From the Django docs,
Generally, if a variable doesn't exist, the template system inserts the value of the TEMPLATE_STRING_IF_INVALID setting, which is set to '' (the empty string) by default.
Filters that are applied to an invalid variable will only be applied if TEMPLATE_STRING_IF_INVALID is set to '' (the empty string). If TEMPLATE_STRING_IF_INVALID is set to any other value, variable filters will be ignored.
This behavior is slightly different for the if, for and regroup template tags. If an invalid variable is provided to one of these template tags, the variable will be interpreted as None. Filters are always applied to invalid variables within these template tags.
If an invalid variable always gets translated to '', for template tags and filters other than if, for and regroup, then what good does the template filter default_if_none do? Obsolete?
A Django template is a text document or a Python string marked-up using the Django template language. Some constructs are recognized and interpreted by the template engine. The main ones are variables and tags. A template is rendered with a context.
Variable names consist of any combination of alphanumeric characters and the underscore ( "_" ) but may not start with an underscore, and may not be a number.
You cannot use python code in django template. This is by design, Django's idea of template is to isolate the presentation logic from the programming code.
Yes, it is possible to do this. In order to render HTML without a template in Django, we'll need to manually build up a string of HTML and send that to the browser as an HttpResponse.
There is a difference between an invalid variable and one that exists but has a value of None
.
Consider the following context:
{'apple':'green','banana':None}`
In your template {{ apple }}
resolves to green
, while {{ banana }}
resolves to None
, and {{ orange }}
resolves to TEMPLATE_STRING_IF_INVALID
.
Now consider {{ banana|default_if_none:'yellow' }}
and you should see the use of the default_if_none
tag.
Here's a case where I have used default_if_none
a few times. I'm querying a secondary database in which I have no control and I'm displaying the data in the template. Most of the times the data looks fine but sometimes, the data value will show None
. In that case, I will use the filter as:
{{ data_value|default_if_none:"N/A" }}
The general public and users of site doesn't usually understand what the None
value means, by replacing it with a more friendly word, the filter default_if_none
comes in handy.
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