Fixed #13408 -- Deprecated silent unpacking exception passing in for template tag.

Thanks peterbe for the suggestion.

django/template/defaulttags.py

@@ -17,6 +17,7 @@ from django.template.base import (Node, NodeList, Template, Context, Library,
     render_value_in_context)
 from django.template.smartif import IfParser, Literal
 from django.template.defaultfilters import date
+from django.utils.deprecation import RemovedInDjango20Warning
 from django.utils.encoding import force_text, smart_text
 from django.utils.safestring import mark_safe
 from django.utils.html import format_html
@@ -158,7 +159,8 @@ class ForNode(Node):
             nodelist = []
             if self.is_reversed:
                 values = reversed(values)
-            unpack = len(self.loopvars) > 1
+            num_loopvars = len(self.loopvars)
+            unpack = num_loopvars > 1
             # Create a forloop value in the context.  We'll update counters on each
             # iteration just below.
             loop_dict = context['forloop'] = {'parentloop': parentloop}
@@ -177,6 +179,21 @@ class ForNode(Node):
                 if unpack:
                     # If there are multiple loop variables, unpack the item into
                     # them.
+
+                    # To complete this deprecation, remove from here to the
+                    # try/except block as well as the try/except itself,
+                    # leaving `unpacked_vars = ...` and the "else" statements.
+                    if not isinstance(item, (list, tuple)):
+                        len_item = 1
+                    else:
+                        len_item = len(item)
+                    # Check loop variable count before unpacking
+                    if num_loopvars != len_item:
+                        warnings.warn(
+                            "Need {0} values to unpack in for loop; got {1}. "
+                            "This will raise an exception in Django 2.0."
+                            .format(num_loopvars, len_item),
+                            RemovedInDjango20Warning)
                     try:
                         unpacked_vars = dict(zip(self.loopvars, item))
                     except TypeError:

docs/internals/deprecation.txt

@@ -22,6 +22,9 @@ about each item can often be found in the release notes of two versions prior.
 
 * ``SimpleTestCase.urls`` will be removed.
 
+* Using an incorrect count of unpacked values in the ``for`` template tag
+  will raise an exception rather than fail silently.
+
 .. _deprecation-removed-in-1.9:
 
 1.9

docs/releases/1.8.txt

@@ -270,3 +270,9 @@ removed in Django 2.0. Use :func:`@override_settings(ROOT_URLCONF=...)
 Related to the previous item, the ``prefix`` argument to
 :func:`django.conf.urls.i18n.i18n_patterns` has been deprecated. Simply pass a
 list of :func:`django.conf.urls.url` instances instead.
+
+Using an incorrect count of unpacked values in the :ttag:`for` template tag
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Using an incorrect count of unpacked values in :ttag:`for` tag will raise an
+exception rather than fail silently in Django 2.0.

tests/template_tests/tests.py

@@ -602,6 +602,9 @@ class TemplateTests(TestCase):
                                     failures.append("Template test (Cached='%s', TEMPLATE_STRING_IF_INVALID='%s', TEMPLATE_DEBUG=%s): %s -- FAILED. Template loading invoked method that shouldn't have been invoked." % (is_cached, invalid_str, template_debug, name))
 
