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First Steps in LHCb
Interactively exploring a DST
10

Learning Objectives {.objectives}

  • Open a DST in an interactive python session
  • Print all nodes in a DST
  • Explore the contents of the TES
  • Inspect a track
  • Inspect a stripping location

Data is stored in files called DSTs, which are processed by DaVinci to make nTuples. However you can also explore them interactively from a python session.

This is particularly useful if you want to quickly find something out, or the more complex processing in DaVinci is not working as expected.

The file we downloaded from the grid contains simulated data, with stripping and trigger decisions and so on. Here we assumed the file you downloaded is called 00035742_00000002_1.allstreams.dst. To take a look at the contents of the TES, we need to write a small Python file:

import sys

import GaudiPython as GP
from GaudiConf import IOHelper
from Configurables import DaVinci

dv = DaVinci()
dv.DataType = '2012'

# Pass file to open as first command line argument
inputFiles = [sys.argv[-1]]
IOHelper('ROOT').inputFiles(inputFiles)

appMgr = GP.AppMgr()
evt = appMgr.evtsvc()

appMgr.run(1)
evt.dump()

Place this into a file called first.py and run the following command in a new terminal:

$ lb-run DaVinci/v41r2 ipython -i first.py 00035742_00000002_1.allstreams.dst

This will open the DST and print out some of the TES locations which exist for this event. We are now ready to explore the TES, which is accessible via the evt variable. For example you could look at the properties of some tracks for the first event by typing inside the python session:

tracks = evt['/Event/Rec/Track/Best']
print tracks[0]

The next question is, how do you know what TES locations that could exist? As we saw evt.dump() prints a few of them, but not all. In addition there are some special ones that only exist if you try to access them. The following snippet allows you to discover most TES locations that are interesting:

def nodes(evt, node=None):
    """List all nodes in `evt`"""
    nodenames = []

    if node is None:
        root = evt.retrieveObject('')
        node = root.registry()

    if node.object():
        nodenames.append(node.identifier())
        for l in evt.leaves(node):
            # skip a location that takes forever to load
            # XXX How to detect these automatically??
            if 'Swum' in l.identifier():
                continue

            temp = evt[l.identifier()]
            nodenames += nodes(evt, l)

    else:
        nodenames.append(node.identifier())

    return nodenames

The easiest way to use it is to add it to your first.py script and re-run it with ipython -i first.py 00035742_00000002_1.allstreams.dst. This will list a large number of TES locations, but even so there are some which you have to know about. Another oddity is that some locations are "packed", for example: /Event/AllStreams/pPhys/Particles. You can not access these directly at this location. Instead you have to know what location the contents will get unpacked to when you want to use it. Often you can just try removing the small p from the location (/Event/AllStreams/Phys/Particles).

You can also inspect the particles and vertices built by your stripping line. However not every event will contain a candidate for your line, so the first tool we need is something that will advance us until the stripping decision was positive:

def advance(decision):
    """Advance until stripping decision is true, returns
    number of events by which we advanced"""
    n = 0
    while True:
        appMgr.run(1)

        if not evt['/Event/Rec/Header']:
            print 'Reached end of input files'
            break

        n += 1
        dec=evt['/Event/Strip/Phys/DecReports']
        if dec.hasDecisionName('Stripping{0}Decision'.format(decision)):
            break

    return n

Add this to your script and restart ipython as before.

Detecting file ends {.callout}

It is not easy to detect that the input file has ended. Especially if you want to get it right for data and simulation. Checking that /Event/Rec/Header exists is a safe bet in simulation and data if your file has been processed by Brunel (the event reconstruction software). It might not work in other cases.

Using the name of our stripping line we can now advance through the DST until we reach an event which contains a candidate:

line = 'D2hhCompleteEventPromptDst2D2RSLine'
advance(line)

The candidates built for you can now be found at /Event/AllStreams/Phys/D2hhCompleteEventPromptDst2D2RSLine/Particles:

cands = evt['/Event/AllStreams/Phys/D2hhCompleteEventPromptDst2D2RSLine/Particles']
print cands.size()

This tells you how many candidates there are in this event and you can access the first one with:

print cands[0]

Which will print out some information about the Particle. In our case a D*. You can access its daughters with cands[0].daughtersVector()[0] and cands[0].daughtersVector()[1], which will be a D0 and a pion.

There is a useful tool for printing out decay trees, which you can pass the top level particle to and it will print out the daughters etc:

print_decay = appMgr.toolsvc().create(
  'PrintDecayTreeTool', interface='IPrintDecayTreeTool'
)
print_decay.printTree(cands[0])

With our candidates in hand, it would be nice to be able to retrieve and compute the variables we need for an analysis. On to LoKi functors!

Fast DST browsing {.callout}

While here we have discussed for pedagogical reasons all the configuration options needed in order to browse a DST file, in your daily life as a physicist it is often useful to use the bender application that belongs to the Bender project.

For example, to explore the DST we could have simply done:

lb-run Bender/latest bender 00035742_00000002_1.allstreams.dst

This leaves us in a prompt in which we can proceed as discussed in this lesson, with the advantage that some functions are already provided for us, such as seekStripDecision (which replaces our advance) or ls and get, which allow to list and get TES locations. Other examples of useful functions are listed in the bender starting banner.

Bender also provides a useful command dst-dump, which is a quick way of figuring out what objects are present on a DST and where. Try out:

lb-run Bender/latest dst-dump -f -n 100 00035742_00000002_1.allstreams.dst

The -f option tells Bender to try and "unpack" the locations such as /Event/AllStreams/pPhys/Particles that we mentioned above, while -n 100 tells it to only process the first 100 events on the DST. Give this a try if you're ever stuck figuring out where your candidates are hiding!