diff --git a/python/core/qgsexpressioncontext.sip b/python/core/qgsexpressioncontext.sip
index 5f67bb68ebe..f4c6dc68b01 100644
--- a/python/core/qgsexpressioncontext.sip
+++ b/python/core/qgsexpressioncontext.sip
@@ -76,7 +76,7 @@ class QgsExpressionContextScope
        * @param value initial variable value
        * @param readOnly true if variable should not be editable by users
        */
-      StaticVariable( const QString &name = QString(), const QVariant &value = QVariant(), bool readOnly = false );
+      StaticVariable( const QString &name = QString(), const QVariant &value = QVariant(), bool readOnly = false, bool isStatic = false  );
 
       /** Variable name */
       QString name;
@@ -86,6 +86,9 @@ class QgsExpressionContextScope
 
       /** True if variable should not be editable by users */
       bool readOnly;
+
+      //! A static variable can be cached for the lifetime of a context
+      bool isStatic;
     };
 
     /** Constructor for QgsExpressionContextScope
@@ -109,7 +112,7 @@ class QgsExpressionContextScope
      * @param value variable value
      * @see addVariable()
      */
-    void setVariable( const QString &name, const QVariant &value );
+    void setVariable( const QString &name, const QVariant &value, bool isStatic = false );
 
     /** Adds a variable into the context scope. If a variable with the same name is already set then its
      * value is overwritten, otherwise a new variable is added to the scope.
@@ -162,6 +165,14 @@ class QgsExpressionContextScope
      */
     bool isReadOnly( const QString &name ) const;
 
+    /**
+     * Tests whether the variable with the specified \a name is static and can
+     * be cached.
+     *
+     * \note Added in QGIS 3.0
+     */
+    bool isStatic( const QString &name ) const;
+
     /** Returns the count of variables contained within the scope.
      */
     int variableCount() const;
diff --git a/src/core/qgsexpression.cpp b/src/core/qgsexpression.cpp
index c144b1d8ae0..4cfe3f6694f 100644
--- a/src/core/qgsexpression.cpp
+++ b/src/core/qgsexpression.cpp
@@ -4322,9 +4322,10 @@ const QList<QgsExpression::Function *> &QgsExpression::Functions()
     varFunction->setIsStaticFunction(
       []( const NodeFunction * node, QgsExpression * parent, const QgsExpressionContext * context )
     {
-      /* A variable node is static if it has a static name and the name is defined at prepare time
-       * (e.g. global or layer variables)
-       * It is not static if a variable is set during iteration (e.g. geom_part variable)
+      /* A variable node is static if it has a static name and the name can be found at prepare
+       * time and is tagged with isStatic.
+       * It is not static if a variable is set during iteration or not tagged isStatic.
+       * (e.g. geom_part variable)
        */
       if ( node->args()->count() > 0 )
       {
@@ -4333,8 +4334,10 @@ const QList<QgsExpression::Function *> &QgsExpression::Functions()
         if ( !argNode->isStatic( parent, context ) )
           return false;
 
-        if ( fcnGetVariable( QVariantList() << argNode->eval( parent, context ), context, parent ).isValid() )
-          return true;
+        QString varName = argNode->eval( parent, context ).toString();
+
+        const QgsExpressionContextScope *scope = context->activeScopeForVariable( varName );
+        return scope ? scope->isStatic( varName ) : false;
       }
       return false;
     }
diff --git a/src/core/qgsexpressioncontext.cpp b/src/core/qgsexpressioncontext.cpp
index 07a9e4ad6a3..469bc27f57e 100644
--- a/src/core/qgsexpressioncontext.cpp
+++ b/src/core/qgsexpressioncontext.cpp
@@ -90,17 +90,18 @@ QgsExpressionContextScope::~QgsExpressionContextScope()
   qDeleteAll( mFunctions );
 }
 
