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8
Interfacing Gaudi with Python
Gaudi Framework Tutorial, 2004
Objectives
After completing this lesson, you should
be able to:
• Understand how Gaudi is interfaced to
Python
• Convince you that Python is a very
good programming language for Rapid
Prototyping
• Write simple Python scripts to access
LHCb data
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Gaudi Framework Tutorial, 2004
Python Language
Python is an Open Source programming language
created by Guido van Rossum. While it has been
available since 1990, it has recently seen a dramatic
increase in popularity in a wide variety of domains
Python is simple, elegant, powerful and reliable
Features:
• Variables & Arithmetic expressions
• String manipulations
• Conditionals (if/else statements), Loops (for/while)
• Functions, Lists, Dictionaries, Classes (Objects),
Exceptions , Modules
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Gaudi Framework Tutorial, 2004
Why Python?
Free, Open-Source, Maintainable
Cross-Platform, Portable
General-Purpose, High-level, Object-Oriented
Interpreted
• However it is quite fast (byte code idea from Java)
Dynamically typed, Introspective
• Not need to declare any variable
Simple syntax, Robust
• Emphasis by the author to minimize typing
Variety of Shells
Powerful built-in types and modules
 Ideal for Scripting and Prototyping
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Gaudi Framework Tutorial, 2004
Python as a “Glue”
Extension Modules
• Very easy to interface to C++ classes (C-API)
Collaborative
• Extension modules can be plugged and used together
Very rich set
specialized generic
modules
LHC modules
Gateways to
other
frameworks
EDG API
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PVSS
XML
Several GUI
toolkits
Database
Python
JPE
PyROOT
gaudipython
Java
Classes
Root
Classes
Gaudi
Very rich set
Framework of Python
Gaudi Framework Tutorial, 2004
math
math
standard
modules
GUI
GUI
shell
Python Binding Techniques
Python provides a very complete C-API
• By hand bindings are possible but even for
medium-sized projects, it becomes cumbersome
and unmaintainable.
Developing Python bindings
• Several existing options: SWIG, Boost.Python, SIP
PyLCGDict: Python binding to the LCG Dictionary
• It allows the user to interact with any C++ class for
which the LCG Dictionary has been generated
• With this module, there is no need to generate
specialized Python bindings or wrapper code to
interact with C++ classes
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Gaudi Framework Tutorial, 2004
PyLCGDict: Mode d’emploi
select.xml
MyClass.h
MyClass.so
lcgdict
MyDict.so
load
PyLCGDict
MyScript.py
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Python
interpreter
Gaudi Framework Tutorial, 2004
From class definitions (.h
files) a “dictionary” library is
produced
• Description of the class
• “stub” functions to class
methods
Absolutely non-intrusive
The PyLCGDict module does
the adaptation between
Python objects and C++
objects in a generic way
• It works for any
dictionary
PyLCGDict: Supported Features
Conversion of C++ and Python primitive types
C++ classes
• Mapped to Python classes and loaded on demand.
Templated classes supported.
C++ namespaces
• Mapped to python scopes. The "::" separator is replaced
by the python "." separator
Class methods
• Static and non static class methods are supported.
Default arguments.
• Method arguments are passed by value or by reference
• The return values are converted into Python types and
new Python classes are created if required. Dynamic
type returned if possible.
• Method overloading works by dispatching sequentially
to the available methods with the same name until a
match with the provided arguments is successful.
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Gaudi Framework Tutorial, 2004
PyLCGDict: Supported Features
Class data members
• Public data members are accessible as Python
properties
Emulation of Python containers
• Container C++ classes (std::vector, std::list, std::map
like) are given the behavior of the Python collections to
be use in iterations and slicing operations.
Operator overloading
• Standard C++ operators are mapped to the
corresponding Python overloading operators
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Gaudi Framework Tutorial, 2004
PyLCGDict: Example (1)
#include <iostream>
namespace Example {
class MyClass {
public:
MyClass() : m_value(0) {}
MyClass(const MyClass& m ) : m_value(m.m_value) {}
~MyClass() {}
int doSomething(const std::string&amp; something ) {
std::cout << “I am doing something with “
<< something << std::endl;
return something.size();
}
int value() { return m_value; }
void setValue(int v) { m_value = v; }
private:
int m_value;
public:
float fprop;
std::string sprop;
};
}
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Gaudi Framework Tutorial, 2004
MyClass.h
PyLCGDict: Example (2)
> python
Python 2.2.2 (#1, Feb 8 2003, 12:11:31) [GCC 3.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import PyLCGDict
>>> PyLCGDict.loadDict('MyClassDict')
>>> m = Example.MyClass()
>>> m.doSomething('care')
I am doing something with care
4
>>> m.setValue(99)
>>> print m.value()
99
>>> n = Example.MyClass(m) # copy constructor
>>> print n.value()
99
>>> n.fprop = 1.2
>>> n.sprop = 'String property'
>>> print n.sprop, n.fprop
String property 1.2
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Gaudi Framework Tutorial, 2004
Another Example: ROOT
The ROOT.py loads the
ROOTDict.so and initializes
the ROOT system
>>>
>>>
>>>
>>>
...
...
...
