Collapse else: if: ... into elif:

Demo/parser/test_unparse.py

@@ -64,6 +64,24 @@ class_decorator = """\
 class Foo: pass
 """
 
+elif1 = """\
+if cond1:
+    suite1
+elif cond2:
+    suite2
+else:
+    suite3
+"""
+
+elif2 = """\
+if cond1:
+    suite1
+elif cond2:
+    suite2
+"""
+
+
+
 class ASTTestCase(unittest.TestCase):
     def assertASTEqual(self, ast1, ast2):
         self.assertEqual(ast.dump(ast1), ast.dump(ast2))
@@ -159,6 +177,10 @@ class UnparseTestCase(ASTTestCase):
     def test_class_definition(self):
         self.check_roundtrip("class A(metaclass=type, *[], **{}): pass")
 
+    def test_elifs(self):
+        self.check_roundtrip(elif1)
+        self.check_roundtrip(elif2)
+
 class DirectoryTestCase(ASTTestCase):
     """Test roundtrip behaviour on all files in Lib and Lib/test."""
 

Demo/parser/unparse.py

@@ -256,9 +256,18 @@ class Unparser:
         self.fill("if ")
         self.dispatch(t.test)
         self.enter()
-        # XXX elif?
         self.dispatch(t.body)
         self.leave()
+        # collapse nested ifs into equivalent elifs.
+        while (t.orelse and len(t.orelse) == 1 and
+               isinstance(t.orelse[0], ast.If)):
+            t = t.orelse[0]
+            self.fill("elif ")
+            self.dispatch(t.test)
+            self.enter()
+            self.dispatch(t.body)
+            self.leave()
+        # final else
         if t.orelse:
             self.fill("else")
             self.enter()