#16335 closed Bug (fixed)
Document why you cannot iterate defaultdict in templates
Reported by: | Owned by: | Nick Meharry | |
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Component: | Documentation | Version: | 1.3 |
Severity: | Normal | Keywords: | |
Cc: | Nick Meharry, akvadrako | Triage Stage: | Ready for checkin |
Has patch: | yes | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | yes |
Easy pickings: | yes | UI/UX: | no |
Description
I am unable to iterate a defaultdict using a for loop in a template.
Code to reproduce issue:
dictionary = defaultdict(list) dictionary['foo'].append('bar') {% for key, value in dictionary.items %} {# Never iterates #} {% endfor %}
Attachments (2)
Change History (21)
comment:1 by , 13 years ago
comment:2 by , 13 years ago
Triage Stage: | Unreviewed → Accepted |
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Indeed, it boils down to the fact that the template language uses the same syntax for dictionary and attribute lookups. The resolution order is documented here: https://docs.djangoproject.com/en/1.3/topics/templates/#variables. It's probably a bad idea to change it.
We could add a note there to warn about objects that accept a dictionary lookup with any name, suggesting to convert them to dict
before passing them to the view, in your example:
context['dictionary'] = dict(dictionary)
comment:3 by , 13 years ago
Cc: | added |
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Needs documentation: | set |
Needs tests: | set |
Owner: | changed from | to
Status: | new → assigned |
How much would break if we were to do the defaultdict -> dict conversion automatically?
by , 13 years ago
Attachment: | defaultdict_conversion.diff added |
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Convert defaultdicts to dicts before template lookups.
comment:4 by , 13 years ago
Has patch: | set |
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Needs tests: | unset |
Patch needs improvement: | set |
by , 13 years ago
Attachment: | defaultdict_docs.diff added |
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Add example of current template behavior w.r.t. defaultdict
s.
comment:5 by , 13 years ago
Needs documentation: | unset |
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After some consideration, perhaps performing the conversion is not such a good idea. I've added an example to the docs that explains why the current behavior happens.
comment:6 by , 13 years ago
Cc: | added |
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comment:7 by , 13 years ago
Component: | Template system → Documentation |
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Triage Stage: | Accepted → Ready for checkin |
Patch with docs explains issue nicely.
comment:8 by , 13 years ago
Patch looks great, however just to be super clear, I think it should include a bit like, "So while this looks like it should do defaultdict.iteritems()
in fact it's doing
defaultdictiteritems
which returns the default value".
comment:9 by , 13 years ago
Hrm, I wonder if the _resolve_lookup
method could be made smarter with something like the following for the dictionary lookup section:
try: if bit not in current: raise KeyError except TypeError: pass current = current[bit]
comment:10 by , 12 years ago
Resolution: | → fixed |
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Status: | assigned → closed |
Fixed #16335 -- Clarified an unintuitive behavior.
The DTL will perform dict lookup before method lookup, which yields
an unexpected result for defaultdicts.
Changeset: d171b3cc0b32374fd5891254b04693e9ec8ed946
comment:11 by , 12 years ago
Fixed #16335 -- Clarified an unintuitive behavior.
The DTL will perform dict lookup before method lookup, which yields
an unexpected result for defaultdicts.
Changeset: d171b3cc0b32374fd5891254b04693e9ec8ed946
comment:12 by , 12 years ago
Fixed #16335 -- Clarified an unintuitive behavior.
The DTL will perform dict lookup before method lookup, which yields
an unexpected result for defaultdicts.
