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--- root:pyroot_tfile [2017/06/13 09:37] – created iwn
+++ root:pyroot_tfile [2017/09/05 10:37] (current) – [Handling TFile files] iwn
@@ Line 1: / Line 1: @@
-====== Handling TH1 histograms ======
+[[root:root|Getting started with ROOT]] -> [[root:pyroot|Useful pyROOT snippets]] → [[root:pyroot_tfile|Handling TFile files]]
-===== Filling a histogram =====
+====== Handling TFile files ======
-Simple enough:
+Class references for:
+  * ''[[https://root.cern.ch/doc/master/classTDirectory.html|TDirectory]]'',
+  * ''[[https://root.cern.ch/doc/master/classTFile.html|TFile]]''.
+===== Adding a directory to a TFile =====
-<code python>
+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!
-hist = TH1D("hist_name","hist title",100,0,100)
-for i in xrange(10000):
-    hist.Fill(gRandom.Gauss())
-</code>
-With some weight:
+<file python addDirectory.py>
-<code python>
+from ROOT import TFile
-weight = gRandom.Gauss(1,0.1)
-hist.Fill(gRandom.Gauss(),weight)
-</code>
-===== Filling a histogram from a tree =====
+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()
-The classic way would be
+# make tree, histogram, ...
-<code python>
-hist = TH1D("hist_name","hist title",100,0,100)
+file.Write("",TFile.kOverwrite)
-for event in tree:
+file.Close()
-    hist.Fill(tree.px)
+</file>
-</code>
-But the simplest and most powerful way to draw some variable into a histogram, is with the [[https://root.cern.ch/doc/master/classTTree.html#a73450649dc6e54b5b94516c468523e45|Draw]] function:
-<code python>
-hist = TH1D("hist_name","hist title",100,0,100)
-tree.Draw("px >> hist_name")
-</code>
-Note that the histogram has been saved into ''ROOT'''s working memory and is therefore accessible via its name in the ''Draw'' function, ''"hist_name"'' in this example.
-You can also use mathematical expressions:
+===== gDirectory =====
-<code python>
-tree.Draw("2*sqrt(px*px+py*py) >> hist_name")
-tree.Draw("abs(eta) >> hist_name")
-</code>
-You can apply some selections using variable, available in the tree's branches:
+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:
-<code python>
-tree.Draw("px >> hist_name","pt>20 && E>20")
-</code>
-You can apply some selections and/or a weight, if a weight variable is available in the tree:
+<file python gDirectory.py>
-<code python>
+from ROOT import TFile, TTree, TH1F, gDirectory
-tree.Draw("px >> hist_name","(pt>20 && E>20)*weight")
-tree.Draw("px >> hist_name","weight")
-</code>
-An equivalent method is creating the histogram in the ''Draw'' method. If you need the ''TH1F'' object, you can still get it with ''gDirectory'' (see [[root:pyroot#gDirectory|below]]):
+def printGDirectory():
-<code python>
+    print ">>> gDirectory.GetName()\n%s" % gDirectory.GetName()
-from ROOT import gDirectory
+    print ">>> gDirectory.pwd()"
-...
+    gDirectory.pwd()
-tree.Draw("pt >> h(100,0,100)")
+    print ">>> gDirectory.ls()"
-hist = gDirectory.Get("h") # will be a TH1F object
+    gDirectory.ls()
-</code>
+    print
-It's also possible to pass some option you would normally use in "TH1::Draw()". It is passed as the third string.
+print "\ndefault"
-<code python>
+printGDirectory()
-tree.Draw("px >> h1(100,0,100)","","E2")
-tree.Draw("py >> h2(100,0,100)","","E2 SAME")
-tree.Draw("px >> h1(100,0,100)","abs(eta)>2","E2")
+print ">>> creating a file with some contents"
-tree.Draw("px >> h2(100,0,100)","abs(eta)<2","E2 SAME")
+file = TFile("test.root","recreate")
-</code>
+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()
+</file>
-__Protip__: This is very useful for quick drawing and comparing, for example in the ''ROOT'' command line.
-<code C++>
-$ root -l tree.root
-[1] tree->Draw("px >> h1(100,0,100)","","E2")
-[2] tree->Draw("py >> h2(100,0,100)","","E2 SAME")
-</code>

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