fixed a case when formatter breaks code (PY-4034 comment)

python/src/com/jetbrains/python/formatter/PyBlock.java

@@ -177,7 +177,7 @@ public class PyBlock implements ASTBlock {
         childIndent = Indent.getNormalIndent();
       }
     }
-    else if (parentType == PyElementTypes.PARENTHESIZED_EXPRESSION) {
+    else if (parentType == PyElementTypes.PARENTHESIZED_EXPRESSION && hasLineBreaksBefore(child, 1)) {
       childIndent = Indent.getNormalIndent();
     }
 
@@ -242,7 +242,8 @@ public class PyBlock implements ASTBlock {
   }
 
   private static boolean hasLineBreaksBefore(ASTNode child, int minCount) {
-    return isWhitespaceWithLineBreaks(TreeUtil.findLastLeaf(child.getTreePrev()), minCount) ||
+    final ASTNode treePrev = child.getTreePrev();
+    return (treePrev != null && isWhitespaceWithLineBreaks(TreeUtil.findLastLeaf(treePrev), minCount)) ||
            isWhitespaceWithLineBreaks(child.getFirstChildNode(), minCount);
   }
 

python/testData/formatter/tupleAssignment.py

@@ -0,0 +1,8 @@
+def bad_autoformat_example():
+    a = 5
+    b = 10
+    print a, b
+    (a, b) = b, a
+    print a, b
+    a, b = b, a
+    print a, b

python/testData/formatter/tupleAssignment_after.py

@@ -0,0 +1,8 @@
+def bad_autoformat_example():
+    a = 5
+    b = 10
+    print a, b
+    (a, b) = b, a
+    print a, b
+    a, b = b, a
+    print a, b

python/testSrc/com/jetbrains/python/PyFormatterTest.java

@@ -113,6 +113,10 @@ public class PyFormatterTest extends PyLightFixtureTestCase {
     doTest();
   }
 
+  public void testTupleAssignment() {  // PY-4034 comment
+    doTest();
+  }
+
   public void testPsiFormatting() { // IDEA-69724
     String initial =
       "def method_name(\n" +