accept trailing comma in list literal

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

@@ -106,6 +106,9 @@ public class ExpressionParsing extends Parsing {
         if (!matchToken(PyTokenTypes.COMMA)) {
           builder.error("expected ',' or ']'");
         }
+        if (atToken(PyTokenTypes.RBRACKET)) {
+          break;
+        }
         if (!parseSingleExpression(isTargetExpression)) {
           builder.error(message("PARSE.expected.expr.or.comma.or.bracket"));
           break;

python/testData/psi/TrailingCommaInList.py

@@ -0,0 +1 @@
+x = ['a','b',]

python/testData/psi/TrailingCommaInList.txt

@@ -0,0 +1,16 @@
+PyFile:TrailingCommaInList.py
+  PyAssignmentStatement
+    PyTargetExpression: x
+      PsiElement(Py:IDENTIFIER)('x')
+    PsiWhiteSpace(' ')
+    PsiElement(Py:EQ)('=')
+    PsiWhiteSpace(' ')
+    PyListLiteralExpression
+      PsiElement(Py:LBRACKET)('[')
+      PyStringLiteralExpression: a
+        PsiElement(Py:STRING_LITERAL)(''a'')
+      PsiElement(Py:COMMA)(',')
+      PyStringLiteralExpression: b
+        PsiElement(Py:STRING_LITERAL)(''b'')
+      PsiElement(Py:COMMA)(',')
+      PsiElement(Py:RBRACKET)(']')

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

@@ -218,6 +218,10 @@ public class PythonParsingTest extends ParsingTestCase {
     doTest();
   }
 
+  public void testTrailingCommaInList() {
+    doTest();
+  }
+
   public void doTest() {
     doTest(LanguageLevel.PYTHON25);
   }