Python function replacing part of variable

Question

I am writing a code for a project in particle physics (using pyroot).

In my first draft, I use the following line

for i in MyTree:    

   pion.SetXYZM(K_plus_PX, K_plus_PY, K_plus_PZ,K_plus_MM)

This basically assigns to the pion the values of variables in the parenthesis, ie momenta and inv. mass of the kaon.

Physics aside, I would like to write a function "of the form":

def myfunc(particle):
    return %s_PX % particle

I know this is wrong. What I would like to achieve is to write a function that allows, for a given particle, to set particle_PX, particle_PY etc to be the arguments of SetXYZM.

Thank you for your help,

B

Could you clarify what %s_PX % particle is supposed to mean? How would you use this function? — sidney
– sidney, Commented Jul 3, 2016 at 18:36
Possible duplicate of Concatenating variable names in Python — jscs
– jscs, Commented Jul 3, 2016 at 18:39
You are setting i as each item in MyTree, but your code doesn't do anything with i. Can you add more explanation or an example of what you want your output to be? — joel goldstick
– joel goldstick, Commented Jul 3, 2016 at 18:40
@sidney and @joel goldstick, SetXYZM() is a function in ROOT that sets the values of the four momenta of a given particle, so that for particle.SetXYZM(0, 0, 0, 0), the particle will have zero spatial momentum and (for natural units) zero mass (silly example). My difficulty lies in the fact that Kplus_PX and variables of the same form are only numbers (possibly long floats) stored in a TTree ( basically the record of a hadron collision). I need to input manually the momenta and mass, and I was hoping there was a way to make this easier to generalise. — Blaise Delaney
– Blaise Delaney, Commented Jul 3, 2016 at 18:47
I am looking to avoid having a function that has a massive if statement where I spell out explictly all the possible combinations of arguments of SetXYZM() for a particle of choice. — Blaise Delaney
– Blaise Delaney, Commented Jul 3, 2016 at 18:48

pseyfert · Accepted Answer · 2016-07-12 09:53:23Z

To access class attributes from string variables you can use python's getattr:

import ROOT
inputfile = ROOT.TFile.Open("somefile.root","read")
inputtree = inputfile.Get("NameOfTTree")
inputtree.Print()
# observe that there are branches
# K_plus_PX
# K_plus_PY
# K_plus_PZ
# K_plus_MM
# K_minus_PX
# K_minus_PY
# K_minus_PZ
# K_minus_MM
# pi_minus_PX
# pi_minus_PY
# pi_minus_PZ
# pi_minus_MM

def getx(ttree,particlename):
    return getattr(ttree,particlename+"_PX")
def gety(ttree,particlename):
    return getattr(ttree,particlename+"_PY")
def getz(ttree,particlename):
    return getattr(ttree,particlename+"_PZ")
def getm(ttree,particlename):
    return getattr(ttree,particlename+"_MM")
def getallfour(ttree,particlename):
    x = getattr(ttree,particlename+"_PX")
    y = getattr(ttree,particlename+"_PY")
    z = getattr(ttree,particlename+"_PZ")
    m = getattr(ttree,particlename+"_MM")
    return x,y,z,m


for entry in xrange(inputtree.GetEntries()):
    inputtree.GetEntry(entry)
    pion1 = ROOT.TLorentzVector()
    x = getx(inputtree,"K_plus")
    y = gety(inputtree,"K_plus")
    z = getz(inputtree,"K_plus")
    m = getm(inputtree,"K_plus")
    pion2.SetXYZM(x,y,z,m)
    x,y,z,m = getallfour(inputtree,"pi_minus")
    pion2 = ROOT.TLorentzVector()
    pion2.SetXYZM(x,y,z,m)

As linked by Josh Caswell, you can similarly access variable names:

def getx(particlename):
    x = globals()[partilcename+"_PX"]

though that might get nasty quickly as of whether your variables are global or local and for local, in which context.

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