Multiple instances of block within template

Andy Gimblett

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Jul 28, 2005, 6:06:54 PM7/28/05

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Hi there,

I don't seem to be able to include multiple instances of a
particularly named block within a template. Doing so seems to raise a
TemplateSyntaxError (django/core/template_loader.py, line 111, in
do_block).

Is there a good reason for this other than sanity checking? If it's
just a sanity check, then I'd like to argue for it being unnecessary
and over-restrictive. I'd like, for example, to use a block called
"title" in both the <title> part of my page, and in an <h1> at the
top. Isn't that reasonable? I'd expect the behaviour to be "wherever
you see this block, fill it in with the supplied value", really. But
maybe I'm missing some reason why this is a bad idea.

My first instinct was to raise a bug about this, since it seems like a
bug to me, but obviously somebody made a design decision that this was
the right thing to do at some point, so I thought I'd ask first. :-)

BTW, other than this, I'm loving the template system. The inheritance
mechanism is particularly nice.

Thanks!

-Andy

--
Andy Gimblett
Computer Science Department
University of Wales Swansea
http://www.cs.swan.ac.uk/~csandy/

Adrian Holovaty

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Jul 28, 2005, 6:42:14 PM7/28/05

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On 7/28/05, Andy Gimblett <g...@gimbo.org.uk> wrote:
> I don't seem to be able to include multiple instances of a
> particularly named block within a template. Doing so seems to raise a
> TemplateSyntaxError (django/core/template_loader.py, line 111, in
> do_block).
>
> Is there a good reason for this other than sanity checking? If it's
> just a sanity check, then I'd like to argue for it being unnecessary
> and over-restrictive. I'd like, for example, to use a block called
> "title" in both the <title> part of my page, and in an <h1> at the
> top. Isn't that reasonable? I'd expect the behaviour to be "wherever
> you see this block, fill it in with the supplied value", really. But
> maybe I'm missing some reason why this is a bad idea.

Hey Andy,

The reason you can't have two {% block %}s in the same template is
because a block tag works in "both" directions. That is, a block tag
doesn't just provide a hole to fill -- it also defines the content
that fills the hole in the *parent*. If there were two similarly-named
{% block %} tags in a template, that template's parent wouldn't know
which one of the blocks' content to use.

For example, say this is your parent template, "base":

"""
<html>
<head><title>{% block title %}{% endblock %}</title></head>
<body>
{% block content %}{% endblock %}
</body>
</html>
"""

And here's the child template:

"""
{% extends "base" %}

{% block title %}Foo{% endblock %}
{% block title %}Bar{% endblock %}
{% block content %}Here's the content{% endblock %}
"""

In this case, the template rendering engine wouldn't know which {%
block title %} to put in the parent template. That's the problem.

Of course, this is only a problem when a *child* template defines two
blocks -- it's not a problem when a parent template defines two
blocks. (In the latter case, it can be useful, as you've pointed out.)
I suppose it would be possible to change the template system's
behavior to allow for duplicate block names only within top-level
*parent* templates...but is the complexity worth the special-case? I'd
love to hear people's opinions on this.

At any rate, thanks for bringing this up!

Adrian

ksmith

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Jul 28, 2005, 8:06:42 PM7/28/05

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AFAIK in a Cheetah Template when a block is defined it can be
referenced as a variable

#block title
A title
#end block

could be reprinted by doing $title

Would this be possible in a Django Template?

{% block title %}
A title
{% endblock %}
{{ title }}

Andy Gimblett

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Jul 29, 2005, 5:43:07 AM7/29/05

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On Thu, Jul 28, 2005 at 05:42:14PM -0500, Adrian Holovaty wrote:
>
> The reason you can't have two {% block %}s in the same template is
> because a block tag works in "both" directions. That is, a block tag
> doesn't just provide a hole to fill -- it also defines the content
> that fills the hole in the *parent*. If there were two
> similarly-named {% block %} tags in a template, that template's
> parent wouldn't know which one of the blocks' content to use.

Ah yes. Gotcha.

I thought there'd be a good reason. :-)

> Of course, this is only a problem when a *child* template defines
> two blocks -- it's not a problem when a parent template defines two
> blocks. (In the latter case, it can be useful, as you've pointed
> out.) I suppose it would be possible to change the template
> system's behavior to allow for duplicate block names only within
> top-level *parent* templates...but is the complexity worth the
> special-case? I'd love to hear people's opinions on this.

Hmmm, yes, good point. Probably not worth it, in my opinion, unless
there's a _very_ clean way it can be expressed in the code.