[Python] Deprecate conversion from Python set to RooArgSet

This conversion can lead to subtle problems because Python sets are
unordered, different from C++ initializer lists with the same syntax.
This can lead to confusion that we should prevent.

Example:
```python
import ROOT

x = ROOT.RooRealVar("x", "x", 1.0)
y = ROOT.RooRealVar("y", "y", -1.0)

ROOT.RooArgSet(set([x, y])).Print()
ROOT.RooArgSet(set([y, x])).Print()
```
Gives on my system:
```txt
RooArgSet:: = (x,y)
RooArgSet:: = (x,y) <--- reverse order expected!
```

This is not even deterministic, because Python doesn't guarantee that
a `set` is ordered in any way.

I introduced this 5 years ago in commit c54c324
(PR #8751) whithout thinking
too much about this footgun.

Author: guitargeek

README/ReleaseNotes/v642/index.md

@@ -47,6 +47,7 @@ The following people have contributed to this new version:
 * The `TSSLSocket` class is now removed following deprecation in ROOT 6.40.
 * The bindings to the R programming language that are enabled with the `r=ON` or `tmva-rmva=ON` build options (`TRInterface`, RMVA, and friends) are removed, following deprecation in ROOT 6.40. Their maintenance is no longer justified, given the broader adoption of the scientific Python ecosystem. Users who still rely on R from C++ are encouraged to call R directly via https://cran.r-project.org/package=RInside, which is what the ROOT bindings were using internally.
 * Several enums that are redundant with `ROOT::ESTLType` are deprecated and will be removed in ROOT 6.44: `TClassEdit::ESTLType`, `TDictionary::ESTLType`, `TStreamerElement::ESTLType`. Please use `ROOT::ESTLType` instead.
+* The conversion from Python set to **RooArgSet** is deprecated and won't work anymore in ROOT 6.44. The problem is that Python sets are unordered while RooArgSets are ordered, and this mismatch can lead to subtle problems later on. Prefer conversion from Python lists or tuples, which are ordered too.
 
 ## Python Interface
 

bindings/pyroot/pythonizations/python/ROOT/_pythonization/_roofit/_rooargset.py

@@ -11,13 +11,13 @@
 
 
 import operator
+import warnings
 
 from ._rooabscollection import RooAbsCollection
 
 
 class RooArgSet(RooAbsCollection):
-
-    __cpp_name__ = 'RooArgSet'
+    __cpp_name__ = "RooArgSet"
 
     def __init__(self, *args, **kwargs):
         """Pythonization of RooArgSet constructor to support implicit
@@ -30,6 +30,16 @@ def __init__(self, *args, **kwargs):
         # argument is added, hence the `len(kwargs) == 0` check breaks the
         # implicit conversion!
         if len(args) == 1 and isinstance(args[0], set):
+            warnings.warn(
+                "Constructing a RooArgSet from a Python set is deprecated and "
+                "will be removed in ROOT 6.44. Python sets are unordered, while "
+                "a RooArgSet is ordered, so this conversion can silently change "
+                "the order of the elements. Construct the RooArgSet from the "
+                "individual RooFit objects, or from a list or tuple of them "
+                "instead.",
+                FutureWarning,
+                stacklevel=2,
+            )
             return self._init(*args[0], **kwargs)
         return self._init(*args, **kwargs)
 

bindings/pyroot/pythonizations/test/root_module.py

@@ -120,12 +120,20 @@ def test_deprecated_features(self):
 
         version = tuple(int(n) for n in ROOT.__version__.split("."))
 
-        if version >= (6, 44, 0):
+        if version >= (6, 43, 0):
             # Verify that SetHeuristicMemoryPolicy is not available
             with self.assertRaises(AttributeError):
                 ROOT.SetHeuristicMemoryPolicy(True)
                 ROOT.SetHeuristicMemoryPolicy(False)
 
+        if version >= (6, 43, 0) and hasattr(ROOT, "RooArgSet"):
+            # Verify that the implicit conversion from Python set to RooArgSet
+            # is gone
+            x = ROOT.RooRealVar("x", "x", 1.0)
+            y = ROOT.RooRealVar("y", "y", -1.0)
+            with self.assertRaises(TypeError):
+                ROOT.RooDataSet("data", "data", {x, y})
+
     def test_lazy_gdirectory(self):
         """Check that gDirectory is always lazy evaluated to the current
         directory.