                                 try:
+                                    with warnings.catch_warnings():
+                                        # Ignore deprecations of using the wrong number of variables with the 'for' tag.
+                                        warnings.filterwarnings("ignore", category=RemovedInDjango20Warning, module="django.template.defaulttags")
                                         output = self.render(test_template, vals)
                                 except ShouldNotExecuteException:
                                     failures.append("Template test (Cached='%s', TEMPLATE_STRING_IF_INVALID='%s', TEMPLATE_DEBUG=%s): %s -- FAILED. Template rendering invoked method that shouldn't have been invoked." % (is_cached, invalid_str, template_debug, name))
@@ -954,18 +957,21 @@ class TemplateTests(TestCase):
             'for-tag-unpack08': ("{% for key,value, in items %}{{ key }}:{{ value }}/{% endfor %}", {"items": (('one', 1), ('two', 2))}, template.TemplateSyntaxError),
             # Ensure that a single loopvar doesn't truncate the list in val.
             'for-tag-unpack09': ("{% for val in items %}{{ val.0 }}:{{ val.1 }}/{% endfor %}", {"items": (('one', 1), ('two', 2))}, "one:1/two:2/"),
-            # Otherwise, silently truncate if the length of loopvars differs to the length of each set of items.
-            'for-tag-unpack10': ("{% for x,y in items %}{{ x }}:{{ y }}/{% endfor %}", {"items": (('one', 1, 'carrot'), ('two', 2, 'orange'))}, "one:1/two:2/"),
-            'for-tag-unpack11': ("{% for x,y,z in items %}{{ x }}:{{ y }},{{ z }}/{% endfor %}", {"items": (('one', 1), ('two', 2))}, ("one:1,/two:2,/", "one:1,INVALID/two:2,INVALID/")),
-            'for-tag-unpack12': ("{% for x,y,z in items %}{{ x }}:{{ y }},{{ z }}/{% endfor %}", {"items": (('one', 1, 'carrot'), ('two', 2))}, ("one:1,carrot/two:2,/", "one:1,carrot/two:2,INVALID/")),
             'for-tag-unpack13': ("{% for x,y,z in items %}{{ x }}:{{ y }},{{ z }}/{% endfor %}", {"items": (('one', 1, 'carrot'), ('two', 2, 'cheese'))}, ("one:1,carrot/two:2,cheese/", "one:1,carrot/two:2,cheese/")),
-            'for-tag-unpack14': ("{% for x,y in items %}{{ x }}:{{ y }}/{% endfor %}", {"items": (1, 2)}, (":/:/", "INVALID:INVALID/INVALID:INVALID/")),
             'for-tag-empty01': ("{% for val in values %}{{ val }}{% empty %}empty text{% endfor %}", {"values": [1, 2, 3]}, "123"),
             'for-tag-empty02': ("{% for val in values %}{{ val }}{% empty %}values array empty{% endfor %}", {"values": []}, "values array empty"),
             'for-tag-empty03': ("{% for val in values %}{{ val }}{% empty %}values array not found{% endfor %}", {}, "values array not found"),
             # Ticket 19882
             'for-tag-filter-ws': ("{% load custom %}{% for x in s|noop:'x y' %}{{ x }}{% endfor %}", {'s': 'abc'}, 'abc'),
 
+            # These tests raise deprecation warnings and will raise an exception
+            # in Django 2.0. The existing behavior is silent truncation if the
+            # length of loopvars differs to the length of each set of items.
+            'for-tag-unpack10': ("{% for x,y in items %}{{ x }}:{{ y }}/{% endfor %}", {"items": (('one', 1, 'carrot'), ('two', 2, 'orange'))}, "one:1/two:2/"),
+            'for-tag-unpack11': ("{% for x,y,z in items %}{{ x }}:{{ y }},{{ z }}/{% endfor %}", {"items": (('one', 1), ('two', 2))}, ("one:1,/two:2,/", "one:1,INVALID/two:2,INVALID/")),
+            'for-tag-unpack12': ("{% for x,y,z in items %}{{ x }}:{{ y }},{{ z }}/{% endfor %}", {"items": (('one', 1, 'carrot'), ('two', 2))}, ("one:1,carrot/two:2,/", "one:1,carrot/two:2,INVALID/")),
+            'for-tag-unpack14': ("{% for x,y in items %}{{ x }}:{{ y }}/{% endfor %}", {"items": (1, 2)}, (":/:/", "INVALID:INVALID/INVALID:INVALID/")),
+
             ### IF TAG ################################################################
             'if-tag01': ("{% if foo %}yes{% else %}no{% endif %}", {"foo": True}, "yes"),
             'if-tag02': ("{% if foo %}yes{% else %}no{% endif %}", {"foo": False}, "no"),