[PyROOT] Add __reduce__ method to ROOTFacade to help serialization tools

bindings/pyroot/pythonizations/python/ROOT/_facade.py

+import importlib
 import types
 import sys
 import os
@@ -75,6 +76,17 @@ def _create_rdf_experimental_distributed_module(parent):
 
     return experimental
 
+
+def _subimport(name):
+    # type: (str) -> types.ModuleType
+    """
+    Import and return the Python module with the input name.
+
+    Helper function for the __reduce__ method of the ROOTFacade class.
+    """
+    return importlib.import_module(name)
+
+
 class ROOTFacade(types.ModuleType):
     """Facade class for ROOT module"""
 
@@ -253,13 +265,41 @@ class ROOTFacade(types.ModuleType):
                 logons = [
                     os.path.join(str(self.TROOT.GetEtcDir()), 'system' + name),
                     os.path.expanduser(os.path.join('~', name))
-                    ]
+                ]
                 if logons[-1] != os.path.join(os.getcwd(), name):
                     logons.append(name)
                 for rootlogon in logons:
                     if os.path.exists(rootlogon):
                         self.TApplication.ExecuteFile(rootlogon)
 
+    def __reduce__(self):
+        # type: () -> types.ModuleType
+        """
+        Reduction function of the ROOT facade to customize the (pickle)
+        serialization step.
+
+        Defines the ingredients needed for a correct serialization of the
+        facade, that is a function that imports a Python module and the name of
+        that module, which corresponds to this facade's __name__ attribute. This
+        method helps serialization tools like `cloudpickle`, especially used in
+        distributed environments, that always need to include information about
+        the ROOT module in the serialization step. For example, the following
+        snippet would not work without this method::
+
+            import ROOT
+            import cloudpickle
+
+            def foo():
+                return ROOT.TH1F()
+
+            cloudpickle.loads(cloudpickle.dumps(foo))
+
+        In particular, it would raise::
+
+            TypeError: cannot pickle 'ROOTFacade' object
+        """
+        return _subimport, (self.__name__,)
+
     # Inject version as __version__ property in ROOT module
     @property
     def __version__(self):