#18742: Template tag length returns invalid value
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Reporter: lukaslipka@… | Owner: nobody
Type: Bug | Status: new
Component: Template system | Version: 1.4
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by kmtracey):
* needs_better_patch: => 0
* needs_tests: => 0
* needs_docs: => 0
Comment:
Not sure what to do about this one. During the evaluation of the filter
expression condition in the if tag, `blabla` is evaluated as a
`TemplateLiteral`, which resolves the value in the context specifying
`ignore_failures=True`:
https://github.com/django/django/blob/stable/1.4.x/django/template/defaulttags.py#L814 With ignore_failures True, the value resolves to None instead of the empty
string it would resolve to if ignore_failures was False. None as the value
for the length filter causes length to return an empty string:
https://github.com/django/django/blob/stable/1.4.x/django/template/defaultfilters.py#L540 Subsequently when evaluating `{{ blabla|length }}`, `blabla` is resolved
with ignore_failures False, so it resolve to the empty string. And the
length of the empty string is 0.
Changing `TemplateLiteral` eval to not specify ignore_failures=True breaks
a bunch of if tag template tests. Changing the length filter to return 0
instead of the empty string for invalid inputs breaks a bunch of length
filter tests. Whether either of these changes would break real code I'm
not sure (length returning an empty string, in particular, seems an odd
thing to me...though I can see how that may actually be used to display no
output at all for non-existent data rather than 0...so I suspect changing
that would in fact break things in some real code). Either of them does
make the template fragment in the description behave sanely, but neither
is backwards-compatible.
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https://code.djangoproject.com/ticket/18742#comment:1>
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