[df] Python interface for new Hist action

tree/dataframe/inc/ROOT/RDF/RInterface.hxx

@@ -2493,7 +2493,31 @@ public:
    /// ROOT::Experimental::RRegularAxis axis(10, {5.0, 15.0});
    /// auto myHist = myDf.Hist({axis}, {"col0"});
    /// ~~~
-   template <typename BinContentType = double, typename ColumnType = RDFDetail::RInferredType, typename... ColumnTypes>
+   template <typename BinContentType = double>
+   RResultPtr<ROOT::Experimental::RHist<BinContentType>>
+   Hist(std::vector<ROOT::Experimental::RAxisVariant> axes, const ColumnNames_t &columnList)
+   {
+      // Note: this overload works around limitations for automatic pythonization of variadic function templates with
+      // template parameter packs.
+      return Hist<BinContentType, RDFDetail::RInferredType>(std::move(axes), columnList);
+   }
+
+   ////////////////////////////////////////////////////////////////////////////
+   /// \brief Fill and return an RHist (*lazy action*).
+   /// \tparam BinContentType The bin content type of the returned RHist.
+   /// \param[in] axes The returned histogram will be constructed using these axes.
+   /// \param[in] columnList A list containing the names of the columns that will be passed when calling `Fill`
+   /// \return the histogram wrapped in a RResultPtr.
+   ///
+   /// This action is *lazy*: upon invocation of this method the calculation is
+   /// booked but not executed. Also see RResultPtr.
+   ///
+   /// ### Example usage:
+   /// ~~~{.cpp}
+   /// ROOT::Experimental::RRegularAxis axis(10, {5.0, 15.0});
+   /// auto myHist = myDf.Hist<double, double>({axis}, {"col0"});
+   /// ~~~
+   template <typename BinContentType, typename ColumnType, typename... ColumnTypes>
    RResultPtr<ROOT::Experimental::RHist<BinContentType>>
    Hist(std::vector<ROOT::Experimental::RAxisVariant> axes, const ColumnNames_t &columnList)
    {
@@ -2587,8 +2611,34 @@ public:
    /// ROOT::Experimental::RRegularAxis axis(10, {5.0, 15.0});
    /// auto myHist = myDf.Hist({axis}, {"col0"}, "colW");
    /// ~~~
-   template <typename BinContentType = ROOT::Experimental::RBinWithError,
-             typename ColumnType = RDFDetail::RInferredType, typename... ColumnTypes>
+   template <typename BinContentType = ROOT::Experimental::RBinWithError>
+   RResultPtr<ROOT::Experimental::RHist<BinContentType>>
+   Hist(std::vector<ROOT::Experimental::RAxisVariant> axes, const ColumnNames_t &columnList, std::string_view wName)
+   {
+      // Note: this overload works around limitations for automatic pythonization of variadic function templates with
+      // template parameter packs.
+      return Hist<BinContentType, RDFDetail::RInferredType>(std::move(axes), columnList, wName);
+   }
+
+   ////////////////////////////////////////////////////////////////////////////
+   /// \brief Fill and return an RHist with weights (*lazy action*).
+   /// \tparam BinContentType The bin content type of the returned RHist.
+   /// \param[in] axes The returned histogram will be constructed using these axes.
+   /// \param[in] columnList A list containing the names of the columns that will be passed when calling `Fill`
+   /// \param[in] wName The name of the column that will provide the weights.
+   /// \return the histogram wrapped in a RResultPtr.
+   ///
+   /// This action is *lazy*: upon invocation of this method the calculation is
+   /// booked but not executed. Also see RResultPtr.
+   ///
+   /// This overload is not available for integral bin content types (see \ref RHistEngine::SupportsWeightedFilling).
+   ///
+   /// ### Example usage:
+   /// ~~~{.cpp}
+   /// ROOT::Experimental::RRegularAxis axis(10, {5.0, 15.0});
+   /// auto myHist = myDf.Hist<ROOT::Experimental::RBinWithError, double, double>({axis}, {"col0"}, "colW");
+   /// ~~~
+   template <typename BinContentType = ROOT::Experimental::RBinWithError, typename ColumnType, typename... ColumnTypes>
    RResultPtr<ROOT::Experimental::RHist<BinContentType>>
    Hist(std::vector<ROOT::Experimental::RAxisVariant> axes, const ColumnNames_t &columnList, std::string_view wName)
    {

