Add checking for missing parentheses for print/exec

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/Python3Core.java

@@ -40,6 +40,8 @@
 import java.util.Map.Entry;
 import java.util.ServiceLoader;
 import java.util.logging.Level;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 import org.graalvm.nativeimage.ImageInfo;
 
@@ -209,6 +211,8 @@ public final class Python3Core implements PythonCore {
     private static final TruffleLogger LOGGER = PythonLanguage.getLogger(Python3Core.class);
     private final String[] coreFiles;
 
+    public static final Pattern MISSING_PARENTHESES_PATTERN = Pattern.compile("^(print|exec) +([^(][^;]*).*");
+
     private static final String[] initializeCoreFiles() {
         // Order matters!
         List<String> coreFiles = new ArrayList<>(Arrays.asList(
@@ -774,6 +778,22 @@ public RuntimeException raiseInvalidSyntax(PythonParser.ErrorType type, Node loc
         } else {
             msg = "invalid syntax";
         }
+        if (section.isAvailable() && type == PythonParser.ErrorType.Generic) {
+            Matcher matcher = MISSING_PARENTHESES_PATTERN.matcher(source.getCharacters(section.getStartLine()));
+            if (matcher.matches()) {
+                String fn = matcher.group(1);
+                if (fn.equals("print")) {
+                    String arg = matcher.group(2).trim();
+                    String maybeEnd = "";
+                    if (arg.endsWith(",")) {
+                        maybeEnd = " end=\" \"";
+                    }
+                    msg = (new ErrorMessageFormatter()).format("Missing parentheses in call to 'print'. Did you mean print(%s%s)?", arg, maybeEnd);
+                } else {
+                    msg = "Missing parentheses in call to 'exec'";
+                }
+            }
+        }
         instance.setAttribute("msg", msg);
         throw PException.fromObject(instance, location);
     }

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/parser/antlr/DescriptiveBailErrorListener.java

@@ -65,7 +65,10 @@ public class DescriptiveBailErrorListener extends BaseErrorListener {
     public void syntaxError(Recognizer<?, ?> recognizer, Object offendingSymbol,
                     int line, int charPositionInLine,
                     String msg, RecognitionException e) {
-
+        // If we already constructed the error, keep it, it should be the most specific one
+        if (e instanceof EmptyRecognitionException) {
+            throw e;
+        }
         String entireMessage = e == null || e.getMessage() == null ? "invalid syntax" : e.getMessage();
 
         IntervalSet expectedTokens = null;