>>>
>>>
from ROOT import gRandom, TCanvas, TH1F
c1 = TCanvas('c1','Example',200,10,700,500)
hpx = TH1F('hpx','px',100,-4,4)
for i in xrange(25000):
px = gRandom.Gaus()
hpx.Fill(px)
hpx.Draw()
c1.Update()
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Gaudi Framework Tutorial, 2004
GaudiPython
Enabling the interaction of Gaudi components from
Python interpreted
• Configuration, Interactivity, etc.
Starting from Gaudi v14r1, GaudiPython has been
re-implemented using PyLCGDict
• Generated dictionaries for most common
Gaudi “Interfaces” and “Base classes” (~80
classes)
• Not need to generate dictionaries for all
classes (in particular the implementations)
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Gaudi Framework Tutorial, 2004
GaudiPython
The end-user module “gaudimodule.py”
hides some of the technicalities and adds
some handy functionality
• Very easy to extern/modify/adapt since
is written in Python
• Basically backward compatible with
previous version
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Gaudi Framework Tutorial, 2004
Using GaudiPython
Add in the requirements file:
use GaudiPython v*
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Gaudi Framework Tutorial, 2004
Starting Methods
Starting a Gaudi application from Python Shell
>>> import gaudimodule
>>> gaudi = gaudimodule.AppMgr()
>>> gaudi.run(100)
...
>>> gaudi.exit()
Starting a Python interpreted from a Gaudi application
Optional
initial script
ApplicationMgr.DLLs
+= { "GaudiPython“ };
ApplicationMgr.Runable = "PythonScriptingSvc";
PythonScriptingSvc.StartupScript = "../options/AnalysisTest.py";
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Gaudi Framework Tutorial, 2004
Configuration
Instead of using “JobOptions” files we could use
Python to completely configure Gaudi applications
• ATLAS is going in this direction
“Properties” are mapped to native Python types (int,
float, str, list, etc)
>>>
>>>
>>>
>>>
>>>
>>>
>>>
gaudi = gaudimodule.AppMgr()
gaudi.EvtMax = 100
gaudi.TopAlg = [‘Alg1’, ‘Alg2’]
alg1 = gaudi.algorithm(‘Alg1’)
alg1.EnergyCut = 10.7
msgsvc = gaudi.service('MessageSvc')
msgsvc.OutputLevel = 3
...
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Gaudi Framework Tutorial, 2004
Interacting with the Event
Since we have the complete set of Dictionaries
we can interact with any Event class
References to Event objects are obtained by
asking the Event data service ( AppMgr.evtSvc() )
• There are currently some small limitations
(i.e. handling directly KeyedContainer<T>,
etc.)
>>>
>>>
>>>
>>>
>>>
...
evt = gaudi.evtSvc()
header = evt['Header']
print ‘Run number = ‘, header.runNum()
mcparts = evt['MC/Particles']
for p in mcparts.containedObjects() :
print p.momentum().px()
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Gaudi Framework Tutorial, 2004
Interacting with Histograms
It is possible to book and fill histograms using
the Histogram data service
• Same interface as in C++
The complete AIDA Histogram interfaces are
available to Python
>>>
>>>
>>>
>>>
...
>>>
import random
his = gaudi.histsvc()
h1 = his.book('h1',‘Histogram title', 40, 0, 1000)
for v in range(1000) :
h.fill(random.uniform(0,1000))
print h1.mean()
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Gaudi Framework Tutorial, 2004
Complete Example: Read a DST file
import gaudimodule
gaudi = gaudimodule.AppMgr(outputlevel=5,
joboptions='$EVENTSYSROOT/options/PoolDicts.opts')
sel = gaudi.evtsel()
sel.open(['PFN:rfio:/castor/cern.ch/lhcb/DC04/00000541_00000001_10.dst'])
evt = gaudi.evtsvc()
his = gaudi.histsvc()
h1 = his.book('h1','# of MCParticles', 40, 0, 5000)
class DumpAlg(gaudimodule.PyAlgorithm):
def execute(self):
evh = evt['Header']
mcps = evt['MC/Particles']
print 'event # = ',evh.evtNum()
h1.fill(mcps.size())
gaudi.addAlgorithm(DumpAlg())
gaudi.run(10)
print h1, h1.contents()
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Gaudi Framework Tutorial, 2004
Ex/PythonExample
Complete Example: Read a DST file
EventSelector
SUCCESS Reading Event record 0. Record number within stream 0: 0
event # = 1
event # = 2
event # = 3
event # = 4
event # = 5
event # = 6
event # = 7
event # = 8
event # = 9
event # = 10
Histogram 1D "# of MCParticles" 40 bins [0.000000,5000.000000]
[5, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
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Gaudi Framework Tutorial, 2004
Building on GaudiPython
It is fairly easy to extern the functionality
provided by GaudiPython
There are already several “products” which are
build on top of GaudiPython
Bender
• Physics Analysis on Python
Panoramix
• Detector and Event Visualization
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Gaudi Framework Tutorial, 2004
Summary
This is an attempt to convince you that Python is a very
good programming language for Rapid Prototyping
• Efficient way to test your Physics Analysis ideas
Interfacing C++ classes to Python is fairly easy
• Several well established technologies exists
(Boost.Python, etc.)
• PyLCGDict is another way of doing it
• PyLCGDict exploits the LCG dictionaries that are
already available due to object persistency (POOL)
GaudiPython
• Is the package that enables the interaction of Gaudi
from Python
• Latest version of GaudiPython is based on PyLCGDict
Examples of the available functionality has been shown
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Gaudi Framework Tutorial, 2004