import ROOT
import os

# We create a data frame on top of the hsimple example.
hsimplePath = os.path.join(str(ROOT.gROOT.GetTutorialDir().Data()), "hsimple.root")
df = ROOT.RDataFrame("ntuple", hsimplePath)

# We apply a simple cut and define a new column.
df_cut = df.Filter("py > 0.f")\
           .Define("px_plus_py", "px + py")

# We cache the content of the dataset. Nothing has happened yet: the work to accomplish
# has been described.
df_cached = df_cut.Cache()

h = df_cached.Histo1D("px_plus_py")

# Now the event loop on the cached dataset is triggered by accessing the histogram.
# This event triggers the loop on the `df` data frame lazily.
c = ROOT.TCanvas()
h.Draw()
c.SaveAs("df019_Cache.png")

print("Saved figure to df019_Cache.png")