fix group_by on layer to be std::string const& and reflect in python

bindings/python/mapnik_layer.cpp

@@ -304,6 +304,14 @@ void export_layer()
                       ">>> lyr.srs = '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs +over'\n"
             )
 
+        .add_property("group_by",
+                      make_function(&layer::group_by,return_value_policy<copy_const_reference>()),
+                      &layer::set_group_by,
+                      "Get/Set the optional layer group name.\n"
+                      "\n"
+                      "More details at https://github.com/mapnik/mapnik/wiki/Grouped-rendering:\n"
+            )
+
         .add_property("styles",
                       make_function(_styles_,return_value_policy<reference_existing_object>()),
                       "The styles list attached to this layer.\n"

include/mapnik/layer.hpp

@@ -171,7 +171,7 @@ public:
     /*!
      * @return The field rendering of this layer is grouped by.
      */
-    std::string group_by() const;
+    std::string const& group_by() const;
 
     /*!
      * @brief Attach a datasource for this layer.

src/layer.cpp

@@ -237,7 +237,7 @@ void layer::set_group_by(std::string column)
     group_by_ = column;
 }
 
-std::string layer::group_by() const
+std::string const& layer::group_by() const
 {
     return group_by_;
 }

tests/python_tests/layer_test.py

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+from nose.tools import *
+import mapnik
+
+# Map initialization
+def test_layer_init():
+    l = mapnik.Layer('test')
+    eq_(l.name,'test')
+    eq_(l.srs,'+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs')
+    eq_(l.envelope(),mapnik.Box2d())
+    eq_(l.clear_label_cache,False)
+    eq_(l.cache_features,False)
+    eq_(l.visible(1),True)
+    eq_(l.active,True)
+    eq_(l.datasource,None)
+    eq_(l.queryable,False)
+    eq_(l.minzoom,0.0)
+    eq_(l.maxzoom > 1e+6,True)
+    eq_(l.group_by,"")
+    eq_(l.maximum_extent,None)
+    eq_(l.buffer_size,0.0)
+    eq_(len(l.styles),0)
+
+if __name__ == "__main__":
+    [eval(run)() for run in dir() if 'test_' in run]