Expression context fixes:

- Fix python API break in QgsExpression::Function
- Add convenience methods for retrieving feature/fields from a
context

python/core/qgsexpression.sip

@@ -290,8 +290,9 @@ class QgsExpression
          * @param context context expression is being evaluated against
          * @param parent parent expression
          * @returns result of function
+         * @note named funcV2 in Python bindings. Will be renamed to func to replace deprecated method in QGIS 3.0.
          */
-     	virtual QVariant func( const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent );
+     	virtual QVariant func( const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent ) /PyName=funcV2/;
 
         virtual bool handlesNull() const;
     };

python/core/qgsexpressioncontext.sip

@@ -346,16 +346,28 @@ class QgsExpressionContext
      * will be set within the last scope of the context, so will override any
      * existing features within the context.
      * @param feature feature for context
+     * @see feature()
      */
     void setFeature( const QgsFeature& feature );
 
+    /** Convenience function for retrieving the feature for the context, if set.
+     * @see setFeature
+     */
+    QgsFeature feature() const;
+
     /** Convenience function for setting a fields for the context. The fields
      * will be set within the last scope of the context, so will override any
      * existing fields within the context.
      * @param fields fields for context
+     * @see fields()
      */
     void setFields( const QgsFields& fields );
 
+    /** Convenience function for retrieving the fields for the context, if set.
+     * @see setFields
+     */
+    QgsFields fields() const;
+
 };
 
 /** \ingroup core

src/core/qgsexpression.h

@@ -399,7 +399,9 @@ class CORE_EXPORT QgsExpression
          * @param context context expression is being evaluated against
          * @param parent parent expression
          * @returns result of function
+         * @note named funcV2 in Python bindings. Will be renamed to func to replace deprecated method in QGIS 3.0.
          */
+        //TODO QGIS 3.0 - rename python method
         virtual QVariant func( const QVariantList& values, const QgsExpressionContext* context, QgsExpression* parent );
 
         bool operator==( const Function& other ) const

src/core/qgsexpressioncontext.cpp

@@ -350,6 +350,11 @@ void QgsExpressionContext::setFeature( const QgsFeature &feature )
   mStack.last()->setFeature( feature );
 }
 
+QgsFeature QgsExpressionContext::feature() const
+{
+  return qvariant_cast<QgsFeature>( variable( QgsExpressionContext::EXPR_FEATURE ) );
+}
+
 void QgsExpressionContext::setFields( const QgsFields &fields )
 {
   if ( mStack.isEmpty() )
@@ -358,6 +363,11 @@ void QgsExpressionContext::setFields( const QgsFields &fields )
   mStack.last()->setFields( fields );
 }
 
+QgsFields QgsExpressionContext::fields() const
+{
+  return qvariant_cast<QgsFields>( variable( QgsExpressionContext::EXPR_FIELDS ) );
+}
+
 
 //
 // QgsExpressionContextUtils

src/core/qgsexpressioncontext.h

@@ -378,16 +378,28 @@ class CORE_EXPORT QgsExpressionContext
      * will be set within the last scope of the context, so will override any
      * existing features within the context.
      * @param feature feature for context
+     * @see feature()
      */
     void setFeature( const QgsFeature& feature );
 
+    /** Convenience function for retrieving the feature for the context, if set.
+     * @see setFeature
+     */
+    QgsFeature feature() const;
+
     /** Convenience function for setting a fields for the context. The fields
      * will be set within the last scope of the context, so will override any
      * existing fields within the context.
      * @param fields fields for context
+     * @see fields()
      */
     void setFields( const QgsFields& fields );
 
+    /** Convenience function for retrieving the fields for the context, if set.
+     * @see setFields
+     */
+    QgsFields fields() const;
+
     static const QString EXPR_FIELDS;
     static const QString EXPR_FEATURE;
 

tests/src/core/testqgsexpressioncontext.cpp

@@ -411,11 +411,13 @@ void TestQgsExpressionContext::setFeature()
 
   //test setting a feature in a context with no scopes
   QgsExpressionContext emptyContext;
+  QVERIFY( !emptyContext.feature().isValid() );
   emptyContext.setFeature( feature );
   //setFeature should have created a scope
   QCOMPARE( emptyContext.scopeCount(), 1 );
   QVERIFY( emptyContext.hasVariable( QgsExpressionContext::EXPR_FEATURE ) );
   QCOMPARE(( qvariant_cast<QgsFeature>( emptyContext.variable( QgsExpressionContext::EXPR_FEATURE ) ) ).id(), 50LL );
+  QCOMPARE( emptyContext.feature(), feature() );
 
   QgsExpressionContext contextWithScope;
   contextWithScope << new QgsExpressionContextScope();
@@ -423,6 +425,7 @@ void TestQgsExpressionContext::setFeature()
   QCOMPARE( contextWithScope.scopeCount(), 1 );
   QVERIFY( contextWithScope.hasVariable( QgsExpressionContext::EXPR_FEATURE ) );
   QCOMPARE(( qvariant_cast<QgsFeature>( contextWithScope.variable( QgsExpressionContext::EXPR_FEATURE ) ) ).id(), 50LL );
+  QCOMPARE( contextWithScope.feature(), feature() );
 }
 
 void TestQgsExpressionContext::setFields()
@@ -438,11 +441,13 @@ void TestQgsExpressionContext::setFields()
 
   //test setting a fields in a context with no scopes
   QgsExpressionContext emptyContext;
+  QVERIFY( emptyContext.fields().isEmpty() );
   emptyContext.setFields( fields );
   //setFeature should have created a scope
   QCOMPARE( emptyContext.scopeCount(), 1 );
   QVERIFY( emptyContext.hasVariable( QgsExpressionContext::EXPR_FIELDS ) );
   QCOMPARE(( qvariant_cast<QgsFields>( emptyContext.variable( QgsExpressionContext::EXPR_FIELDS ) ) ).at( 0 ).name(), QString( "testfield" ) );
+  QCOMPARE( emptyContext.fields().at( 0 ).name(), QString( "testfield" ) );
 
   QgsExpressionContext contextWithScope;
   contextWithScope << new QgsExpressionContextScope();
@@ -450,6 +455,7 @@ void TestQgsExpressionContext::setFields()
   QCOMPARE( contextWithScope.scopeCount(), 1 );
   QVERIFY( contextWithScope.hasVariable( QgsExpressionContext::EXPR_FIELDS ) );
   QCOMPARE(( qvariant_cast<QgsFields>( contextWithScope.variable( QgsExpressionContext::EXPR_FIELDS ) ) ).at( 0 ).name(), QString( "testfield" ) );
+  QCOMPARE( contextWithScope.fields().at( 0 ).name(), QString( "testfield" ) );
 }
 
 void TestQgsExpressionContext::globalScope()