Allow '{%+' to be valid syntax (but NOP) when lstrip_blocks == False. F…

…ixes pallets#748.

CHANGES.rst

@@ -16,8 +16,11 @@ unreleased
 - The ``map`` filter in async mode now automatically awaits
 - Added a new ``ChainableUndefined`` class to support getitem
   and getattr on an undefined object. (`#977`_)
+- Allow `'{%+'` syntax (with NOP behavior) when
+  `lstrip_blocks == False` (`#748`_)
 
 .. _#765: https://github.com/pallets/jinja/issues/765
+.. _#748: https://github.com/pallets/jinja/issues/748
 .. _#977: https://github.com/pallets/jinja/issues/977
 
 
@@ -28,6 +31,7 @@ _Unreleased_
 
 -  Fix Python 3.7 deprecation warnings.
 
+
 Version 2.10.1
 --------------
 

jinja2/lexer.py

@@ -445,22 +445,21 @@ def __init__(self, environment):
 
         # strip leading spaces if lstrip_blocks is enabled
         prefix_re = {}
+        no_lstrip_re = e('+')
+        # detect overlap between block and variable or comment strings
+        block_diff = c(r'^%s(.*)' % e(environment.block_start_string))
+        # make sure we don't mistake a block for a variable or a comment
+        m = block_diff.match(environment.comment_start_string)
+        no_lstrip_re += m and r'|%s' % e(m.group(1)) or ''
+        m = block_diff.match(environment.variable_start_string)
+        no_lstrip_re += m and r'|%s' % e(m.group(1)) or ''
+        # detect overlap between comment and variable strings
+        comment_diff = c(r'^%s(.*)' % e(environment.comment_start_string))
+        m = comment_diff.match(environment.variable_start_string)
+        no_variable_re = m and r'(?!%s)' % e(m.group(1)) or ''
+
         if environment.lstrip_blocks:
             # use '{%+' to manually disable lstrip_blocks behavior
-            no_lstrip_re = e('+')
-            # detect overlap between block and variable or comment strings
-            block_diff = c(r'^%s(.*)' % e(environment.block_start_string))
-            # make sure we don't mistake a block for a variable or a comment
-            m = block_diff.match(environment.comment_start_string)
-            no_lstrip_re += m and r'|%s' % e(m.group(1)) or ''
-            m = block_diff.match(environment.variable_start_string)
-            no_lstrip_re += m and r'|%s' % e(m.group(1)) or ''
-
-            # detect overlap between comment and variable strings
-            comment_diff = c(r'^%s(.*)' % e(environment.comment_start_string))
-            m = comment_diff.match(environment.variable_start_string)
-            no_variable_re = m and r'(?!%s)' % e(m.group(1)) or ''
-
             lstrip_re = r'^[ \t]*'
             block_prefix_re = r'%s%s(?!%s)|%s\+?' % (
                     lstrip_re,
@@ -474,10 +473,21 @@ def __init__(self, environment):
                     no_variable_re,
                     e(environment.comment_start_string),
                     )
-            prefix_re['block'] = block_prefix_re
-            prefix_re['comment'] = comment_prefix_re
         else:
-            block_prefix_re = '%s' % e(environment.block_start_string)
+            # If lstrip_blocks is False, then '{%+' is allowed but a NOP
+            block_prefix_re = r'%s(?!%s)|%s\+?' % (
+                e(environment.block_start_string),
+                no_lstrip_re,
+                e(environment.block_start_string),
+            )
+            comment_prefix_re = r'%s%s|%s\+?' % (
+                e(environment.comment_start_string),
+                no_variable_re,
+                e(environment.comment_start_string),
+            )
+
+        prefix_re['block'] = block_prefix_re
+        prefix_re['comment'] = comment_prefix_re
 
         self.newline_sequence = environment.newline_sequence
         self.keep_trailing_newline = environment.keep_trailing_newline

tests/test_lexnparse.py

@@ -503,6 +503,14 @@ def test_no_lstrip(self, env):
         tmpl = env.from_string('''    {%+ if True %}\n    {%+ endif %}''')
         assert tmpl.render() == "    \n    "
 
+    def test_lstrip_blocks_false_with_no_lstrip(self, env):
+        # Test that + is a NOP (but does not cause an error) if lstrip_blocks=False
+        env = Environment(lstrip_blocks=False, trim_blocks=False)
+        tmpl = env.from_string('''    {% if True %}\n    {% endif %}''')
+        assert tmpl.render() == "    \n    "
+        tmpl = env.from_string('''    {%+ if True %}\n    {%+ endif %}''')
+        assert tmpl.render() == "    \n    "
+
     def test_lstrip_endline(self, env):
         env = Environment(lstrip_blocks=True, trim_blocks=False)
         tmpl = env.from_string(