doc/master/tfile__context__manager_8py.html

import os


import ROOT

from ROOT import TFile, gROOT


# By default, objects of some ROOT types such as `TH1` and its derived types

# are automatically attached to a ROOT.TDirectory when they are created.

# Specifically, at any given point of a ROOT application, the ROOT.gDirectory

# object tells which is the current directory where objects will be attached to.

# The next line will print 'PyROOT' as the name of the current directory.

# That is the global directory created when using ROOT from Python, which is

# the ROOT.gROOT object.

print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))


# We can check to which directory a newly created histogram is attached.

histo_1 = ROOT.TH1F("histo_1", "histo_1", 10, 0, 10)

print("Histogram '{}' is attached to: '{}'.\n".format(histo_1.GetName(), histo_1.GetDirectory().GetName()))


# For quick saving and forgetting of objects into ROOT files, it is possible to

# open a TFile as a Python context manager. In the context, objects can be

# created, modified and finally written to the file. At the end of the context,

# the file will be automatically closed.

path = str(gROOT.GetTutorialDir()) + '/io/'

filename = path+"tfile_1.root"

with TFile.Open(filename, "recreate") as f:

    histo_2 = ROOT.TH1F("histo_2", "histo_2", 10, 0, 10)

    # Inside the context, the current directory is the open file

    print("Current directory: '{}'.\n".format(ROOT.gDirectory.GetName()))

    # And the created histogram is automatically attached to the file

    print("Histogram '{}' is attached to: '{}'.\n".format(histo_2.GetName(), histo_2.GetDirectory().GetName()))

    # Before exiting the context, objects can be written to the file

    f.WriteObject(histo_2, "my_histogram")


# When the TFile.Close method is called, the current directory is automatically

# set again to ROOT.gROOT. Objects that were attached to the file inside the

# context are automatically deleted, so they can't be accessed anymore after

# the file is closed.

print("Status after the first TFile context manager:")

print(" Current directory: '{}'.".format(ROOT.gDirectory.GetName()))

# print(histo_2) # the object is deleted at this point, so don't use it anymore!


# Also reading data from a TFile can be done in a context manager. Information

# stored in the objects of the file can be queried and used inside the context.

# After the context, the objects are not usable anymore because the file is

# automatically closed. This means you should use this pattern as a quick way

# to get information or modify objects from a certain file, without needing to

# keep the histograms alive afterwards.


with TFile.Open(filename, "read") as f:

    # Retrieve histogram using the name given to f.WriteObject in the previous

    # with statement

    histo_2_fromfile = f["my_histogram"]

    print("Retrieved '{}' histogram from file '{}'.\n".format(histo_2_fromfile.GetName(), f.GetName()))


# Cleanup the file created for this tutorial

os.remove(filename)

format
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
Definition TGWin32VirtualXProxy.cxx:249

TFile::Open
static TFile * Open(const char *name, Option_t *option="", const char *ftitle="", Int_t compress=ROOT::RCompressionSetting::EDefaults::kUseCompiledDefault, Int_t netopt=0)
Create / open a file.
Definition TFile.cxx:4086