-void QgsExpressionContextScope::setVariable( const QString &name, const QVariant &value )
+void QgsExpressionContextScope::setVariable( const QString &name, const QVariant &value, bool isStatic )
 {
   if ( mVariables.contains( name ) )
   {
     StaticVariable existing = mVariables.value( name );
     existing.value = value;
+    existing.isStatic = isStatic;
     addVariable( existing );
   }
   else
   {
-    addVariable( QgsExpressionContextScope::StaticVariable( name, value ) );
+    addVariable( QgsExpressionContextScope::StaticVariable( name, value, false, isStatic ) );
   }
 }
 
@@ -179,6 +180,11 @@ bool QgsExpressionContextScope::isReadOnly( const QString &name ) const
   return hasVariable( name ) ? mVariables.value( name ).readOnly : false;
 }
 
+bool QgsExpressionContextScope::isStatic( const QString &name ) const
+{
+  return hasVariable( name ) ? mVariables.value( name ).isStatic : false;
+}
+
 bool QgsExpressionContextScope::hasFunction( const QString &name ) const
 {
   return mFunctions.contains( name );
@@ -545,19 +551,19 @@ QgsExpressionContextScope *QgsExpressionContextUtils::globalScope()
 
   for ( QVariantMap::const_iterator it = customVariables.constBegin(); it != customVariables.constEnd(); ++it )
   {
-    scope->setVariable( it.key(), it.value() );
+    scope->setVariable( it.key(), it.value(), true );
   }
 
   //add some extra global variables
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_version" ), Qgis::QGIS_VERSION, true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_version_no" ), Qgis::QGIS_VERSION_INT, true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_short_version" ), QStringLiteral( "%1.%2" ).arg( Qgis::QGIS_VERSION_INT / 10000 ).arg( Qgis::QGIS_VERSION_INT / 100 % 100 ), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_release_name" ), Qgis::QGIS_RELEASE_NAME, true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_platform" ), QgsApplication::platform(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_os_name" ), QgsApplication::osName(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_locale" ), QgsApplication::locale(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "user_account_name" ), QgsApplication::userLoginName(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "user_full_name" ), QgsApplication::userFullName(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_version" ), Qgis::QGIS_VERSION, true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_version_no" ), Qgis::QGIS_VERSION_INT, true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_short_version" ), QStringLiteral( "%1.%2" ).arg( Qgis::QGIS_VERSION_INT / 10000 ).arg( Qgis::QGIS_VERSION_INT / 100 % 100 ), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_release_name" ), Qgis::QGIS_RELEASE_NAME, true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_platform" ), QgsApplication::platform(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_os_name" ), QgsApplication::osName(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "qgis_locale" ), QgsApplication::locale(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "user_account_name" ), QgsApplication::userLoginName(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "user_full_name" ), QgsApplication::userFullName(), true, true ) );
 
   return scope;
 }
@@ -715,17 +721,17 @@ QgsExpressionContextScope *QgsExpressionContextUtils::projectScope( const QgsPro
 
   for ( ; it != vars.constEnd(); ++it )
   {
-    scope->setVariable( it.key(), it.value() );
+    scope->setVariable( it.key(), it.value(), true );
   }
 
   //add other known project variables
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_title" ), project->title(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_path" ), project->fileInfo().filePath(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_folder" ), project->fileInfo().dir().path(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_filename" ), project->fileInfo().fileName(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_title" ), project->title(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_path" ), project->fileInfo().filePath(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_folder" ), project->fileInfo().dir().path(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_filename" ), project->fileInfo().fileName(), true, true ) );
   QgsCoordinateReferenceSystem projectCrs = project->crs();
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs" ), projectCrs.authid(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs_definition" ), projectCrs.toProj4(), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs" ), projectCrs.authid(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "project_crs_definition" ), projectCrs.toProj4(), true, true ) );
 
   scope->addFunction( QStringLiteral( "project_color" ), new GetNamedProjectColor( project ) );
   return scope;
@@ -772,14 +778,14 @@ QgsExpressionContextScope *QgsExpressionContextUtils::layerScope( const QgsMapLa
 
