Fixed #18400 -- Modified length template filter to return 0 for unknown variables.

Thanks Florian for the bug report, luyikei for the initial code patch, and
Bouke for the code review feedback.

django/template/defaultfilters.py

@@ -578,7 +578,7 @@ def length(value):
     try:
         return len(value)
     except (ValueError, TypeError):
-        return ''
+        return 0
 
 
 @register.filter(is_safe=False)

docs/ref/templates/builtins.txt

@@ -1688,6 +1688,11 @@ For example::
 If ``value`` is ``['a', 'b', 'c', 'd']`` or ``"abcd"``, the output will be
 ``4``.
 
+.. versionchanged:: 1.8
+
+    The filter returns ``0`` for an undefined variable. Previously, it returned
+    an empty string.
+
 .. templatefilter:: length_is
 
 length_is

docs/releases/1.8.txt

@@ -267,6 +267,9 @@ Miscellaneous
 
 * ``django.contrib.gis`` dropped support for GEOS 3.1 and GDAL 1.6.
 
+* The :tfilter:`length` template filter now returns ``0`` for an undefined
+  variable, rather than an empty string.
+
 .. _deprecated-features-1.8:
 
 Features deprecated in 1.8

tests/template_tests/filters.py

@@ -322,8 +322,8 @@ def get_filter_tests():
         'length04': ('{{ string|length }}', {'string': 'django'}, '6'),
         'length05': ('{% if string|length == 6 %}Pass{% endif %}', {'string': mark_safe('django')}, 'Pass'),
         # Invalid uses that should fail silently.
-        'length06': ('{{ int|length }}', {'int': 7}, ''),
+        'length06': ('{{ int|length }}', {'int': 7}, '0'),
-        'length07': ('{{ None|length }}', {'None': None}, ''),
+        'length07': ('{{ None|length }}', {'None': None}, '0'),
 
         # length_is filter.
         'length_is01': ('{% if some_list|length_is:"4" %}Four{% endif %}', {'some_list': ['4', None, True, {}]}, 'Four'),