ntuple

Python package for ntuple production

What is an n-tuple?

In its broad sense an n-tuple is a sequence (or ordered list) of n elements, where n is a non-negative integer (see Wikipedia). In the case of High Energy Particle Physics (HEP) an n-tuple is considered a 'flat' TTree as defined by the ROOT framework. This is an ordered list of elements (we call them events), where each event property is defined as a 'leaf':

tree
	- property1
	- property2
	...

In an n-tuple the properties are either basic types (and string) or sequences of them (e.g. vector<int>). From a programming perspective, the Tree behaves much like a list of Python's namedtuple:

for event in tree:
    print event.property1
    for e in event.collection1:
    	print e.pt()

What can I do with this package.

The main goal of this package is to translate a TTree with C++ object elements into a TTree with basic types (or sequences of them). This module provides three components to implement that goal:

• n-tuple map: a simple way to map functions and properties of C++ objects in a TTree to simple types
• converter for input files and their content
• a summary component for bookkeeping

The n-tuple map

The n-tuple map relies on the three classes NTupleContent, NTupleVariable and NTupleCollection. NTupleContent is a container that represents the final content of your n-tuple and constists of one or more NTupleVariable/NTupleCollection. NTupleVariable defines a simple 1:1 mapping between original content for basic types and string. The NTupleCollection constists of one or more NTupleVariable and maps a vector of objects onto vectors of basic types (n:m mapping, where n can be equal to m).

For clarification an example is provided:

n = NTupleContent(tree_name = 'events', output_file = 'ntuple.root')
isRealData = NTupleVariable = (output_name = "isRealData", 
					   vtype = 'bool', extract_function = lambda event: event.isRealData(), help_doc = 'Maps event.isRealData() to event.isRealData')
# this will produce the vectors electron.pt, electron.eta and electron.charge
electrons = NTupleCollection('electron', source = lambda event: event.slimmedElectrons, 
							help_doc = 'maps event.slimmedElectrons.<> onto event.electron.<> where <> mappings are defined by added NTupleVariables',
							variables = [
							NTupleVariable = ("pt", vtype = 'float', extract_function = lambda e: e.pt(), help_doc = 'electron transverse momentum (pt)'),
					   		NTupleVariable = ("eta", vtype = 'float', extract_function = lambda e: e.eta(), help_doc = 'electron pseudorapidity'),
					   		NTupleVariable = ("charge", vtype = 'int', extract_function = lambda e: e.charge(), help_doc = 'electron charge'),
							]
							)
n.add_variable(isRealData)
n.add_collection(electrons)

Converters

Converters are meant to normalise the input to the ntuple component. The following is an example of the CMS software (CMSSW) converter.

The CMSSWConverter is a wrapper for the n-tuple component which maps

• file input using edmFileUtil which will translate a global path, i.e. a path starting with /store into a local path (file://) if the file is available at the local storage element or into a remote file path (root://) if it is not. Paths on the local file system are not changed
• event._event.getRun() to event.getRun() (also isRealData, bunchCrossing, luminosityBlock, orbitNumber, time)
• handle = Handle ('std::vector<pat::Muon>');label = ("slimmedMuons");event.getByLabel (label, handle);muons = handle.product() to event.slimmedMuons

In a full example (taking the NTupleContent definition from the previous section) is shown below:

events = CMSSWConverter(file_to_process)
events.prepare_sources(n.get_sources())
map(n.fill, events)
# write new tree to file
n.save()

# summary over
summary = n.summary()
summary.extend(events.summary())
summary.save()