processing, it can prevent the creation of the thread by setting PyConfig.StartGUIThread to False.

Enabling batch mode

When running in batch mode, PyROOT will not display any graphics. There are two ways in which a user can activate the batch mode:

  1. Passing -b as a command line argument, e.g. python -b For this to work, PyConfig.IgnoreCommandLineOptions must be set to False inside the PyROOT script, as discussed here.

  2. Calling gROOT.SetBatch in the PyROOT script, right after importing ROOT:

import ROOT

Low-level manipulation of objects

When instantiating a C++ class from Python via PyROOT, both a C++ object and its Python proxy object are created. Such Python object forwards any access to its internal C++ object, thus acting as a proxy. PyROOT offers some functions to inspect or manipulate Python proxies and their C++ counterparts, based on functionality provided by cppyy. All these functions can be accessed with the pattern ROOT.NameOfFunction and they are listed next:

  • AddressOf(obj): when applied to a Proxy object obj, it returns an indexable buffer of length 1, whose only element is the address of the C++ object proxied by obj. The address of the buffer is the same as the address of the address of the C++ object. This function is kept for backwards compatibility with old PyROOT versions.

  • addressof(obj, field, byref): similarly to AddressOf, it can be used to obtain the address of an internal C++ object from its Python proxy. However, addressof returns that address as an integer, not as an indexable buffer. Furthermore, addressof accepts two more parameters: field and byref. field can be used to specify the name of a field in a struct, in order to get the address of that field. If byref is set to true, addressof returns the address of the address of the C++ object. This function is equivalent to cppyy’s addressof. Example usage:

>>> import ROOT
>>> ROOT.gInterpreter.Declare("struct MyStruct { int foo; float bar; };")
>>> mys = ROOT.MyStruct()
>>> ROOT.addressof(mys) # Address of mys' C++ object
>>> ROOT.addressof(mys, 'foo') # Address of "foo" field (same as address of object)
>>> ROOT.addressof(mys, 'bar') # Address of "bar" field
>>> ROOT.addressof(mys, byref = True) # Address of address of mys' C++ object

Moreover, addressof can be used in conjunction with TTree::Branch from Python as shown here.

  • AsCObject: equivalent to cppyy’s as_cobject.

  • BindObject: equivalent to cppyy’s bind_object.

  • MakeNullPointer(class_proxy): equivalent to BindObject(0, class_proxy), it returns a Proxy object of the class represented by class_proxy that is bound to a C++ null pointer. For example, MakeNullPointer(TTree) returns a TTree proxy object which internally points to null. This function is kept for backwards compatibility with old PyROOT versions.

  • SetOwnership(obj, python_owns): a Python proxy can either own or not own its internal C++ object. If a Python proxy owns its C++ object and the proxy is being destroyed, the C++ object will be deleted too. This ownership can be modified for a given Python proxy with SetOwnership. For example, when calling SetOwnership(obj, False), the user makes sure that the C++ object proxied by obj will not be deleted by PyROOT when obj is garbage collected; this is useful e.g. if the user knows the deletion will happen in C++ and wants to prevent a double delete. This functionality should be used with care not to produce memory leaks.

Loading user libraries & Just-In-Time compilation (JITting)

PyROOT allows to use any C++ library from Python, not only ROOT’s libraries. This is possible thanks to the automatic and dynamic bindings between Python and C++ that PyROOT provides. Without any prior generation of wrappers, at execution time, PyROOT can load C++ code and call into it.

This allows for writing high-performance C++, compiling it, and using it from Python. The following options are available, ordered by complexity and performance:

  1. Just-in-time compilation of small strings: Small functions and classes to be used from Python. Especially useful for testing and debugging.
  2. Just-in-time compilation of entire files: Small or medium-size C++ code in single files
  3. Loading C++ libraries, JITting headers: Larger C++ code in libraries. Allows for optimising code for high performance. Headers compiled just in time.
  4. Loading C++ libraries with dictionaries: Load large/very large, high-performance C++ projects, no just-in-time compilation

1. Jitting strings with code

ROOT comes with a C++ interpreter, which can process pieces of C++ code. Sometimes, if such code is short (e.g. the definition of a small function or class) or for rapid exploration/debugging, it is enough the place the C++ code in a Python string, which is passed to the interpreter. The code will be just-in-time compiled (jitted) and is immediately available for invocation, as shown in the example below. Here, the constructor of the C++ class A and the function f are called from Python after defining them via the interpreter.

