Simon Willison’s Weblog

json-flatten, now with format documentation. json-flatten is a fun little Python library I put together a few years ago for converting JSON data into a flat key-value format, suitable for inclusion in an HTML form or query string. It lets you take a structure like this one:

{"foo": {"bar": [1, True, None]}

And convert it into key-value pairs like this:

foo.bar.[0]$int=1
foo.bar.[1]$bool=True
foo.bar.[2]$none=None

The flatten(dictionary) function function converts to that format, and unflatten(dictionary) converts back again.

I was considering the library for a project today and realized that the 0.3 README was a little thin - it showed how to use the library but didn't provide full details of the format it used.

On a hunch, I decided to see if files-to-prompt plus LLM plus Claude 3.5 Sonnet could write that documentation for me. I ran this command:

files-to-prompt *.py | llm -m claude-3.5-sonnet --system 'write detailed documentation in markdown describing the format used to represent JSON and nested JSON as key/value pairs, include a table as well'

That *.py picked up both json_flatten.py and test_json_flatten.py - I figured the test file had enough examples in that it should act as a good source of information for the documentation.

This worked really well! You can see the first draft it produced here.

It included before and after examples in the documentation. I didn't fully trust these to be accurate, so I gave it this follow-up prompt:

llm -c "Rewrite that document to use the Python cog library to generate the examples"

I'm a big fan of Cog for maintaining examples in READMEs that are generated by code. Cog has been around for a couple of decades now so it was a safe bet that Claude would know about it.

This almost worked - it produced valid Cog syntax like the following:

[[[cog
example = {
"fruits": ["apple", "banana", "cherry"]
}

cog.out("```json\n")
cog.out(str(example))
cog.out("\n```\n")
cog.out("Flattened:\n```\n")
for key, value in flatten(example).items():
    cog.out(f"{key}: {value}\n")
cog.out("```\n")
]]]
[[[end]]]

But that wasn't entirely right, because it forgot to include the Markdown comments that would hide the Cog syntax, which should have looked like this:

<!-- [[[cog -->
...
<!-- ]]] -->
...
<!-- [[[end]]] -->

I could have prompted it to correct itself, but at this point I decided to take over and edit the rest of the documentation by hand.

The end result was documentation that I'm really happy with, and that I probably wouldn't have bothered to write if Claude hadn't got me started.