Fix template errors with empty blocks.

bdarnell · bdarnell · commit 54d91d5e1312 · 2012-08-26T18:03:12.000-07:00
Emits a "pass" statement in every generated block whether it's empty or not; I verified with the disassembler that the unnecessary pass statements don't generate extra bytecodes. Closes tornadoweb#546.
diff --git a/tornado/template.py b/tornado/template.py
@@ -501,6 +501,8 @@ def generate(self, writer):
         writer.write_line("%s:" % self.statement, self.line)
         with writer.indent():
             self.body.generate(writer)
+            # Just in case the body was empty
+            writer.write_line("pass", self.line)
 
 
 class _IntermediateControlBlock(_Node):
@@ -509,6 +511,8 @@ def __init__(self, statement, line):
         self.line = line
 
     def generate(self, writer):
+        # In case the previous block was empty
+        writer.write_line("pass", self.line)
         writer.write_line("%s:" % self.statement, self.line, writer.indent_size() - 1)
 
 
diff --git a/tornado/test/template_test.py b/tornado/test/template_test.py
@@ -101,6 +101,10 @@ def test_if(self):
         self.assertEqual(template.generate(x=5), b("yes"))
         self.assertEqual(template.generate(x=3), b("no"))
 
+    def test_if_empty_body(self):
+        template = Template(utf8("{% if True %}{% else %}{% end %}"))
+        self.assertEqual(template.generate(), b(""))
+
     def test_try(self):
         template = Template(utf8("""{% try %}
 try{% set y = 1/x %}
