[expression] Add try() function to provide a way to detect and handle

expressions which can intermittently fail.

Author: nirvn

resources/function_help/json/try

@@ -0,0 +1,14 @@
+{
+  "name": "try",
+  "type": "function",
+  "description": "Tries an expression and returns its value if error-free. If the expression returns an error, an alternative value will be returned when provided otherwise the function will return null.",
+  "arguments": [
+    {"arg":"expression","description":"the expression which should be run"},
+    {"arg":"alternative","optional":true,"description":"the result which will be returned if the expression returns an error."}
+  ],
+  "examples": [
+    { "expression":"try( to_int( '1' ), 0 )", "returns":"1"},
+    { "expression":"try( to_int( 'a' ), 0 )", "returns":"0"},
+    { "expression":"try( to_date( 'invalid_date' ) )", "returns":"NULL"}
+  ]
+}

src/core/expression/qgsexpressionfunction.cpp

@@ -3688,6 +3688,21 @@ static QVariant fcnColorRgb( const QVariantList &values, const QgsExpressionCont
   return QStringLiteral( "%1,%2,%3" ).arg( color.red() ).arg( color.green() ).arg( color.blue() );
 }
 
+static QVariant fcnTry( const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent, const QgsExpressionNodeFunction * )
+{
+  QgsExpressionNode *node = QgsExpressionUtils::getNode( values.at( 0 ), parent );
+  QVariant value = node->eval( parent, context );
+  if ( parent->hasEvalError() )
+  {
+    parent->setEvalErrorString( QString() );
+    node = QgsExpressionUtils::getNode( values.at( 1 ), parent );
+    ENSURE_NO_EVAL_ERROR;
+    value = node->eval( parent, context );
+    ENSURE_NO_EVAL_ERROR;
+  }
+  return value;
+}
+
 static QVariant fcnIf( const QVariantList &values, const QgsExpressionContext *context, QgsExpression *parent, const QgsExpressionNodeFunction * )
 {
   QgsExpressionNode *node = QgsExpressionUtils::getNode( values.at( 0 ), parent );
@@ -4763,6 +4778,7 @@ const QList<QgsExpressionFunction *> &QgsExpression::Functions()
         << new QgsStaticExpressionFunction( QStringLiteral( "coalesce" ), -1, fcnCoalesce, QStringLiteral( "Conditionals" ), QString(), false, QSet<QString>(), false, QStringList(), true )
         << new QgsStaticExpressionFunction( QStringLiteral( "nullif" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "value1" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "value2" ) ), fcnNullIf, QStringLiteral( "Conditionals" ) )
         << new QgsStaticExpressionFunction( QStringLiteral( "if" ), 3, fcnIf, QStringLiteral( "Conditionals" ), QString(), false, QSet<QString>(), true )
+        << new QgsStaticExpressionFunction( QStringLiteral( "try" ), QgsExpressionFunction::ParameterList() << QgsExpressionFunction::Parameter( QStringLiteral( "expression" ) ) << QgsExpressionFunction::Parameter( QStringLiteral( "alternative" ), true, QVariant() ), fcnTry, QStringLiteral( "Conditionals" ), QString(), false, QSet<QString>(), true )
 
         << new QgsStaticExpressionFunction( QStringLiteral( "aggregate" ),
                                             QgsExpressionFunction::ParameterList()

tests/src/core/testqgsexpression.cpp

@@ -1177,6 +1177,9 @@ class TestQgsExpression: public QObject
       QTest::newRow( "regexp match false" ) << "regexp_match('abc DEF','\\\\s[a-z]+')" << false << QVariant( 0 );
       QTest::newRow( "if true" ) << "if(1=1, 1, 0)" << false << QVariant( 1 );
       QTest::newRow( "if false" ) << "if(1=2, 1, 0)" << false << QVariant( 0 );
+      QTest::newRow( "try valid" ) << "try(to_int('1'),0)" << false << QVariant( 1 );
+      QTest::newRow( "try invalid with alternative" ) << "try(to_int('a'),0)" << false << QVariant( 0 );
+      QTest::newRow( "try invalid without alternative" ) << "try(to_int('a'))" << false << QVariant();
 
       // Datetime functions
       QTest::newRow( "to date" ) << "todate('2012-06-28')" << false << QVariant( QDate( 2012, 6, 28 ) );