Resolve "EXTERN: Slicing a Container returns a List, not a Container"

Summary

Slicing a Container used to return a list instead of a Container (issue #183 (closed), issue #104 on GitLab.com). This is fixed by adding a Container.__getitem__ method.

Focus

Checkout Container.__getitem__ and the corresponding test test_container_slicing in test_container.py.

Test Environment

Regular unittests.

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unittests/test_container.py

@@ -178,3 +178,13 @@ def test_container_deletion_with_references():
     assert len(deps14a) == 1 and deps14a.pop() == -1
     assert len(deps14b) == 1 and deps14b.pop() == -1
     assert len(deps15) == 1 and deps15.pop() == -1
+
+
+def test_container_slicing():
+    cont = db.Container()
+    cont.extend([db.Record(name=f"TestRec{ii+1}") for ii in range(5)])
+    assert isinstance(cont, db.common.models.Container)
+    container_slice = cont[:2]
+    assert isinstance(container_slice, db.common.models.Container), \
+        f"Container slice should be Container, was {type(container_slice)}"
+    # cont[:2].retrieve()