tcontext_context_manager.py File Reference

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 tfile_context_manager.py tutorial.

import os
import sys
 
import ROOT
from ROOT import TDirectory, TFile, gROOT
 
# 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.
path = str(gROOT.GetTutorialDir()) + '/io/'
file_1 = TFile(path+"tcontext_1.root", "recreate")
file_2 = TFile(path+"tcontext_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(path+"tcontext_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
file_1.Close();
file_2.Close();
 
for i in range(1, 4):
    os.remove(path+"tcontext_{}.root".format(i))

Date

March 2022

Author

Vincenzo Eduardo Padulano CERN/UPV

Definition in file tcontext_context_manager.py.