import ROOT

# Write some C++ code in a string
cpp_code = """
// Function definition
int f(int i) { return i*i; }

// Class definition
class A {
    A() { cout << "Hello PyROOT!" << endl; }

# Inject the code in the ROOT interpreter

# We find all the C++ entities in Python, right away!
a = ROOT.A()   # prints Hello PyROOT!
x = ROOT.f(3)  # x = 9

2. Including a header

If the C++ code we want to use is in a header, we can also ask the interpreter to include and compile it on the fly. Let’s assume we have a header called my_header.h with the following content:

int f(int i) { return i*i; }

class A {
    A() { cout << "Hello PyROOT!" << endl; }

In Python, we can access it like this:

 # Make the header known to the interpreter
 ROOT.gInterpreter.ProcessLine('#include "my_header.h"')

 # We find all the C++ entities in Python, right away!
 a = ROOT.A()   # prints Hello PyROOT!
 x = ROOT.f(3)  # x = 9

3. Loading a library, but JITting the headers

It is also possible to dynamically load C++ libraries with PyROOT, and call functions from the library. The advantage of this method is that all the code in the library needs to be compiled only once, possibly with optimisations (-O2, -O3), and it can be used again and again from Python without further JITting. It is, however, necessary to JIT the headers to make the code usable in Python.

With the following header my_header.h:

int f(int i);

class A {

and the source file:

#include "my_header.h"

int f(int i) { return i*i; }

A::A() { cout << "Hello PyROOT!" << endl; }

one can create the library In Python, we can load and use it like so:

 # First include header, then load C++ library
ROOT.gInterpreter.ProcessLine('#include "my_header.h"')`

# We find all the C++ entities in Python, right away!
a = ROOT.A()   # prints Hello PyROOT!
x = ROOT.f(3)  # x = 9

4. Loading a library with ROOT dictionaries

For larger analysis frameworks, one may not want to compile the headers each time the Python interpreter is started. One may also want to read or write custom C++/C objects in ROOT files, and use them with RDataFrame or similar tools. A large analysis framework might further have multiple libraries. In these cases, one generates ROOT dictionaries, and adds these to the libraries, which will provide ROOT with the necessary information to generate Python bindings on the fly. This is what the large LHC experiments do to steer their analysis frameworks from Python.

  1. Create one or multiple C++ libraries, e.g. as a CMake project that uses ROOT. CMake details
  2. [Optional] Add ClassDef macros for classes that should be read/written from/into files.
  3. Have ROOT generate a dictionary of all classes that should receive I/O capabilities, i.e. that can be written into ROOT files. Use a LinkDef.h file to select which classes or functions ROOT should include in the dictionary.

    The corresponding cmake instructions would look similar to this:

    # Generate G__AnalysisLib.cxx with all the necessary class info:
    ROOT_GENERATE_DICTIONARY(G__AnalysisLib inc/classA.h inc/classB.h ...
                              LINKDEF LinkDef.h)
     add_library(AnalysisLib SHARED
        G__AnalysisLib.cxx) # Important: Here, all dictionary info is directly compiled into the library.
     # Ensure dictionary is generated before compiling the library:
     add_dependencies(AnalysisLib G__AnalysisLib)
  4. Implement the C++ side, and compile the library using CMake.
  5. On the Python side, load the library/libraries with high-performance C++ in one go:
    import ROOT

    NB If either ROOT or the headers that were used to create the dictionaries were moved to a different location, and ROOT cannot find them (it will issue an error in this case), the location of the headers has to be communicated to ROOT:

    import ROOT

    The loading of C++ libraries can even be automated using the of a Python package.

TPython: running Python from C++

ROOT also allows to run Python code from C++ via the TPython class.

The example below shows how to use Exec to run a Python statement, Eval to evaluate a Python expression and get its result back in C++ and Prompt to start an interactive Python session.

root [0] TPython::Exec( "print(1 + 1)" )
root [1] auto b = (TBrowser*)TPython::Eval( "ROOT.TBrowser()" )
(TBrowser *) @0x7ffec81094f8
root [2] TPython::Prompt()
>>> i = 2
>>> print(i)

For a more complete description of the TPython interface, please check the reference guide for TPython .

JupyROOT: (Py)ROOT for Jupyter notebooks

Jupyter notebooks provide a web-based interface that can be used to do interactive computing with ROOT.

How to use it

Two kernels (or language flavours) allow to run ROOT from Jupyter: Python (2 and 3) and C++. In order to use such kernels, there are mainly two options:

  • Local ROOT installation: to install ROOT locally, please follow the instructions here. In addition to ROOT, two Python packages are required, which can be installed e.g. with pip:
pip install jupyter
pip install metakernel

Once all requirements are installed, a Jupyter server with the Python and C++ kernels can be started by running from a terminal:

root --notebook
  • SWAN: CERN provides an online service to create and run Jupyter notebooks, called SWAN. With this option no installation is required: a browser and a working Internet connection are enough. Both the Python and C++ kernels are available in SWAN.