tree/dataframe/test/CMakeLists.txt

@@ -167,6 +167,9 @@ if(pyroot)
     ROOT_ADD_PYUNITTEST(dataframe_merge_results dataframe_merge_results.py)
     ROOT_ADD_PYUNITTEST(dataframe_snapshot_py dataframe_snapshot.py)
   endif()
+  if(root7)
+    ROOT_ADD_PYUNITTEST(dataframe_hist dataframe_hist.py)
+  endif()
 endif()
 
 configure_file(.rootrc . COPYONLY)

tree/dataframe/test/dataframe_hist.cxx

@@ -70,6 +70,10 @@ TEST_P(RDFHist, RegularJit)
       EXPECT_EQ(hist->GetBinContent(index), 1.0);
    }
 
+   // The user can template explicitly only the bin content type.
+   hist = dfX.Hist</*BinContentType=*/double>({axis}, {"x"});
+   EXPECT_EQ(hist->GetNEntries(), 10);
+
    // The one-dimensional specialization returns the same type.
    hist = dfX.Hist(10, {5.0, 15.0}, "x");
    EXPECT_EQ(hist->GetNEntries(), 10);
@@ -335,6 +339,10 @@ TEST_P(RDFHist, WeightJit)
       EXPECT_FLOAT_EQ(bin.fSum2, weight * weight);
    }
 
+   // The user can template explicitly only the bin content type.
+   hist = dfXW.Hist</*BinContentType=*/RBinWithError>({axis}, {"x"}, "w");
+   EXPECT_EQ(hist->GetNEntries(), 10);
+
    // The one-dimensional specialization returns the same type.
    hist = dfXW.Hist(10, {5.0, 15.0}, "x", "w");
    EXPECT_EQ(hist->GetNEntries(), 10);

tree/dataframe/test/dataframe_hist.py

@@ -0,0 +1,34 @@
+import unittest
+
+import ROOT
+
+RRegularAxis = ROOT.Experimental.RRegularAxis
+RVariableBinAxis = ROOT.Experimental.RVariableBinAxis
+
+
+class RDFHist(unittest.TestCase):
+    def test_Regular(self):
+        df = ROOT.RDataFrame(10)
+        dfX = df.Define("x", "rdfentry_ + 5.5")
+        hist = dfX.Hist(10, (5.0, 15.0), "x")
+        self.assertEqual(hist.GetNEntries(), 10)
+
+    def test_MultiDim(self):
+        df = ROOT.RDataFrame(10)
+        dfXY = df.Define("x", "rdfentry_ + 5.5").Define("y", "2 * rdfentry_ + 0.5")
+
+        regularAxis = RRegularAxis(10, (5.0, 15.0))
+        bins = [i for i in range(0, 21)]
+        variableBinAxis = RVariableBinAxis(bins)
+        hist = dfXY.Hist([regularAxis, variableBinAxis], ["x", "y"])
+        self.assertEqual(hist.GetNEntries(), 10)
+
+    def test_Weight(self):
+        df = ROOT.RDataFrame(10)
+        dfXW = df.Define("x", "rdfentry_ + 5.5").Define("w", "0.1 + rdfentry_ * 0.03")
+        hist = dfXW.Hist(10, (5.0, 15.0), "x", "w")
+        self.assertEqual(hist.GetNEntries(), 10)
+
+        regularAxis = RRegularAxis(10, (5.0, 15.0))
+        hist = dfXW.Hist([regularAxis], ["x"], "w")
+        self.assertEqual(hist.GetNEntries(), 10)