Fixed #21741 -- Fixed render_to_string to stop pushing empty dictionaries to its Context

Thanks to kezabelle for the report and original patch
and to numerodix for his improved patch.

django/template/loader.py

@@ -164,13 +164,14 @@ def render_to_string(template_name, dictionary=None, context_instance=None,
     get_template, or it may be a tuple to use select_template to find one of
     the templates in the list. Returns a string.
     """
-    dictionary = dictionary or {}
     if isinstance(template_name, (list, tuple)):
         t = select_template(template_name, dirs)
     else:
         t = get_template(template_name, dirs)
     if not context_instance:
         return t.render(Context(dictionary))
+    if not dictionary:
+        return t.render(context_instance)
     # Add the dictionary to the context stack, ensuring it gets removed again
     # to keep the context_instance in the same state it started in.
     with context_instance.push(dictionary):

tests/template_tests/templates/test_context_stack.html

+{% load custom %}
+{% context_stack_length %}

tests/template_tests/templatetags/custom.py

@@ -22,6 +22,11 @@ def noop(value, param=None):
     return value
 
 
+@register.simple_tag(takes_context=True)
+def context_stack_length(context):
+    return len(context.dicts)
+
+
 @register.simple_tag
 def no_params():
     """Expected no_params __doc__"""

tests/template_tests/test_loaders.py

@@ -171,6 +171,18 @@ class RenderToStringTest(TestCase):
             'No template names provided$',
             loader.select_template, [])
 
+    def test_no_empty_dict_pushed_to_stack(self):
+        """
+        No empty dict should be pushed to the context stack when render_to_string
+        is called without any argument (#21741).
+        """
+
+        # The stack should have a length of 1, corresponding to the builtins
+        self.assertEqual('1',
+            loader.render_to_string('test_context_stack.html').strip())
+        self.assertEqual('1',
+            loader.render_to_string('test_context_stack.html', context_instance=Context()).strip())
+
 
 class TemplateDirsOverrideTest(unittest.TestCase):