python - Error: "dictionary update sequence element #0 has length 1; 2 is required" on Django 1.4

Question

I have an error message on Django 1.4:

dictionary update sequence element #0 has length 1; 2 is required

It happened when I tried using a template tag like: {% for v in values %}:

dictionary update sequence element #0 has length 1; 2 is required

Request Method:     GET
Request URL:    ...
Django Version:     1.4.5
Exception Type:     ValueError
Exception Value:    

dictionary update sequence element #0 has length 1; 2 is required

Exception Location:     /usr/local/lib/python2.7/dist-packages/djorm_hstore/fields.py in __init__, line 21
Python Executable:  /usr/bin/uwsgi-core
Python Version:     2.7.3
Python Path:    

['/var/www/',
 '.',
 '',
 '/usr/lib/python2.7',
 '/usr/lib/python2.7/plat-linux2',
 '/usr/lib/python2.7/lib-tk',
 '/usr/lib/python2.7/lib-old',
 '/usr/lib/python2.7/lib-dynload',
 '/usr/local/lib/python2.7/dist-packages',
 '/usr/lib/python2.7/dist-packages',
 '/usr/lib/python2.7/dist-packages/PIL',
 '/usr/lib/pymodules/python2.7']

Server time:    sam, 13 Jul 2013 16:15:45 +0200
Error during template rendering

In template /var/www/templates/app/index.html, error at line 172
dictionary update sequence element #0 has length 1; 2 is required

172     {% for product in products %}

Traceback Switch to copy-and-paste view

/usr/lib/python2.7/dist-packages/django/core/handlers/base.py in get_response

                            response = callback(request, *callback_args, **callback_kwargs)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/contrib/auth/decorators.py in _wrapped_view

                    return view_func(request, *args, **kwargs)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/views/decorators/http.py in inner

                return func(request, *args, **kwargs)

    ...
? Local vars
./app/views.py in index

            context_instance=RequestContext(request))

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/shortcuts/__init__.py in render_to_response

        return HttpResponse(loader.render_to_string(*args, **kwargs), **httpresponse_kwargs)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/loader.py in render_to_string

            return t.render(context_instance)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/base.py in render

                return self._render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/base.py in _render

            return self.nodelist.render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/base.py in render

                    bit = self.render_node(node, context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/debug.py in render_node

                return node.render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/loader_tags.py in render

            return compiled_parent._render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/base.py in _render

            return self.nodelist.render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/base.py in render

                    bit = self.render_node(node, context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/debug.py in render_node

                return node.render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/loader_tags.py in render

                result = block.nodelist.render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/base.py in render

                    bit = self.render_node(node, context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/debug.py in render_node

                return node.render(context)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/template/defaulttags.py in render

            len_values = len(values)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/core/paginator.py in __len__

            return len(self.object_list)

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/db/models/query.py in __len__

                    self._result_cache = list(self.iterator())

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/db/models/query.py in iterator

                        obj = model(*row[index_start:aggregate_start])

    ...
? Local vars
/usr/lib/python2.7/dist-packages/django/db/models/base.py in __init__

                    setattr(self, field.attname, val)

    ...
? Local vars
/usr/local/lib/python2.7/dist-packages/djorm_hstore/fields.py in __set__

                value = self.field._attribute_class(value, self.field, obj)

    ...
? Local vars
/usr/local/lib/python2.7/dist-packages/djorm_hstore/fields.py in __init__

            super(HStoreDictionary, self).__init__(value, **params)

    ...
? Local vars

It happens too when I try to access on a hstore queryset:

Traceback (most recent call last):
File "manage.py", line 14, in <module>
    execute_manager(settings)

File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 459, in execute_manager
    utility.execute()

File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 382, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)

File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 196, in run_from_argv
    self.execute(*args, **options.__dict__)

File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 232, in execute
    output = self.handle(*args, **options)

File "/home/name/workspace/project/app/data/commands/my_command.py", line 60, in handle
    item_id = tmp[0].id,

File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 207, in __getitem__
    return list(qs)[0]

File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 87, in __len__
    self._result_cache.extend(self._iter)

File "/usr/local/lib/python2.7/dist-packages/django/db/models/query.py", line 301, in iterator
    obj = model(*row[index_start:aggregate_start])

File "/usr/local/lib/python2.7/dist-packages/django/db/models/base.py", line 300, in __init__
    setattr(self, field.attname, val)

File "/usr/local/lib/python2.7/dist-packages/djorm_hstore/fields.py", line 38, in __set__
    value = self.field._attribute_class(value, self.field, obj)

File "/usr/local/lib/python2.7/dist-packages/djorm_hstore/fields.py", line 21, in __init__
    super(HStoreDictionary, self).__init__(value, **params)

ValueError: dictionary update sequence element #0 has length 1; 2 is required

The code is:

tmp = Item.objects.where(HE("kv").contains({'key':value}))

if tmp.count() > 0:
            
    item_id = tmp[0].id,

I'm just trying to access the value. I don't understand the "update sequence" message. When I use a cursor instead of hstore queryset, the function works. The error comes on template rendering too. I just restarted uwsgi and everything works well, but the error comes back later.

Answer 1

Just ran into this problem. I don't know if it's the same thing that hit your code, but for me the root cause was because I forgot to put name= on the last argument of the url (or path in Django 2.0+) function call.

For instance, the following functions throw the error from the question:

url(r'^foo/(?P<bar>[A-Za-z]+)/$', views.FooBar.as_view(), 'foo')
path('foo/{slug:bar}/', views.FooBar, 'foo')

But these actually work:

url(r'^foo/(?P<bar>[A-Za-z]+)/$', views.FooBar.as_view(), name='foo')
path('foo/{slug:bar}/', views.FooBar, name='foo')

The reason why the traceback is unhelpful is because internally, Django wants to parse the given positional argument as the keyword argument kwargs, and since a string is an iterable, an atypical code path begins to unfold. Always use name= on your urls!