[Exp PyROOT] Implement TCollection iteration with a generator function

After the upgrade of CPyCppyy to v1.5.1, the pythonisations of cppyy
do not inject anymore the __iter__ method on TCollections. Before the
update, cppyy was injecting __iter__ on any class that had a `begin`
and an `end` methods, which is the case of TCollection.

Since now cppyy does not inject __iter__, we can inject it ourselves
in our TCollection pythonisation and the subclasses of TCollection
will keep that pythonisation.

bindings/pyroot_experimental/PyROOT/python/ROOT/pythonization/_tcollection.py

@@ -87,12 +87,15 @@ def _imul_pyz(self, n):
 
 # Python iteration
 
-def _begin_pyz(self):
+def _iter_pyz(self):
+    # Generator function to iterate on TCollections
     # Parameters:
     # - self: collection to be iterated
-    # Returns:
-    # - TIter iterator on collection
-    return TIter(self)
+    it = TIter(self)
+    o = it.Next()
+    while o:
+        yield o
+        o = it.Next()
 
 
 @pythonization()
@@ -117,20 +120,7 @@ def pythonize_tcollection(klass, name):
         klass.__rmul__ = _mul_pyz
         klass.__imul__ = _imul_pyz
 
-        # Make TCollections iterable.
-        # In Pythonize.cxx, cppyy injects an `__iter__` method into
-        # any class that has a `begin` and an `end` methods.
-        # This is the case of TCollection and its subclasses, where
-        # cppyy associates `__iter__` with a function that returns the
-        # result of calling `begin`, i.e. a TIter that already points
-        # to the first element of the collection.
-        # That setting breaks the iteration on TCollections, since the
-        # first time `Next` is called on the iterator it will return
-        # the second element of the collection (if any), thus skipping
-        # the first one.
-        # By pythonising `begin` here, we make sure the iterator returned
-        # points to nowhere, and it will return the first element of the
-        # collection on the first invocation of `Next`.
-        klass.begin = _begin_pyz
+        # Make TCollections iterable
+        klass.__iter__ = _iter_pyz
 
     return True

bindings/pyroot_experimental/PyROOT/python/ROOT/pythonization/_titer.py

-# Author: Enric Tejedor CERN  01/2019
-
-################################################################################
-# Copyright (C) 1995-2019, Rene Brun and Fons Rademakers.                      #
-# All rights reserved.                                                         #
-#                                                                              #
-# For the licensing terms see $ROOTSYS/LICENSE.                                #
-# For the list of contributors see $ROOTSYS/README/CREDITS.                    #
-################################################################################
-
-from ROOT import pythonization
-import cppyy
-
-def _next_pyz(self):
-    # Parameters:
-    # - self: iterator on a collection
-    # Returns:
-    # - next object in the collection, or raises StopIteration if none
-    o = self.Next()
-    if o:
-        return o
-    else:
-        raise StopIteration()
-
-# The TIter class does not go through the mechanism of lazy pythonisations of
-# cppyy, since it is used before such mechanism is put in place. Therefore, we
-# define here the pythonisations for TIter as immediate, i.e. executed upon
-# import of the ROOT module
-@pythonization(lazy = False)
-def pythonize_titer():
-    klass = cppyy.gbl.TIter
-
-    # Make TIter a Python iterator.
-    # This makes it possible to iterate over TCollections, since Cppyy
-    # injects on them an `__iter__` method that returns a TIter.
-    klass.__next__ = _next_pyz  # Py3
-    klass.next     = _next_pyz  # Py2
-
-    return True

bindings/pyroot_experimental/PyROOT/test/CMakeLists.txt

@@ -17,9 +17,6 @@ ROOT_ADD_PYUNITTEST(pyroot_pyz_ttree_setbranchaddress ttree_setbranchaddress.py
 ROOT_ADD_PYUNITTEST(pyroot_pyz_ttree_branch ttree_branch.py
                     COPY_TO_BUILDDIR TreeHelper.h)
 
-# TIter pythonisations
-ROOT_ADD_PYUNITTEST(pyroot_pyz_titer_iterator titer_iterator.py)
-
 # TCollection and subclasses pythonizations
 ROOT_ADD_PYUNITTEST(pyroot_pyz_tcollection_len tcollection_len.py)
 ROOT_ADD_PYUNITTEST(pyroot_pyz_tcollection_listmethods tcollection_listmethods.py)

bindings/pyroot_experimental/PyROOT/test/titer_iterator.py

-import unittest
-
-import ROOT
-from libcppyy import SetOwnership
-
-
-class TIterIterator(unittest.TestCase):
-    """
-    Test for the pythonization that allows instances of TIter to
-    behave as Python iterators.
-    """
-
-    num_elems = 3
-
-    # Helpers
-    def create_tcollection(self):
-        c = ROOT.TList()
-        for _ in range(self.num_elems):
-            o = ROOT.TObject()
-            # Prevent immediate deletion of C++ TObjects
-            SetOwnership(o, False)
-            c.Add(o)
-
-        return c
-
-    # Tests
-    def test_iterator(self):
-        c = self.create_tcollection()
-
-        itc1 = ROOT.TIter(c)
-        itc2 = ROOT.TIter(c)
-        for _ in range(c.GetEntries()):
-            self.assertEqual(next(itc1), itc2.Next())