Add Python example for thread-safe Define using rdfslot_

tutorials/dataframe/DefineSlotPython.py

@@ -0,0 +1,46 @@
+"""
+DefineSlotPython.py
+
+Demonstrate a thread-safe Define pattern for Python users using `rdfslot_`.
+
+This shows how to declare a small C++ functor and use it from PyROOT via
+`RDataFrame.Define` so each thread gets a thread-local RNG seeded by the slot.
+
+Run with:
+
+  python3 DefineSlotPython.py
+
+Requires a working PyROOT installation (ROOT with Python bindings).
+"""
+from __future__ import print_function
+
+import ROOT
+
+def main():
+    # Declare a small C++ function that uses a thread_local TRandom3 per slot.
+    ROOT.gInterpreter.Declare("""
+    double myfunctor(unsigned int rdfslot) {
+      static thread_local TRandom3 r3(rdfslot);
+      return r3.Gaus();
+    }
+    """)
+
+    # Create an RDataFrame and use Define passing the C++ functor by name.
+    # Note: the expression uses the helper column `rdfslot_` which is different
+    # for each thread when implicit multi-threading is enabled.
+    # Enable implicit multi-threading for example (optional).
+    # This must happen before constructing the RDataFrame.
+    try:
+        ROOT.EnableImplicitMT()
+    except Exception:
+        # If ROOT was built without implicit MT, continue single-threaded
+        pass
+
+    rdf = ROOT.RDataFrame(100000)
+    rdf_x = rdf.Define("x", "myfunctor(rdfslot_)")
+    h = rdf_x.Histo1D(("h", "Gaussian per-slot test", 100, -5, 5), "x")
+    # Trigger event loop by accessing the filled histogram.
+    print("Histogram entries:", int(h.GetValue().GetEntries()))
+
+if __name__ == '__main__':
+    main()