Many ROOT tutorials are available in the form of Jupyter notebooks. For example, most PyROOT tutorials are listed with two buttons below to obtain their notebook version and to open them in SWAN, respectively.

JavaScript graphics

The ROOT graphics are also available in Jupyter, both in Python and C++. Moreover, the user can choose between two modes:

  • Static (default): the graphics are displayed as a static image in the output of the notebook cell.
  • Interactive: the graphics are shown as an interactive JavaScript display. In order to activate this mode, the %jsroot on line needs to be included in a cell. Once enabled, the mode will stay on until it is disabled with %jsroot off (i.e. no need to enable it in every cell). This is an example for a Python notebook:
%jsroot on
c = ROOT.TCanvas()
f1 = ROOT.TF1("func1", "sin(x)", 0, 10)
c.Draw() # Necessary to make the graphics show!

Executing the code above in a cell will make the following interactive canvas appear as output:

Note that the creation and drawing of a canvas are necessary when displaying ROOT graphics in a notebook. If no canvas is drawn in the cell, no graphics output will be shown!

New PyROOT: Backwards-Incompatible Changes

The new PyROOT has some backwards-incompatible changes with respect to its predecessor, which are listed next:

  • Instantiation of function templates must be done using square brackets instead of parentheses. For example, if we consider the following code snippet:
>>> import ROOT

>>> ROOT.gInterpreter.Declare("""
template<typename T> T foo(T arg) { return arg; }

>>>['int']  # instantiation
cppyy template proxy (internal)

>>>'int')  # call (auto-instantiation from arg type)

Note that the above does not affect class templates, which can be instantiated either with parenthesis or square brackets:

>>> ROOT.std.vector['int'] # instantiation
<class cppyy.gbl.std.vector<int> at 0x5528378>

>>> ROOT.std.vector('int') # instantiation
<class cppyy.gbl.std.vector<int> at 0x5528378>
  • Overload resolution in new cppyy has been significantly rewritten, which sometimes can lead to a different overload choice (but still a compatible one!). For example, for the following overloads of std::string:
string (const char* s, size_t n)                           (1)
string (const string& str, size_t pos, size_t len = npos)  (2)

when invoking ROOT.std.string(s, len(s)), where s is a Python string, the new PyROOT will pick (2) whereas the old would pick (1).

  • The conversion between None and C++ pointer types is not allowed anymore. Instead, ROOT.nullptr should be used:
>>> ROOT.gInterpreter.Declare("""
class A {};
void foo(A* a) {}

>>>  # ok

>>>          # fails
TypeError: void ::foo(A* a) =>
TypeError: could not convert argument 1
  • Old PyROOT has ROOT.Long and ROOT.Double to pass integer and floating point numbers by reference. In the new PyROOT, ctypes must be used instead.
>>> ROOT.gInterpreter.Declare("""
void foo(int& i) { ++i; }
void foo(double& d) { ++d; }

>>> import ctypes

>>> i = ctypes.c_int(1)
>>> d = ctypes.c_double(1.)

>>>; i

>>>; d
  • When a character array is converted to a Python string, the new PyROOT only considers the characters before the end-of-string character:
>>> ROOT.gInterpreter.Declare('char MyWord[] = "Hello";')

>>> mw = ROOT.MyWord

>>> type(mw)
<class 'str'>

>>> mw  # '\x00' is not part of the string
  • Any Python class derived from a base C++ class now requires the base class to define a virtual destructor:
>>> ROOT.gInterpreter.Declare("class CppBase {};")
>>> class PyDerived(ROOT.CppBase): pass
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: CppBase not an acceptable base: no virtual destructor
  • There are several name changes for what concerns cppyy APIs and proxy object attributes, including:
Old PyROOT/cppyy New PyROOT/cppyy
cppyy.gbl.MakeNullPointer(klass) cppyy.bind_object(0, klass)
cppyy.gbl.BindObject / cppyy.bind_object cppyy.bind_object
cppyy.AsCObject libcppyy.as_cobject
cppyy.gbl.nullptr cppyy.nullptr
cppyy.gbl.PyROOT.TPyException cppyy.gbl.CPyCppyy.TPyException
buffer.SetSize(N) buffer.reshape((N,))
obj.__cppname__ type(obj).__cpp_name__
obj._get_smart_ptr obj.__smartptr__
callable._creates callable.__creates__
callable._mempolicy callable.__mempolicy__
callable._threaded callable.__release_gil__
  • New PyROOT does not parse command line arguments by default anymore. For example, when running python -b, the -b argument will not be parsed by PyROOT, and therefore the batch mode will not be activated. If the user wants to enable the PyROOT argument parsing again, they can do so by starting their Python script with:
import ROOT
ROOT.PyConfig.IgnoreCommandLineOptions = False
  • In new PyROOT, addressof should be used to retrieve the address of fields in a struct, for example:
>>> ROOT.gInterpreter.Declare("""
struct MyStruct {
  int a;
  float b;