Changeset: d171b3cc0b32374fd5891254b04693e9ec8ed946
comment:13 by , 11 years ago
Having this case:
my_dict = collections.defaultdict(lambda: collections.defaultdict(list)) dictionary['foo']['foo1'].append(obj)
and doing
return Response({'my_dict ': dict(my_dict)}, template_name='_internal_template.html')
I'm able to loop the first for only
{% for groups in my_dict.itervalues %} groups = {{ groups }} <br><br> //It prints: groups = defaultdict(<type 'list'>, {1: [<Obj: Obj 1 by me>]}) {% for cols in groups.itervalues %} cols = {{ cols }} <br><br> {% for maps in cols.itervalues %}
comment:14 by , 11 years ago
Resolution: | fixed |
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Django 1.6, Pyhton 3.3.3. Even in the simpliest case:
{% for key, values in dict.items %}
{{ key }}
{% endfor %}
there is nothing in "key".
While
{% for key in dict %}
{{ key }}
{% endfor %}
works well.
Also preliminary conversion of a defaultdict to dict helps.
comment:15 by , 10 years ago
I am also seeing this for Django 1.6.1 python 2.7.6. Conversion to standard dict does not help in my case. Otherwise same symptoms as the post from krommail.
comment:16 by , 10 years ago
After upgrading to 1.6.5 still with python 2.7.6 the workaround (converting to standard dictionary prior to handing dict to the template) functions.
As a Django newbie I have sunk 5 hours on this silly issue. I need a break today.
comment:17 by , 10 years ago
Status: | closed → new |
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comment:18 by , 10 years ago
Resolution: | → fixed |
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Status: | new → closed |
Summary: | Cannot iterate defaultdict in template → Document why you cannot iterate defaultdict in templates |
The documentation that was added as part of the ticket [d171b3cc0b32374fd5891254b04693e9ec8ed946] explains why it doesn't work.
comment:19 by , 10 years ago
I am actually getting a MemoryError (which sometimes end in segfault) when trying to iterate the values in a defaultdict
In views.py:
totals = defaultdict(Decimal) add_stuff_to_it(totals)
In the template:
{% for t in totals.values %} {{ t }} {% endfor %}
Results in
$ python manage.py runserver 0.0.0.0:8000 --settings=xxx.settings_debug Validating models... 0 errors found July 30, 2014 - 09:57:05 Django version 1.6.5, using settings 'xxx.settings_debug' Starting development server at http://0.0.0.0:8000/ Quit the server with CONTROL-C. Traceback (most recent call last): File "/usr/lib/python2.7/wsgiref/handlers.py", line 85, in run self.result = application(self.environ, self.start_response) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 67, in __call__ return self.application(environ, start_response) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 206, in __call__ response = self.get_response(request) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 194, in get_response response = self.handle_uncaught_exception(request, resolver, sys.exc_info()) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 229, in handle_uncaught_exception return debug.technical_500_response(request, *exc_info) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/views/debug.py", line 69, in technical_500_response html = reporter.get_traceback_html() File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/views/debug.py", line 324, in get_traceback_html return t.render(c) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py", line 140, in render return self._render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py", line 134, in _render return self.nodelist.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py", line 840, in render bit = self.render_node(node, context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py", line 78, in render_node return node.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py", line 305, in render return nodelist.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py", line 840, in render bit = self.render_node(node, context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py", line 78, in render_node return node.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py", line 36, in render output = self.nodelist.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py", line 840, in render bit = self.render_node(node, context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/debug.py", line 78, in render_node return node.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/defaulttags.py", line 212, in render return nodelist.render(context) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/template/base.py", line 844, in render return mark_safe(''.join(bits)) File "/home/vagrant/venv/local/lib/python2.7/site-packages/django/utils/safestring.py", line 116, in mark_safe return SafeText(s) MemoryError [30/Jul/2014 09:57:16] "GET /statistics/order_book_and_wip/ HTTP/1.1" 500 59
Strangely, iterating over the keys seems to work fine.
Setting the default_factory to None as detailed here makes it work as expected. It would seem like a cheap and sensible workaround to apply that to defaultdicts. Or otherwise at least raise an exception so users can know what's going on.
Right, because dictionary key access takes precedence in template variable lookups, so
{% ... dictionary.items %}
adds an empty list named 'items' to the dictionary. Not sure there's a good workaround here.