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


Answer: 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.

