>
> I suspect it has to do with {% if %} being interpreted at runtime while
> {% block %} is interpreted at compile time.
>
> I never investigated this fully. If someone does, I’m interested :-)
>
> --
> Aymeric.
>
I took a look at how this works. The issue comes from a naive approach to
how block nodes are selected to be overridden in the ExtendsNode. I wrote
some pseudocode below that I think clarifies what's happening:
Compile step:
parser.parse() is called
- if got extends node
- call do_extends
- call parser.parse() to get rest of nodelist for current template *
- ExtendsNode.__init__
- get block nodes with nodelist.get_nodes_by_type(BlockNode) **
- ExtendsNode.render
- add BlockContext instance to context ***
- add block nodes from __init__ call to BlockContext
- compile parent
- add parent blocks to BlockContext **
- call parent._render(context)
* At this point, it's possible nodelist has IfNodes that contain BlockNodes.
** nodelist.get_nodes_by_type(BlockNode) returns all block nodes, ignoring
any conditionals
*** BlockContext is a defaultdict of block name -> block list, where
block_context[name][-1] is the winning block.
Render step:
When BlockNodes are rendered they check for a BlockContext in the context.
If an entry exists in BlockContext with the same name, it is rendered in
place of the current block. Because block_context[name][-1] is the one that
wins, and blocks are added to BlockContext regardless of the condition
around it, the parent block is always overridden.
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