PY-21823 Align bound and stride expressions inside slices

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

@@ -327,6 +327,10 @@ public class PyBlock implements ASTBlock {
         childIndent = Indent.getNoneIndent();
       }
     }
+    // Note that colons are aligned together with bounds and stride
+    else if (myNode.getElementType() == PyElementTypes.SLICE_ITEM) {
+      childAlignment = getChildAlignment();
+    }
     else if (parentType == PyElementTypes.GENERATOR_EXPRESSION || parentType == PyElementTypes.PARENTHESIZED_EXPRESSION) {
       final boolean tupleOrGenerator = parentType == PyElementTypes.GENERATOR_EXPRESSION ||
                                        myNode.getPsi(PyParenthesizedExpression.class).getContainedExpression() instanceof PyTupleExpression;
@@ -934,7 +938,7 @@ public class PyBlock implements ASTBlock {
 
   @Nullable
   private Alignment getChildAlignment() {
-    if (ourListElementTypes.contains(myNode.getElementType())) {
+    if (ourListElementTypes.contains(myNode.getElementType()) || myNode.getElementType() == PyElementTypes.SLICE_ITEM) {
       if (isInControlStatement()) {
         return null;
       }

python/testData/formatter/sliceAlignment.py

@@ -0,0 +1,10 @@
+xs1 = ys[42:
+5:
+-1]
+
+xs2 = ys[:
+2:
+3]
+
+xs3 = ys[::
+3]

python/testData/formatter/sliceAlignment_after.py

@@ -0,0 +1,10 @@
+xs1 = ys[42:
+         5:
+         -1]
+
+xs2 = ys[:
+         2:
+         3]
+
+xs3 = ys[::
+         3]

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

@@ -847,6 +847,11 @@ public class PyFormatterTest extends PyTestCase {
     doTest();
   }
 
+  // PY-21823
+  public void testSliceAlignment() {
+    doTest();
+  }
+
   public void testVariableAnnotations() {
     runWithLanguageLevel(LanguageLevel.PYTHON36, this::doTest);
   }