tweak Python parser to handle incomplete code better (PY-4053)

python/src/com/jetbrains/python/parsing/ExpressionParsing.java

@@ -460,6 +460,7 @@ public class ExpressionParsing extends Parsing {
         }
         if (!parseSingleExpression(false)) {
           myBuilder.error(message("PARSE.expected.expression"));
+          break;
         }
       }
     }

python/src/com/jetbrains/python/parsing/Parsing.java

@@ -36,13 +36,13 @@ public class Parsing {
     return getParsingContext().getFunctionParser();
   }
 
-  protected void checkMatches(final IElementType token, final String message) {
+  protected boolean checkMatches(final IElementType token, final String message) {
     if (myBuilder.getTokenType() == token) {
       myBuilder.advanceLexer();
+      return true;
     }
-    else {
-      myBuilder.error(message);
-    }
+    myBuilder.error(message);
+    return false;
   }
 
   protected boolean checkMatches(final TokenSet tokenSet, final String message) {

python/src/com/jetbrains/python/parsing/StatementParsing.java

@@ -532,8 +532,9 @@ public class StatementParsing extends Parsing implements ITokenTypeRemapper {
     final PsiBuilder.Marker ifPart = myBuilder.mark();
     myBuilder.advanceLexer();
     getExpressionParser().parseExpression();
-    checkMatches(PyTokenTypes.COLON, "colon expected");
-    parseSuite();
+    if (checkMatches(PyTokenTypes.COLON, "colon expected")) {
+      parseSuite();
+    }
     ifPart.done(PyElementTypes.IF_PART_IF);
     PsiBuilder.Marker elifPart = myBuilder.mark();
     while (myBuilder.getTokenType() == PyTokenTypes.ELIF_KEYWORD) {

python/testData/psi/MissingParenInCall.py

@@ -0,0 +1,5 @@
+def get_key():
+    if isinstance(self.instance,
+
+def clean():
+    pass

python/testData/psi/MissingParenInCall.txt

@@ -0,0 +1,41 @@
+PyFile:MissingParenInCall.py
+  PyFunction('get_key')
+    PsiElement(Py:DEF_KEYWORD)('def')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:IDENTIFIER)('get_key')
+    PyParameterList
+      PsiElement(Py:LPAR)('(')
+      PsiElement(Py:RPAR)(')')
+    PsiElement(Py:COLON)(':')
+    PsiWhiteSpace('\n    ')
+    PyStatementList
+      PyIfStatement
+        PyIfPartIf
+          PsiElement(Py:IF_KEYWORD)('if')
+          PsiWhiteSpace(' ')
+          PyCallExpression: isinstance
+            PyReferenceExpression: isinstance
+              PsiElement(Py:IDENTIFIER)('isinstance')
+            PyArgumentList
+              PsiElement(Py:LPAR)('(')
+              PyReferenceExpression: instance
+                PyReferenceExpression: self
+                  PsiElement(Py:IDENTIFIER)('self')
+                PsiElement(Py:DOT)('.')
+                PsiElement(Py:IDENTIFIER)('instance')
+              PsiElement(Py:COMMA)(',')
+              PsiErrorElement:expression expected
+                <empty list>
+      PsiWhiteSpace('\n\n')
+      PyFunction('clean')
+        PsiElement(Py:DEF_KEYWORD)('def')
+        PsiWhiteSpace(' ')
+        PsiElement(Py:IDENTIFIER)('clean')
+        PyParameterList
+          PsiElement(Py:LPAR)('(')
+          PsiElement(Py:RPAR)(')')
+        PsiElement(Py:COLON)(':')
+        PsiWhiteSpace('\n    ')
+        PyStatementList
+          PyPassStatement
+            PsiElement(Py:PASS_KEYWORD)('pass')

python/testSrc/com/jetbrains/python/PythonParsingTest.java

@@ -279,6 +279,10 @@ public class PythonParsingTest extends ParsingTestCase {
     doTest();
   }
 
+  public void testMissingParenInCall() {  // PY-4053
+    doTest();
+  }
+
   public void doTest() {
     doTest(LanguageLevel.PYTHON25);
   }