import ROOT

# start by creating an empty workspace
ws = ROOT.RooWorkspace("workspace")

# the RooJSONFactoryWSTool is responsible for importing and exporting things to and from your workspace
tool = ROOT.RooJSONFactoryWSTool(ws)

# use it to import the information from your JSON file
tool.importJSON(ROOT.gROOT.GetTutorialDir().Data() + "/roofit/roofit/rf515_hfJSON.json")
ws.Print()

# now, you can easily use your workspace to run your fit (as you usually would)
# the model config is named after your pdf, i.e. <the pdf name>_modelConfig
model = ws["ModelConfig"]

# for resetting the parameters after the fit
params = model.GetPdf().getParameters(ws["observed"])
ROOT.SetOwnership(params, True)
params_initial = params.snapshot()
ROOT.SetOwnership(params_initial, True)

# we are fitting a clone of the model now,
result = model.fitTo(ws["observed"], ROOT.RooFit.Save(), ROOT.RooFit.PrintLevel(-1))
ROOT.SetOwnership(result, True)
result.Print()
# reset parameters, such that we are not double-fitting the model in the
# closure check.
params.assign(params_initial)

# in the end, you can again write to json
# the result will be not completely identical to the JSON file you used as an input, but it will work just the same
tool.exportJSON("myWorkspace.json")

# You can again import it if you want and check for closure
ws_2 = ROOT.RooWorkspace("workspace")
tool_2 = ROOT.RooJSONFactoryWSTool(ws_2)
tool_2.importJSON("myWorkspace.json")
ws_2.Print()
model_2 = ws_2["ModelConfig"]
result = model_2.fitTo(ws_2["observed"], ROOT.RooFit.Save(), ROOT.RooFit.PrintLevel(-1))
ROOT.SetOwnership(result, True)
result.Print()