<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _build_block_context(template, context):
 """Populate the block context with BlockNodes from parent templates."""


    # Ensure there's a BlockContext before rendering. This allows blocks in
    # ExtendsNodes to be found by sub-templates (allowing {{ block.super }} and
    # overriding sub-blocks to work).
    if BLOCK_CONTEXT_KEY not in context.render_context:
        context.render_context[BLOCK_CONTEXT_KEY] = BlockContext()
    block_context = context.render_context[BLOCK_CONTEXT_KEY]

    for node in template.nodelist:
        if isinstance(node, ExtendsNode):
            compiled_parent = node.get_parent(context)

            # Add the parent node's blocks to the context. (This ends up being
            # similar logic to ExtendsNode.render(), where we're adding the
            # parent's blocks to the context so a child can find them.)
            block_context.add_blocks(
                {n.name: n for n in compiled_parent.nodelist.get_nodes_by_type(BlockNode)})

            _build_block_context(compiled_parent, context)
            return compiled_parent

        # The ExtendsNode has to be the first non-text node.
        if not isinstance(node, TextNode):
            break