Custom Jinja template tags with attributes

I decided to implement a custom Jinja template block tag for my datasette-render-markdown plugin. I wanted the tag to work like this:

{% markdown %}
# This will be rendered as markdown

- Bulleted
- List
{% endmarkdown %}

A basic Jinja extension

After some fiddling around with GitHub Code Search and ChatGPT I settled on this as the simplest possible skeleton for a custom Jinja tag:

from jinja2 import nodes
from jinja2.exceptions import TemplateSyntaxError
from jinja2.ext import Extension

class MarkdownExtension(Extension):
    tags = set(["markdown"])

    def __init__(self, environment):
        super(MarkdownExtension, self).__init__(environment)

    def parse(self, parser):
        # We need this for reporting errors
        lineno = next(
        body = parser.parse_statements(
            ["name:endmarkdown"], drop_needle=True
        return nodes.CallBlock(

    async def _render_markdown(self, caller):
        return render_markdown(await caller())

Then add it to the Jinja environment like this:


Note that my _render_makdown() method there is async def. This appeared to be necessary because I run Jinja for Datasette in async mode. If I didn't I think this would work like this instead:

    def _render_markdown(self, caller):
        return render_markdown(caller())

I'm not sure of the best way to build an extension that works in both async and regular modes.

Adding support for attributes

My render_markdown() function takes optional arguments for specifying if certain Markdown extensions should be enabled, or which additional tags and attributes should be allowed rather than being stripped by Bleach.

I decided to use the following syntax for that:

{% markdown
  extra_tags="table thead tr th td tbody" 
  extra_attrs="p:id,class a:name,href" %}
## Markdown table

First Header  | Second Header
------------- | -------------
Content Cell  | Content Cell
Content Cell  | Content Cell

<a href="" name="namehere">Example</a>
<p id="paragraph" class="klass">Paragraph</p>
{% endmarkdown %}

Adding key="value" attribute support to a custom Jinja tag was trickier than I expected!

You have to work directly with the parser.

After spending some time in the Python debugger, I figured out that the tokens in my test document looked something like this:

[Token(lineno=6, type='name', value='markdown'),
 Token(lineno=6, type='name', value='foo'),
 Token(lineno=6, type='assign', value='='),
 Token(lineno=6, type='string', value='bar'),
 Token(lineno=6, type='name', value='baz'),
 Token(lineno=6, type='assign', value='='),
 Token(lineno=6, type='string', value='bar2'),
 Token(lineno=6, type='block_end', value='%}'),
 Token(lineno=6, type='data', value='\n# This is markdown'),
 Token(lineno=11, type='block_begin', value='{%'),
 Token(lineno=11, type='name', value='endmarkdown'),
 Token(lineno=11, type='block_end', value='%}'),
 Token(lineno=11, type='data', value='\n')]

To turn key="value" syntax into a dictionary of attributes, I would need to read every token up to the block_end ("%}") token, then look for sequences of three tokens - a name, an assign (=) and a string.

I ended up writing this code to do that:

    def parse(self, parser):
        # We need this for reporting errors
        lineno = next(

        # Gather tokens up to the next block_end ('%}')
        gathered = []
        while != "block_end":

        # If all has gone well, we will have a sequence of triples of tokens:
        #   (type='name, value='attribute name'),
        #   (type='assign', value='='),
        #   (type='string', value='attribute value')
        # Anything else is a parse error

        if len(gathered) % 3 != 0:
            raise TemplateSyntaxError("Invalid syntax for markdown tag", lineno)
        attrs = {}
        for i in range(0, len(gathered), 3):
            if (
                gathered[i].type != "name"
                or gathered[i + 1].type != "assign"
                or gathered[i + 2].type != "string"
                raise TemplateSyntaxError(
                        "Invalid syntax for markdown attribute - got "
                        "'{}', should be name=\"value\"".format(
                            "".join([str(t.value) for t in gathered[i : i + 3]]),
            attrs[gathered[i].value] = gathered[i + 2].value

This did the trick! At the end of that block, attrs is a dictionary of all of the key="value" attributes that were included in that open tag.

Validating the attributes

For my particular template tag, I only wanted three optional attributes to be supported. I added some code to validate them (and handle their slightly weird custom syntax):

        # Validate the attributes
        kwargs = {}
        for attr, value in attrs.items():
            if attr in ("extensions", "extra_tags"):
                kwargs[attr] = value.split()
            elif attr == "extra_attrs":
                # Custom syntax: tag:attr1,attr2 tag2:attr3,attr4
                extra_attrs = {}
                for tag_attrs in value.split():
                    tag, attrs = tag_attrs.split(":")
                    extra_attrs[tag] = attrs.split(",")
                kwargs["extra_attrs"] = extra_attrs
                raise TemplateSyntaxError("Unknown attribute '{}'".format(attr), lineno)

Raising TemplateSyntaxError is a clean way to report errors in Jinja - and you pass the current template lineno to that exception to ensure it is reported back to the user.

Passing attributes to the render method

At the end of this block I had kwargs, ready to be passed to my own render_template(value, **kwargs) function.

But there was one last problem: I needed to call this code:

return nodes.CallBlock(

While passing through the kwargs I had collected to that _render_markdown() method.

I eventually found a pattern that worked, but it's kind of gross:

        body = parser.parse_statements(["name:endmarkdown"], drop_needle=True)

        return nodes.CallBlock(
            # I couldn't figure out how to send attrs to the _render_markdown
            # method other than json.dumps and then passing as a nodes.Const
            self.call_method("_render_markdown", [nodes.Const(json.dumps(kwargs))]),

    async def _render_markdown(self, kwargs_json, caller):
        kwargs = json.loads(kwargs_json)
        return render_markdown(await caller(), **kwargs)

I'm serializing the kwargs dictionary to a JSON string, then wrapping that in nodes.Const(). I can then pass that as a list to the .call_method() method call.

Anything passed in that list becomes available to that _render_markdown() method as a positional argument - so I can take kwargs and json.loads() it to get the data back.

I don't know why I had to do it this way, and I'd be delighted to find a cleaner mechanism for this - but it does work.

The finished code

You can see the finished code here in the datasette-render-markdown GitHub repository.


Created 2023-07-01T18:08:36-07:00, updated 2023-07-01T19:47:56-07:00 · History · Edit