page.meta["template"] throws error on mkdocs serve since updating to 9.6.1. Why?

[Andre601]

Since I updated to 9.6.1 of mkdocs-material am I encountering the following error whenever I try to mkdocs serve (Produced from a reproduction setup):

Error

ERROR   -  Error reading page 'index.md': 'template'
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Scripts\mkdocs.exe\__main__.py", line 7, in <module>
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\click\core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\click\core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\click\core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\click\core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\click\core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\__main__.py", line 268, in serve_command
    serve.serve(**kwargs)
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\commands\serve.py", line 85, in serve
    builder(config)
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\commands\serve.py", line 67, in builder
    build(config, serve_url=None if is_clean else serve_url, dirty=is_dirty)
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\commands\build.py", line 310, in build
    _populate_page(file.page, config, files, dirty)
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\commands\build.py", line 163, in _populate_page
    page.markdown = config.plugins.on_page_markdown(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\plugins.py", line 635, in on_page_markdown
    return self.run_event('page_markdown', markdown, page=page, config=config, files=files)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\Andreas\AppData\Local\Programs\Python\Python311\Lib\site-packages\mkdocs\plugins.py", line 566, in run_event
    result = method(item, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^
  File "D:\GitHub\temp\test_hook.py", line 4, in on_page_markdown
    if page.meta["template"] != "blog-post.html":
       ~~~~~~~~~^^^^^^^^^^^^
KeyError: 'template'

This error points to a hook I created, which simply listens for on_page_markdown and has page.meta["template"] != "blog-post.html" as an if-condition set.
Here's a setup from the reproduction:

from mkdocs.structure.pages import Page

def on_page_markdown(markdown: str, /, *, page: Page, config, files) -> str | None:
    if page.meta["template"] != "blog-post.html":
        return
    
    return f"{markdown}\n\nMore Lorem Ipsum"

What is interesting, is that this error won't show, if I add the if-condition while serve is already running (So it only updates)... Tho, I cannot reproduce that behaviour on reproduction unfortunately.

The hook was working fine before updating mkdocs-material, so something must've changed that makes this no longer work reliably.

I've not made this an issue, as I'm unsure on whether this truly is an issue in mkdocs-material, or a bad coding design on my end...

[kamilkrzyskow]

Hey @Andre601,
the value comes from MkDocs, if you updated the theme then it might've updated the MkDocs base.

meta should always be a dict-like structure so replace it with page.meta.get("template") to not crash.

Adding Python code after running mkdocs serve won't reload the Python code. You need to completely shut it down, and run mkdocs serve again.

[Andre601]

That seems to do the trick. Thank you.

Also, from what I can check was there no MkDocs update between 9.6.1 and 9.5.50 which I was using before the update.
My guess is, that some of the changes in the blog plugin had an effect here, as the issue appears with the meta check against the blog-post.html template...

But either way, the issue is resolved.