Add infer_sq_for_un_vr config option (#1616)

* add infer_sq_for_un_vr config option
* add test and release notes

doc/release_notes/v2.3.0.rst

@@ -29,7 +29,9 @@ Enhancements
   :func:`~pydicom.pixel_data_handlers.utils.unpack_bits`(:pr:`1594`)
 * Added :func:`~pydicom.pixel_data_handlers.util.expand_ybr422` for expanding
   uncompressed ``YBR_FULL_422`` data to ``YBR_FULL`` (:pr:`1593`)
-
+* Replacement of ``UN`` VR with ``SQ`` VR for undefined length data elements
+  (introduced in 2.2.2), can now be configured via
+  :attr:`~pydicom.config.settings.infer_sq_for_un_vr`
 
 Fixes
 -----

pydicom/config.py

@@ -194,6 +194,7 @@ def __init__(self) -> None:
         self._writing_validation_mode: Optional[int] = (
             RAISE if _use_future else None
         )
+        self._infer_sq_for_un_vr: bool = True
 
     @property
     def reading_validation_mode(self) -> int:
@@ -227,6 +228,20 @@ def writing_validation_mode(self) -> int:
     def writing_validation_mode(self, value: int) -> None:
         self._writing_validation_mode = value
 
+    @property
+    def infer_sq_for_un_vr(self) -> bool:
+        """If ``True``, and the VR of a known data element is encoded as
+        **UN** in an explicit encoding for an undefined length data element,
+        the VR is changed to SQ per PS 3.5, section 6.2.2. Can be set to
+        ``False`` where the content of the tag shown as **UN** is not DICOM
+        conformant and would lead to a failure if accessing it.
+        """
+        return self._infer_sq_for_un_vr
+
+    @infer_sq_for_un_vr.setter
+    def infer_sq_for_un_vr(self, value: bool) -> None:
+        self._infer_sq_for_un_vr = value
+
 
 settings = Settings()
 """The global configuration object of type :class:`Settings` to access some

pydicom/filereader.py

@@ -236,7 +236,7 @@ def data_element_generator(
         else:
             # VR UN with undefined length shall be handled as SQ
             # see PS 3.5, section 6.2.2
-            if vr == VR_.UN:
+            if vr == VR_.UN and config.settings.infer_sq_for_un_vr:
                 vr = VR_.SQ
             # Try to look up type to see if is a SQ
             # if private tag, won't be able to look it up in dictionary,

pydicom/tests/conftest.py

@@ -65,6 +65,14 @@ def dont_replace_un_with_known_vr():
     config.replace_un_with_known_vr = old_value
 
 
+@pytest.fixture
+def dont_replace_un_with_sq_vr():
+    old_value = config.settings.infer_sq_for_un_vr
+    config.settings.infer_sq_for_un_vr = False
+    yield
+    config.settings.infer_sq_for_un_vr = old_value
+
+
 @pytest.fixture
 def dont_raise_on_writing_invalid_value():
     old_value = config.settings.writing_validation_mode

pydicom/tests/test_filereader.py

@@ -460,6 +460,15 @@ def test_un_sequence(self, dont_replace_un_with_known_vr):
         assert len(seq_element.value) == 1
         assert len(seq_element.value[0].ReferencedSeriesSequence) == 1
 
+    def test_un_sequence_dont_infer(
+            self,
+            dont_replace_un_with_sq_vr,
+            dont_replace_un_with_known_vr
+    ):
+        ds = dcmread(get_testdata_file("UN_sequence.dcm"))
+        seq_element = ds[0x4453100c]
+        assert seq_element.VR == "UN"
+
     def test_no_meta_group_length(self, no_datetime_conversion):
         """Read file with no group length in file meta."""
         # Issue 108 -- iView example file with no group length (0002,0002)