     QVariant varValue = variableValues.at( varIndex );
     varIndex++;
-    scope->setVariable( variableName, varValue );
+    scope->setVariable( variableName, varValue, true );
   }
 
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer_name" ), layer->name(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer_id" ), layer->id(), true ) );
-  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer" ), QVariant::fromValue<QgsWeakMapLayerPointer >( QgsWeakMapLayerPointer( const_cast<QgsMapLayer *>( layer ) ) ), true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer_name" ), layer->name(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer_id" ), layer->id(), true, true ) );
+  scope->addVariable( QgsExpressionContextScope::StaticVariable( QStringLiteral( "layer" ), QVariant::fromValue<QgsWeakMapLayerPointer >( QgsWeakMapLayerPointer( const_cast<QgsMapLayer *>( layer ) ) ), true, true ) );
 
-  const QgsVectorLayer *vLayer = dynamic_cast< const QgsVectorLayer * >( layer );
+  const QgsVectorLayer *vLayer = qobject_cast< const QgsVectorLayer * >( layer );
   if ( vLayer )
   {
     scope->setFields( vLayer->fields() );
diff --git a/src/core/qgsexpressioncontext.h b/src/core/qgsexpressioncontext.h
index 78fcb012405..76dbb6eb606 100644
--- a/src/core/qgsexpressioncontext.h
+++ b/src/core/qgsexpressioncontext.h
@@ -127,10 +127,11 @@ class CORE_EXPORT QgsExpressionContextScope
        * \param value initial variable value
        * \param readOnly true if variable should not be editable by users
        */
-      StaticVariable( const QString &name = QString(), const QVariant &value = QVariant(), bool readOnly = false )
+      StaticVariable( const QString &name = QString(), const QVariant &value = QVariant(), bool readOnly = false, bool isStatic = false )
         : name( name )
         , value( value )
         , readOnly( readOnly )
+        , isStatic( isStatic )
       {}
 
       //! Variable name
@@ -141,6 +142,9 @@ class CORE_EXPORT QgsExpressionContextScope
 
       //! True if variable should not be editable by users
       bool readOnly;
+
+      //! A static variable can be cached for the lifetime of a context
+      bool isStatic;
     };
 
     /** Constructor for QgsExpressionContextScope
@@ -166,7 +170,7 @@ class CORE_EXPORT QgsExpressionContextScope
      * \param value variable value
      * \see addVariable()
      */
-    void setVariable( const QString &name, const QVariant &value );
+    void setVariable( const QString &name, const QVariant &value, bool isStatic = false );
 
     /** Adds a variable into the context scope. If a variable with the same name is already set then its
      * value is overwritten, otherwise a new variable is added to the scope.
@@ -219,6 +223,14 @@ class CORE_EXPORT QgsExpressionContextScope
      */
     bool isReadOnly( const QString &name ) const;
 
+    /**
+     * Tests whether the variable with the specified \a name is static and can
+     * be cached.
+     *
+     * \note Added in QGIS 3.0
+     */
+    bool isStatic( const QString &name ) const;
+
     /** Returns the count of variables contained within the scope.
      */
     int variableCount() const { return mVariables.count(); }
diff --git a/tests/src/python/test_qgsatlascomposition.py b/tests/src/python/test_qgsatlascomposition.py
index bf746b38572..193c1651c6b 100644
--- a/tests/src/python/test_qgsatlascomposition.py
+++ b/tests/src/python/test_qgsatlascomposition.py
@@ -122,7 +122,7 @@ class TestQgsAtlasComposition(unittest.TestCase):
         for i in range(0, self.mAtlas.numFeatures()):
             self.mAtlas.prepareForFeature(i)
             expected = "output_%d" % (i + 1)
-            assert self.mAtlas.currentFilename() == expected
+            self.assertEqual(self.mAtlas.currentFilename(), expected)
         self.mAtlas.endRender()
 
     def autoscale_render_test(self):