How do I use variable values in frontmatter?

[haxfn]

Description

something like the following is not working:

_variables.yml:

categories:
  A: computing
  B: informatics

index.qmd:

---
title: Emacs
description: Thoughts and References when Learning Emacs.

categories:
  - "{{< var categories.A >}}"
  - "{{< var categories.B >}}"
---

{{< var categories.A >}}

{{< var categories.B >}}

This produces:

Where I was expecting the categories.

[cscheid]

I'm sorry you're running into this issue. That's an unfortunate consequence of the way in which Quarto resolves some of its information. The problem is that categories is resolved in typescript; var shortcodes are resolved in Lua, but the metadata generated for categories is not actually rewritten by the time it gets to the part of the typescript code that will handle it.

You might be able to work around this if you generate the entire categories entry in code. Since you're using {{< var engine.jupyter >}}, I'm going to assume you're using python. In that case, this works:

---
title: "Post With Code"
author: "Harlow Malloc"
date: "2025-05-05"
image: "image.jpg"
keep-md: true
---

This is a post with executable code.

```{python}
#| echo: false
import yaml
from IPython.display import Markdown
categories = {"categories": ["news", "code", "analysis", "and-what-else"]}
Markdown("---\n" + yaml.dump(categories) + "---")
```

Compare this to the example blog post you get with quarto create project blog and you'll see what I mean. The trick here is that you're allowed to split the declaration of metadata across multiple blocks, and so you can emit a metadata block that sets a single subfield programmatically, in this case categories.

Ideally, Quarto should detect that the categories field is being specified dynamically, and then resolve it appropriately. But that's a very hard thing to do in general, because (for example) shortcodes can run arbitrary code.

The delineation of what parts of metadata need to be available for Quarto during what parts of its processing is definitely something that we need to both specify and communicate better - thanks for the question!

[haxfn]

you're allowed to split the declaration of metadata across multiple blocks

I did not know that. This is one hack.

... Quarto should detect that the categories field is being specified dynamically...But that's a very hard thing to do in general, because (for example) shortcodes can run arbitrary code.

I am not exactly sure what you mean here, the variables do not have to be shortcodes, rather they will be static strings from a yaml file, no?

[cscheid]

Sorry. In your case, yes! I was explaining what it would take for Quarto to implement a feature that makes it so you can just use shortcodes inside categories like you were doing.

[mcanouil]

@haxfn why ddid you not follow up on the issue about this you referenced in another thread?

• Variables in categories (in frontmatter) do not parse properly when categories is set to true in listing #5578

(Cc @cscheid for your awareness of the related opened issue)

[albertomercurio]

Hello, I have a similar problem, but related to the external-links entry.

I have the following variables in _variables.yml

github:
  user: Daniele-PyBECTN
  repo: Computational_quantum_optics_lectures
  branch: notebooks

I tried the following

---
other-links:
  - text: Run notebook in Binder
    href: "https://mybinder.org/v2/gh/{{< var github.user >}}/{{< var github.repo >}}/{{< var github.branch >}}/?urlpath=lab/tree/notebooks/lecture1/introduction.ipynb"
    icon: file-code
---

Some text...

but it creates a wrong link with weird symbols. I then tried

---
title: Introduction to Python
keep-md: true
---

```{python}
#| echo: false
import yaml
from IPython.display import Markdown
yaml_code = """
other-links:
  - text: Run notebook in Binder
    href: https://mybinder.org/v2/gh/{{< var github.user >}}/{{< var github.repo >}}/{{< var github.branch >}}/?urlpath=lab/tree/notebooks/lecture1/introduction.ipynb
    icon: file-code
"""

data = yaml.safe_load(yaml_code)
Markdown("---\n" + yaml.dump(data) + "---")
```

But it doesn't show the link button at all.

I have also noticed that this second method doesn't work if I don't set the title in the frontmatter. I'm working with a book project, so I don't need to set the title page by page, as I only define sections and subsections.

[mcanouil]

In most cases shortcodes are not evaluated inside YAML, especially YAML keys that aren't "text".
Some YAML keys are tied up to the structure/layout of the document thus can't be generated, like format for the most obvious one which are required before computations happen.

Note that you an reproduce this behaviour (not a bug) using a simpler example:

---
title: "Reproducible Quarto Document"
quarto-url: "quarto.org"
engine: jupyter
format: html
# other-links:
#   - text: Quarto
#     href: "https://{{< meta quarto-url >}}/"
---

{{< lipsum 1 >}}

```{python}
from IPython.display import Markdown
yaml_code = """
---
other-links:
  - text: Quarto
    href: 'https://{{< meta quarto-url >}}/'
---
"""
Markdown(yaml_code)
```

Alternatively, you can generate buttons/links with whatever you want.

[albertomercurio]

Thank you for your message. I didn't get if the example you showed is supposed to work, because I don't see any external link if I run it locally. If so, the only option seems to be to either run a pre-render script or create a button, right?

[mcanouil]

I didn't get if the example you showed is supposed to work,

It's not. It was simply to show that it's not with a simplet example, easier to read.

If so, the only option seems to be to either run a pre-render script or create a button, right?

Yes.