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Advanced, Shareable Cards with Card Templates

The built-in Card Components allow you to create a visual report with a few lines of Python code. This is by far the easiest way to output visualizations using Metaflow’s default visual style and layout.

This section describes a more advanced concept of Card Templates which are more flexible than Default Cards and Card Components, but they require more upfront effort to create. However, using an existing Card Template is very easy, as shown below. They are a good match for use cases such as

  • Using off-the-shelf Javascript libraries to create advanced visualizations.
  • Creating fully customized reports with any visual style and layout.
  • Creating a project-specific card template.
  • Sharing generally useful card templates publicly.

For instance, if your project involves extracting features from video, you can create a card template that shows metadata, frames from the video, and a sample of features in a predefined format. Everyone working on the project can use the same card template to make it easy to catalogue and compare various approaches.

Using a Card Template

A Card Template is a normal Python package, hosted in a Git repository of its own, optionally published to a private or public package repository. By convention, public Card Templates have a metaflow-card prefix, so you can easily find public card templates on PyPi.

Let’s start with a simple starter template, metaflow-card-html, which simply converts HTML stored in an artifact to a static non-updating card.

First, install the template using pip:

pip install metaflow-card-html

Now we can use the card in any flow by adding a decorator, @card(type=’html’). The type attribute refers to the template name. Let’s test it:

from metaflow import FlowSpec, step, card

class HtmlCardFlow(FlowSpec):

def start(self):
self.html = """
<body style='color: blue'>
Hello World!

def end(self):

if __name__ == "__main__":

Note that this is a basic example of a custom template. Other custom templates don't require writing HTML by hand. Save the flow in Then, you can run it

python run

and view the card

python card view start

You should see a blank page with a blue “Hello World!” text.

A particularly useful feature of card templates is that they work in any compute environment, even when executing tasks remotely. For instance, if you have AWS Batch set up, you can run the flow as follows:

python run --with batch

The card will get produced without you having to worry about installing anything on the remote instances! You can deploy flows to production with custom templates too:

python step-functions create

Now, every time a production run executes, cards will get produced exactly as during prototyping. Behind the scenes, Metaflow takes care of packaging any card templates whenever you execute code remotely.

Developing a static Card Template

If you want to develop a card template of your own, it is useful to have a mental model of how cards work under the hood. Let's start with internals of a static, non-updating card:

The blue box is a Metaflow task executing a step from the user’s flow. It is decorated with a @card decorator that has a type attribute referring to your custom template, e.g. mycard. The task executes before the card template. After the task has finished, a new subprocess is started that executes a card template. This ensures that even if the template fails for any reason, it won’t crash the task.

The card template is given the Task ID of the task that the card corresponds to. Using this Task ID, the template can use the Client API to query any information about the task, its parent run, and any past runs. Using this information, the template needs to output a single stand-alone HTML file - the actual card. Note that the HTML file can’t depend on any other local files. In particular, you must include any images as Data URIs in the file itself.

The template itself is a Python class, derived from MetaflowCard, which needs to implement one method, render, which is given a Task object from the Client API - see the MetaflowCard API reference for details.

This is the complete implementation of the @card(type='html') which we used above:

from import MetaflowCard

class HTMLCard(MetaflowCard):

type = 'html'

def __init__(self, options={"artifact":"html"}, **kwargs):
self._attr_nm = options.get("artifact", "html")

def render(self, task):
if self._attr_nm in task:
return str(task[self._attr_nm].data)


The example above used the default self.html artifact to pass HTML code to the template. You can choose another artifact by specifying an artifact name in the options dictionary that is passed to the template: @card(type='html', options={'artifact': 'other_html').

The render method needs to return a self-contained HTML as a string. This template has it easy, since all it has to do is to return the user-defined artifact. Other templates can do much more complex processing to produce a suitable HTML page.

To implement and publish a template of your own, take a look at the metaflow-card-html repository which shows how to structure the package, as well as step-by-step instructions on how to create one of your own. If you create a Card Template that other people might benefit from, let our Slack community know about it!

Developing a dynamic Card Template

Dynamic cards, aka cards that update during task execution, extend the MetaflowCard class presented above with two new methods: render_runtime and refresh.

render_runtime is called periodically during task execution. It is a close cousin of the render method that produces the final card HTML. render_runtime produces a card HTML as well, but it doesn't have access to artifacts produced by the currently executing Task as they are only available upon task completion. Instead, render_runtime produces the HTML based on a data object that is passed to it by the user via the refresh method.

Calling render_runtime to re-render the whole HTML every time e.g. a progress bar updates would be excessive. Instead, small intermediate updates that don't change the page layout are handled by a refresh method in MetaflowCard, which simply converts data passed to it via the task-side refresh method into a JSON object, which is then sent to the card.

To update the card content on the client side, the viewer calls a function metaflow_card_update in Javascript, which is responsible for updating the card's contents (e.g. moving a progress bar) based on the data it receives.

The following schematic illustrates the process:

If you want to develop a dynamic Card Template of your own, you can use metaflow-card-scatter3d as a starter template. See ScatterFlow for an example how to use the custom card. Also don't hesitate to contact Metaflow Slack for advice.

Managing dependencies in Card Templates

Card templates may rely on 3rd party libraries for their functionality, say, to produce advanced visualizations. To make sure the card can be rendered in remote environments that might not have all dependencies already installed, Metaflow takes care of packaging any files included directly in the template itself. However, it can’t handle 3rd party dependencies automatically. Hence, to make sure your template works without friction, you need to pay attention to its dependencies.

Here are recommended strategies for handling 3rd party library dependencies in card templates:

  1. You can rely on Javascript libraries to move functionality to the frontend side. For instance, instead of producing visualizations in Python, you can produce them in Javascript. Take a look at metaflow-card-uplot-timeseries template to see how to use a Javascript library in your template.
  2. You can include small Python libraries in the template package itself, aka vendor them.

If these approaches don’t work, you can instruct users to include the dependencies of the template in their @conda or @pypi libraries. For templates shared privately, you may also rely on dependencies included in a common Docker image.