>>> s = ROOT.MyStruct()

>>> ROOT.addressof(s, 'a')

>>> ROOT.addressof(s, 'b')

>>> ROOT.addressof(s)

In old PyROOT, AddressOf could be used for that purpose too, but its behaviour was inconsistent. AddressOf(o) returned a buffer whose first position contained the address of object o, but Address(o, 'field') returned a buffer whose address was the address of the field, instead of such address being contained in the first position of the buffer. Note that, in the new PyROOT, AddressOf(o) can still be invoked, and it still returns a buffer whose first position contains the address of object o.

  • In old PyROOT, there were two C++ classes called TPyMultiGenFunction and TPyMultiGradFunction which inherited from ROOT::Math::IMultiGenFunction and ROOT::Math::IMultiGradFunction, respectively. The purpose of these classes was to serve as a base class for Python classes that wanted to inherit from the ROOT::Math classes. This allowed to define Python functions that could be used for fitting in Fit::Fitter. In the new PyROOT, TPyMultiGenFunction and TPyMultiGradFunction do not exist anymore, since their functionality is automatically provided by new cppyy: when a Python class inherits from a C++ class, a wrapper C++ class is automatically generated. That wrapper class will redirect any call from C++ to the methods implemented by the Python class. Therefore, the user can make their Python function classes inherit directly from the ROOT::Math C++ classes, for example:
import ROOT

from array import array

class MyMultiGenFCN( ROOT.Math.IMultiGenFunction ):
    def NDim( self ):
        return 1

    def DoEval( self, x ):
        return (x[0] - 42) * (x[0] - 42)

    def Clone( self ):
        x = MyMultiGenFCN()
        ROOT.SetOwnership(x, False)
        return x

def main():
    fitter = ROOT.Fit.Fitter()
    myMultiGenFCN = MyMultiGenFCN()
    params = array('d', [1.])
    fitter.FitFCN(myMultiGenFCN, params)
    fitter.Result().Print(ROOT.std.cout, True)

if __name__ == '__main__':
  • Inheritance of Python classes from C++ classes is not working in some cases. This is described in ROOT-10789 and ROOT-10582. This affects the creation of GUIs from Python, e.g. in the Python GUI tutorial, where the inheritance from TGMainFrame is not working at the moment. Future releases of ROOT will fix these issues and provide a way to program GUIs from Python, including a replacement for TPyDispatcher, which is no longer provided.

  • When iterating over an std::vector<std::string> from Python, the elements returned by the iterator are no longer of type Python str, but cppyy.gbl.std.string. This is an optimization to make the iteration faster (copies are avoided) and it allows to call modifier methods on the std::string objects.

>>> import cppyy

>>> cppyy.cppdef('std::vector<std::string> foo() { return std::vector<std::string>{"foo","bar"};}')

>>> v =

>>> type(v)
<class cppyy.gbl.std.vector<string> at 0x4ad8220>

>>> for s in v:
...   print(type(s))  # s is no longer a Python string, but an std::string
<class cppyy.gbl.std.string at 0x4cd41b0>
<class cppyy.gbl.std.string at 0x4cd41b0>
  • When obtaining the boolean value of a C++ instance proxy, both old and new PyROOT return False when such proxy points to null. On the other hand, when the proxy points to a C++ object, old PyROOT just returns True, while new PyROOT has slightly modified this behaviour: in new cppyy, if __len__ is available, the result of __len__ is used to determine truth. This is done to comply with the normal Python behaviour, where __len__ is tried if __bool__ is not present (see here). For example, the following code shows how the insertion of __len__ changes the boolean value of an instance proxy:
>>> import ROOT
>>> ROOT.gInterpreter.Declare('class A {};')

>>> a = ROOT.A()
>>> bool(a)

>>> ROOT.A.__len__ = lambda self : 0
>>> bool(a)  # False because len(a) == 0
  • In new cppyy, buffer objects are represented by the LowLevelView type. If such a buffer points to null, it is not iterable, unlike in the old PyROOT. For example, in the following code, GetX() returns a null pointer and therefore an exception will be thrown when calling list(x):
>> import ROOT

>> g = ROOT.TGraph()
>> x = g.GetX()

>> xl = list(x)  # throws ReferenceError

The code above can be protected by checking for the validity of x:

>>> xl = list(x) if x else 'your default'