Why is _repr_mimebundle_ metadata currently ignored?

[daizutabi]

I noticed that marimo currently discards the optional metadata returned by the _repr_mimebundle_ method, which is also noted in the documentation:

Note: marimo currently does not handle any optional metadata returned by _repr_mimebundle_.

(Related PR #6165)

I'm curious about the reasoning behind this design choice. Implementing metadata preservation seems straightforward and would improve compatibility with libraries that rely on the Jupyter metadata spec to provide information like image dimensions.

In marimo/_output/formatters/repr_formatters.py, the metadata dict is unpacked into an unused variable:

if isinstance(contents, tuple) and len(contents) == 2:
    contents, _metadata = cast(
        tuple[dict[str, Any], dict[str, Any]], contents
    )
    # _metadata is currently discarded

We can preserve this information by merging it into the contents dictionary using METADATA_KEY:

if isinstance(contents, tuple) and len(contents) == 2:
    contents, metadata = cast(
        tuple[dict[str, Any], dict[str, Any]], contents
    )
    # preserve metadata for the frontend
    if metadata:
        contents[METADATA_KEY] = metadata

Is there an architectural, security, or performance-related reason why this metadata is intentionally omitted?

[manzt]

Implementing metadata preservation seems straightforward and would improve compatibility with libraries that rely on the Jupyter metadata spec to provide information like image dimensions.

Would you be able to provide a specific example?

[daizutabi]

Thank you for your comment!

Users can define a class like this:

import requests

class PngImage:
    def __init__(self, url, size=None):
        response = requests.get(url)
        self.content = response.content
        self.size = size

    def _repr_mimebundle_(self):
        data = {"image/png": self.content}
        if not self.size:
            return data
        metadata = {"image/png": {"width": self.size, "height": self.size}}
        return data, metadata

The current implementation ignores metadata:

With metadata support, it will look like this:

[manzt]

Awesome. Thank you for the context. I was only familiar with metadata for colab and anywidgets. We should support this. Would you be interested in contributing a PR?

[daizutabi]

Thanks! I'll create a PR for this.