c_store does not convert to big endian explicit

[kwijk]

Describe the bug

Created an scu that on association offers the default 4 transfer syntaxes. The peer selects explicit big endian. But at cstore time pynetdicom does not convert the dataset to explicit big endian:

_File "pynetdicom\association.py", line 490, in get_valid_context
"ValueError: No presentation context for 'MR Image Storage' has been accepted by the peer with 'Explicit VR Little Endian' transfer syntax for the SCU role ".

ae = AE(ae_title="DICOM")
ae.add_requested_context(abstract_syntax='1.2.840.10008.5.1.4.1.1.4') 
ae.associae(...)  # association is accepted. peer chooses 'explicit big endian'

ds = some_dataset_in_little_endian_explicit  # e.g.  MR_small.dcm
assoc.send_c_store(dataset=ds)  # ValueError in get_valid_context

The low level association logs say:

  Context ID:        1 (Accepted) [_handlers.py:479]
    Abstract Syntax: =MR Image Storage [_handlers.py:479]
    Accepted SCP/SCU Role: Default [_handlers.py:479]
    Accepted Transfer Syntax: =Explicit VR Big Endian [_handlers.py:479]
Accepted Extended Negotiation: None [_handlers.py:479]
Accepted Asynchronous Operations Window Negotiation: None [_handlers.py:479]
User Identity Negotiation Response: None [_handlers.py:479]

Context ID:        1 (Proposed) [_handlers.py:902]
  Abstract Syntax: =MR Image Storage [_handlers.py:902]
  Proposed SCP/SCU Role: Default [_handlers.py:902]
  Proposed Transfer Syntaxes: [_handlers.py:902]
    =Implicit VR Little Endian [_handlers.py:902]
    =Explicit VR Little Endian [_handlers.py:902]
    =Deflated Explicit VR Little Endian [_handlers.py:902]
    =Explicit VR Big Endian [_handlers.py:902]
...
  Context ID:        1 (Accepted) [_handlers.py:479]
    Abstract Syntax: =MR Image Storage [_handlers.py:479]
    Accepted SCP/SCU Role: Default [_handlers.py:479]
    Accepted Transfer Syntax: =Explicit VR Big Endian [_handlers.py:479]
Accepted Extended Negotiation: None [_handlers.py:479]
Accepted Asynchronous Operations Window Negotiation: None [_handlers.py:479]
User Identity Negotiation Response: None [_handlers.py:479]
========================== END A-ASSOCIATE-AC PDU ========================== [_handlers.py:479]
Association Accepted [acse.py:518]

Expected behavior

I would expect pynetdicom to do the (little to big endian) conversion automatically.

To be more precise in class Association._get_valid_context there will be no match if a conversion from little_endian to big_endian needs to take place. I think this is unfortunate as in Association.send_c_store the call to encode would have worked fine when converting little endian to big endian. That is for me the following code works:

    ds_explicit_little_endian = pydicom.data.get_testdata_file("CT_small.dcm", read=True)
    assert False == ds_explicit_little_endian.is_implicit_VR
    assert True == ds_explicit_little_endian.is_little_endian

    from io import BytesIO
    from pynetdicom.dsutils import encode

    data = encode(ds=ds_explicit_little_endian, is_implicit_vr=False, is_little_endian=False, deflated=False)
    ds_explicit_big_endian = pydicom.dcmread(BytesIO(data), force=True)
    assert False == ds_explicit_big_endian.is_implicit_VR
    assert False == ds_explicit_big_endian.is_little_endian

Steps To Reproduce
See above

Your environment

Linux-6.16.8-arch3-1-x86_64-with-glibc2.42
Python 3.12.8 (main, Jan 2 2025, 10:00:11) [GCC 14.2.1 20240910]
pydicom 2.4.4
pynetdicom 2.1.0 (actually 2.1.1 according to pip show)

(I also suspect the issue to occur with latest pynetdicom 3.0.4, but could not verify)

[kwijk]

The following standalone test reproduces the error (in latest pydicom 3.0.1 and pynetdicom 3.0.4)

def test_send_cstore_little_to_big_endian_encoding():
    from pynetdicom import AE, debug_logger, evt, sop_class, _config
    from pydicom import uid

    _config.UNRESTRICTED_STORAGE_SERVICE = False
    _config.LOG_HANDLER_LEVEL = 'standard'
    debug_logger()
    logger = logging.getLogger("pynetdicom")
    logger.setLevel(logging.DEBUG)

    def handle_c_store(event):
        # scu must have sent data in supported transfer syntax
        assert event.file_meta.TransferSyntaxUID == uid.ExplicitVRBigEndian
        return 0x0000

    handlers = [
        (evt.EVT_C_STORE, handle_c_store),
    ]
    scp = AE('SCP')
    scp.add_supported_context(
        abstract_syntax=sop_class.MRImageStorage,  # MR images
        transfer_syntax=uid.ExplicitVRBigEndian         # explicit big endian
    )
    scp.start_server(address=('127.0.0.1', 4242), block=False, evt_handlers=handlers)

    scu = AE('SCU')
    scu.add_requested_context(
        abstract_syntax='1.2.840.10008.5.1.4.1.1.4',  # MR images, with default TS
    )

    assoc = scu.associate(addr='127.0.0.1', port=4242)
    assert assoc.is_established

    result = assoc.send_c_store(pydicom.data.get_testdata_file('MR_small.dcm', read=True))
    assert 0 == result.Status

    assoc.release()

Disabling the check in on line 487 in association.py::_get_valid_context makes the test pass:

                # Filter out contexts where the endianness doesn't match
                if tr_syntax.is_little_endian != cx_syntax.is_little_endian:
                    continue

Is this check a leftover for older versions of pynetdicom/pydicom or needed in other situations?

[kwijk]

I just see similar topic discussion: #1049

If I understand correctly, the pynetdicom.dsutils.encode will not do proper endian conversion (via pydicom.filewriter.write_dataset) of the elements in the dataset use binary encodings.