#20745: Document the template language's silencing of TypeError
---------------------------------+------------------------------------
Reporter: robin | Owner: nobody
Type: Bug | Status: new
Component: Template system | Version: 1.5
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
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Changes (by bmispelon):
* easy: 0 => 1
Comment:
After some digging, I think I'm starting to understand what's happening
here.
When django finds a callable element in a template (which `{{ block.super
}}` is, since it refers to a `BlockNode`'s `super` method), it attempts to
call it (without passing any argument), wrapping the calling in a
`try/except` and catching a `TypeError` (which happens when you call a
function/method without enough arguments).
In our case, a `TypeError` is raised from inside the function and
subsequently caught and silenced in the `except` clause.
Before 4397c587a43ff9bfddd295d48d850676778c6e77, any exception raised
while rendering a template would be wrapped in a `TemplateSyntaxError`.
This `TemplateSyntaxError` would then not be caught by the aforementionned
`except TypeError` clause and would bubble up.
Note that this would only happen with `settings.TEMPLATE_DEBUG = True`
(this discrepancy between the two values of `TEMPLATE_DEBUG` is actually
one of the reason why this change was introduced in the first place).
This is therefore a limitation of django's implementation, which stems
from the fact that it can't distinguish between a `TypeError` caused by
calling a function without enough arguments and a `TypeError` raised from
inside the function.
This limitation is noted in a comment in the code [1], but as far as I can
tell, it's not documented.
I'm leaving this ticket open but I see it as a documentation issue, unless
someone can come up with a clever way of side-stepping the problem but I'm
not sure there is one.
Since this details is somehow technical, I think a good place for it would
be this section: https://docs.djangoproject.com/en/1.5/ref/templates/api
/#variables-and-lookups
[1]
https://github.com/django/django/blob/028db9750357d504c55a7ac686c9abaa3c847ac6/django/template/base.py#L791-L794
--
Ticket URL: <https://code.djangoproject.com/ticket/20745#comment:5>
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