Class references for:

• TDirectory,
• TFile.

Here is a basic example creating a file with on directory. If you want to create and save an object in this directory, e.g. a tree or histogram, use dir.cd(). Pay attention to the order of creating an object, changing directory (cd()) and writing the object!

addDirectory.py

from ROOT import TFile
 
file = TFile("tree.root",'update')
 
# make directory if it does not exist
dir = file.GetDirectory("dir")
if not dir:
    print ">>> created dir"
    dir = file.mkdir("dir")
 
# make this directory the current one: everything you write will be saved here
dir.cd()
 
# make tree, histogram, ...
 
file.Write("",TFile.kOverwrite)
file.Close()

Note that gDirectory is the current directory containing all newly created saved objects, or those saved to this directory. If you just opened a file, this will be your current directory until you close it or use cd() on some TDirectory or other TFile. Test the behavior with this macro:

gDirectory.py

from ROOT import TFile, TTree, TH1F, gDirectory
 
def printGDirectory():
    print ">>> gDirectory.GetName()\n%s" % gDirectory.GetName()
    print ">>> gDirectory.pwd()"
    gDirectory.pwd()
    print ">>> gDirectory.ls()"
    gDirectory.ls()
    print
 
print "\ndefault"
printGDirectory()
 
print ">>> creating a file with some contents"
file = TFile("test.root","recreate")
tree = TTree("tree","tree")
hist = TH1F("hist","hist",100,0,100)
dir1 = file.mkdir("dir1")
printGDirectory()
 
print ">>> gDirectory.Delete(\"hist\")"
gDirectory.Delete("hist")
printGDirectory()
 
print ">>> dir1.cd()"
dir1.cd()
printGDirectory()
 
print ">>> file.Close()"
file.Close()
printGDirectory()