[roottest] Inherit Python executable from main process in nbdiff.py

This makes sure that we're using the exact Python version in the
notebook tests that ROOT was built against.

Also, always use the new canonical `jupyter-nbconvert` executable for
notebook conversion.

Author: guitargeek

roottest/python/JupyROOT/nbdiff.py

@@ -2,22 +2,11 @@
 import json
 import os
 import shutil
-import subprocess
 import sys
 import tempfile
 
 nbExtension = ".ipynb"
-convCmdTmpl = (
-    "%s nbconvert "
-    "--to notebook "
-    "--ExecutePreprocessor.kernel_name=%s "
-    "--ExecutePreprocessor.enabled=True "
-    "--ExecutePreprocessor.timeout=3600 "
-    "--ExecutePreprocessor.startup_timeout=180 "
-    "%s "
-    "--output %s"
-)
-pythonInterpName = "python3"
+pythonInterpName = sys.executable
 
 rootKernelFileContent = (
     """{
@@ -149,28 +138,28 @@ def addEtcToEnvironment(inNBDirName):
     return ipythondir
 
 
-def getInterpreterName():
-    """Find if the 'jupyter' executable is available on the platform. If
-    yes, return its name else return 'ipython'
-    """
-    ret = subprocess.call("type jupyter", shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    return "jupyter" if ret == 0 else "i%s" % pythonInterpName
-
-
 def getKernelName(inNBName):
     with open(inNBName) as f:
         nbj = json.load(f)
     if nbj["metadata"]["kernelspec"]["language"] == "python":
-        return pythonInterpName
+        return "python3"
     else:  # we support only Python and C++
         return "root"
 
 
-def canReproduceNotebook(inNBName, kernelName, needsCompare):
+def canReproduceNotebook(inNBName, needsCompare):
     tmpDir = addEtcToEnvironment(os.path.dirname(inNBName))
     outNBName = inNBName.replace(nbExtension, "_out" + nbExtension)
-    interpName = getInterpreterName()
-    convCmd = convCmdTmpl % (interpName, kernelName, inNBName, outNBName)
+    convCmd = (
+        "%s -m jupyter-nbconvert "
+        "--to notebook "
+        "--ExecutePreprocessor.kernel_name=%s "
+        "--ExecutePreprocessor.enabled=True "
+        "--ExecutePreprocessor.timeout=3600 "
+        "--ExecutePreprocessor.startup_timeout=180 "
+        "%s "
+        "--output %s" % (pythonInterpName, getKernelName(inNBName), inNBName, outNBName)
+    )
     exitStatus = os.system(convCmd)  # we use system to inherit the environment in os.environ
     shutil.rmtree(tmpDir)
     if needsCompare:
@@ -206,7 +195,5 @@ def isInputNotebookFileName(filename):
     except:
         raise ImportError("Cannot import jupyter")
 
-    kernelName = getKernelName(nbFileName)
-
-    retCode = canReproduceNotebook(nbFileName, kernelName, needsCompare)
+    retCode = canReproduceNotebook(nbFileName, needsCompare)
     sys.exit(retCode)