diff --git a/Phys/FunctorCore/python/Functors/__init__.py b/Phys/FunctorCore/python/Functors/__init__.py
index 9a6f0ea1bac8c1718b91bdbbd93255c05ee670e9..83eae7fd3a8ab43775e9dadb91756798fa5fc8b8 100644
--- a/Phys/FunctorCore/python/Functors/__init__.py
+++ b/Phys/FunctorCore/python/Functors/__init__.py
@@ -2476,6 +2476,40 @@ def BPVDLS(Vertices: DataHandle = None):
     return VTX_DLS.bind(BPV(Vertices), FORWARDARGS)
 
 
+# See test_constants.py:test_DataTypesToType for the origin of these constants
+DataTypesToType = {
+    "nPVs": 0,
+    "nLongTracks": 10,
+    "nDownstreamTracks": 11,
+    "nUpstreamTracks": 12,
+    "nVeloTracks": 13,
+    "nTTracks": 14,
+    "nBackTracks": 15,
+    "nTracks": 16,
+    "nGhosts": 17,
+    "nRich1Hits": 20,
+    "nRich2Hits": 21,
+    "nVeloClusters": 30,
+    "nVPClusters": 31,
+    "nITClusters": 40,
+    "nTTClusters": 50,
+    "nUTClusters": 51,
+    "nOTClusters": 60,
+    "nFTClusters": 41,
+    "nSPDhits": 70,
+    "eCalTot": 71,
+    "hCalTot": 72,
+    "nEcalClusters": 73,
+    "nMuonCoordsS0": 80,
+    "nMuonCoordsS1": 91,
+    "nMuonCoordsS2": 92,
+    "nMuonCoordsS3": 93,
+    "nMuonCoordsS4": 94,
+    "nMuonTracks": 95,
+    "TypeUnknown": 1000,
+}
+
+
 def RECSUMMARY_INFO(rec_summary: DataHandle, DataType: Union[str, int]):
     """Extracts DataType information from LHCb::RecSummary.DataType
 
@@ -2483,24 +2517,14 @@ def RECSUMMARY_INFO(rec_summary: DataHandle, DataType: Union[str, int]):
         rec_summary: DataHandle of the LHCb::RecSummary
         DataType: Name or enum of datatype as defined in https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/Event/RecEvent/include/Event/RecSummary.h
     """
-    # Workaround for cppyy warning with ROOT-10769
-    # (see https://gitlab.cern.ch:8443/lhcb/LHCb/-/merge_requests/2637)
-    import warnings
-
-    with warnings.catch_warnings():
-        warnings.simplefilter("ignore")
-        from cppyy.gbl import LHCb
-
     if isinstance(DataType, int):  # or maybe check directly for enum type?
-        if DataType == LHCb.RecSummary.TypeUnknown:
+        if DataType == DataTypesToType["TypeUnknown"]:
             raise ValueError("RecSummary type unknown")
         return RECSUMMARY_INFO._F(DataType) @ TES(rec_summary)
     elif isinstance(DataType, str):
-        if LHCb.RecSummary.DataTypesToType(DataType) == LHCb.RecSummary.TypeUnknown:
+        if DataTypesToType[DataType] == DataTypesToType["TypeUnknown"]:
             raise ValueError("RecSummary type unknown")
-        return RECSUMMARY_INFO._F(LHCb.RecSummary.DataTypesToType(DataType)) @ TES(
-            rec_summary
-        )
+        return RECSUMMARY_INFO._F(DataTypesToType[DataType]) @ TES(rec_summary)
     else:
         raise TypeError("RECSUMMARY_INFO expects string or enum value")
 
diff --git a/Phys/FunctorCore/python/Functors/tests/test_constants.py b/Phys/FunctorCore/python/Functors/tests/test_constants.py
new file mode 100644
index 0000000000000000000000000000000000000000..cb7aa842730d14ec5f7b532279f272173b1946c6
--- /dev/null
+++ b/Phys/FunctorCore/python/Functors/tests/test_constants.py
@@ -0,0 +1,39 @@
+###############################################################################
+# (c) Copyright 2025 CERN for the benefit of the LHCb Collaboration           #
+#                                                                             #
+# This software is distributed under the terms of the GNU General Public      #
+# Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   #
+#                                                                             #
+# In applying this licence, CERN does not waive the privileges and immunities #
+# granted to it by virtue of its status as an Intergovernmental Organization  #
+# or submit itself to any jurisdiction.                                       #
+###############################################################################
+"""Ensure that constants in the Functors module are consistent with the C++.
+
+In principle these constants could be loaded at runtime using cppyy (as is done
+for the tests in this module) however this is extremely slow to load and has
+significant memory overhead. Instead we just hard-code the values and use these
+tests to ensure they stay in sync with the C++ code.
+"""
+
+import warnings
+
+
+def test_DataTypesToType():
+    """Ensure that the DataTypesToType mapping is consistent with the C++ code."""
+    from Functors import DataTypesToType
+
+    # Workaround for cppyy warning with ROOT-10769
+    # (see https://gitlab.cern.ch:8443/lhcb/LHCb/-/merge_requests/2637)
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        from cppyy.gbl import LHCb
+
+    expected = {
+        k: int(v)
+        for k, v in LHCb.RecSummary.DataTypes.__dict__.items()
+        if not k.startswith("_")
+    }
+    assert DataTypesToType == expected, (
+        "DataTypesToType mapping is not consistent with the C++ code, please update it!"
+    )