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pyroot006_tcontext_context_manager.py File Reference

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This tutorial demonstrates the usage of the TContext class as a Python context manager. This functionality is related with how TFile works, so it is suggested to also take a look at the pyroot005 tutorial.

import os
import ROOT
from ROOT import TDirectory, TFile
# Sometimes it is useful to have multiple open files at once. In such cases,
# the current directory will always be the file that was open last.
file_1 = TFile("pyroot006_file_1.root", "recreate")
file_2 = TFile("pyroot006_file_2.root", "recreate")
print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))
# Changing directory into another file can be safely done through a TContext
# context manager.
with TDirectory.TContext(file_1):
# Inside the statement, the current directory is file_1
print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))
histo_1 = ROOT.TH1F("histo_1", "histo_1", 10, 0, 10)
file_1.WriteObject(histo_1, "my_histogram")
# After the context, the current directory is restored back to file_2. Also, the
# two files are kept open. This means that objects read, written or modified
# inside the context are still available afterwards.
print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))
if file_1.IsOpen() and file_2.IsOpen():
print("'{}' and '{}' are open.\n".format(file_1.GetName(), file_2.GetName()))
# TContext and TFile context managers can also be used in conjunction, allowing
# for safely:
# - Opening a file, creating, modifying, writing and reading objects in it.
# - Closing the file, storing it on disk.
# - Restoring the previous value of gDirectory to the latest file opened before
# this context, rather than to the global ROOT.gROOT
# Remember that the TContext must be initialized before the TFile, otherwise the
# current directory would already be set to the file opened for this context.
with TDirectory.TContext(), TFile("pyroot006_file_3.root", "recreate") as f:
print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))
histo_2 = ROOT.TH1F("histo_2", "histo_2", 10, 0, 10)
f.WriteObject(histo_2, "another_histogram")
print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))
# Cleanup the files created for this tutorial
for i in range(1, 4):
Option_t Option_t TPoint TPoint const char GetTextMagnitude GetFillStyle GetLineColor GetLineWidth GetMarkerStyle GetTextAlign GetTextColor GetTextSize void char Point_t Rectangle_t WindowAttributes_t Float_t Float_t Float_t Int_t Int_t UInt_t UInt_t Rectangle_t Int_t Int_t Window_t TString Int_t GCValues_t GetPrimarySelectionOwner GetDisplay GetScreen GetColormap GetNativeEvent const char const char dpyName wid window const char font_name cursor keysym reg const char only_if_exist regb h Point_t winding char text const char depth char const char Int_t count const char ColorStruct_t color const char Pixmap_t Pixmap_t PictureAttributes_t attr const char char ret_data h unsigned char height h Atom_t Int_t ULong_t ULong_t unsigned char prop_list Atom_t Atom_t Atom_t Time_t format
TDirectory::TContext keeps track and restore the current directory.
Definition TDirectory.h:89
A ROOT file is an on-disk file, usually with extension .root, that stores objects in a file-system-li...
Definition TFile.h:53
Current directory: 'pyroot006_file_2.root'.
Current directory: 'pyroot006_file_1.root'.
Current directory: 'pyroot006_file_2.root'.
'pyroot006_file_1.root' and 'pyroot006_file_2.root' are open.
Current directory: 'pyroot006_file_3.root'.
Current directory: 'pyroot006_file_2.root'.
March 2022
Vincenzo Eduardo Padulano CERN/UPV

Definition in file pyroot006_tcontext_context_manager.py.