Simon Willison’s Weblog

On cms, https, explorables, mysql, python3, ...


Recent entries

Datasette Facets one month ago

Datasette 0.22 is out with the most significant new feature I’ve added since the initial release: faceted browse.

Datasette lets you deploy an instant web UI and JSON API for any SQLite database. csvs-to-sqlite makes it easy to create a SQLite database out of any collection of CSV files. Datasette Publish is a web app that can run these combined tools against CSV files you upload from your browser. And now the new Datasette Facets feature lets you explore any CSV file using faceted navigation with a couple of clicks.

Exploring characters from Marvel comics

Let’s use facets to explore every character in the Marvel Universe.

FiveThirtyEight have published a CSV file of 16,376 characters from Marvel comics, scraped from Wikia as part of the research for their 2014 story Comic Books Are Still Made By Men, For Men And About Men.

Here’s that CSV file loaded into the latest version of Datasette:

Marvel characters explored using Datasette Facets

We start by applying the identity status, alignment and gender facets. Then we filter down to just the bad characters with a public identity, and apply the eye colour facet. Now we can filter to just the 20 bad characters with a public identity and yellow eyes.

At each stage along the way we could see numerical summaries of the other facets. That’s a pretty sophisticated piece of analysis we’ve been able to run with just a few clicks (and it works responsively on mobile as well).

I’ve published a full copy of everything else in the FiveThirtyEight data repository, which means you can find plenty more examples of facets in action at—one example: Actions under the Antiquities Act, faceted by states, pres_or_congress, action and current_agency.

Analyzing GSA IT Standards with Datasette Publish

The US government’s General Services Administration have a GitHub account, and they use it to publish a repository of assorted data as CSVs.

Let’s take one of those CSVS and analyze it with Datasette Facets, using the Datasette Publish web app to upload and process the CSV.

We’ll start with the it-standards.csv file, downloaded from their repo. We’ll upload it to Datasette Publish and add some associated metadata:

GSA IT standards published using Datasette Publish

Here’s the result, with the Status and Deployment Type facets applied. And here’s a query showing just SaaS tools with status Approved—Preferred.

European Power Stations

The Open Power System Data project publishes data about electricity systems. They publish data in a number of formats, including SQLite databases. Let’s take their conventional_power_plants.sqlite file and explore it with Datasette. With Datasette installed, run the following commands in your terminal:

datasette conventional_power_plants.sqlite

This will start Datasette running at ready for you to explore the data.

Next we can publish the SQLite database directly to the internet using the datasette publish command-line tool:

$ datasette publish now conventional_power_plants.sqlite \
    --source="Open Power System Data. 2018. Data Package Conventional power plants. Version 2018-02-27" \
    --source_url="" \
    --title="Conventional power plants" \
> Deploying /private/var/folders/jj/fngnv0810tn2lt_kd3911pdc0000gp/T/tmpufvxrzgp/datasette under simonw
> [in clipboard] (sfo1) [11s]
> Synced 3 files (1.28MB) [11s]
> Building…
> ▲ docker build
Sending build context to Docker daemon 1.343 MBkB
> Step 1/7 : FROM python:3
> 3: Pulling from library/python
> 3d77ce4481b1: Already exists
> 534514c83d69: Already exists
> Successfully built da7ac223e8aa
> Successfully tagged
> ▲ Storing image
> Build completed
> Verifying instantiation in sfo1
> [0] Serve! files=('conventional_power_plants.sqlite',) on port 8001
> [0] [2018-05-20 22:51:51 +0000] [1] [INFO] Goin' Fast @
> [0] [2018-05-20 22:51:51 +0000] [1] [INFO] Starting worker [1]

Finally, let’s give it a nicer URL using now alias:

 now alias

The result can now be seen at

Here’s every conventional power plant in Europe faceted by country, energy source and technology.

Power Plant Facets

Implementation notes

I love faceted search engines. One of my first approaches to understanding any new large dataset has long been to throw it into a faceted search engine and see what comes out. In the past I’ve built them using Solr, Elasticsearch, PostgreSQL and even Whoosh. I guess it was inevitable that I’d try to build one with SQLite.

You can follow the development of Datasette Facets in the now-closed issue #255 on GitHub.

Facets are requested by appending one or more ?_facet=colname parameters to the URL. This causes Datasette to run the following SQL query for each of those specified columns:

select colname as value, count(*) as count
from tablename where (current where clauses)
group by colname order by count desc limit 31

For large tables, this could get expensive. Datasette supports time limits for SQLite queries, and facets are given up to 200ms (by default, this limit can be customized) to finish executing. If the query doesn’t complete in the given time the user sees a warning that the facet could not be displayed.

We ask for 31 values in the limit clause even though we only display 30. This lets us detect if there are more values available and show a ... indicator to let the user know that the facets were truncated.

Datasette also suggests facets that you might want to apply. This is implemented using another query, this time run against every column that is not yet being used as a facet. If a table has 20 columns this means 20 queries, so they run with an even tighter 50ms time limit. The query looks like this:

select distinct colname
from tablename where (current where clauses)
limit 31

All we are doing here is trying to determine if the column in question has less than 30 unique values. The limit clause here means that if you run this query against a column with entirely distinct values (the primary key for example) the query will terminate extremely quickly—after it has found just the first 31 values.

Once the query has executed, we count the distinct values and check to see if this column, when used as a facet:

  • Will return 30 or less unique options
  • Will return more than one unique option
  • Will return less unique options than the current total number of filtered rows

If the query takes longer than 50ms we terminate it and do not suggest that column as a potential facet.

Facets via JSON

As with everything in Datasette, the facets you can view in your browser are also available as part of the JSON API (which ships with CORS headers so you can easily fetch data from JavaScript running in a browser on any web page).

To get back JSON, add .json to the path (before the first ?). Here’s that power plants example returned as JSON:

Please let me know if you build something interesting with Datasette Facets!

Notes from my appearance on the Changelog podcast one month ago

After I spoke at Zeit Day SF last weekend I sat down with Adam Stacoviak to record a 25 minute segment for episode 296 of the Changelog podcast, talking about Datasette. We covered a lot of ground!

A transcript should be available soon (the Changelog have excellent transcripts) but in the meantime here are my own notes on the topics that we covered.

My section starts 54:45 minutes into the episode.

From the intro

Datasette was inspired by my work at the Guardian with Simon Rogers, where we started the The Guardian Data Blog.

Here’s Datasette on GitHub, and Datasette: instantly create and publish an API for your SQLite databases where I first announced the project.

My website, which is powered by this Datasette API using data from the San Francisco Department of Public Works.

The hosting platform I use for immutable deployments of Docker containers is Zeit Now.

The Datasette tool suite

Here’s the full Datasette suite of tools:

  • csvs-to-sqlite is a command-line tool for converting CSV files into a SQLite database.
  • Datasette is a web application for exploring SQLite databases and interacting with them via a read-only JSON API. It also includes the datasette publish command-line tool for publishing those databases using Zeit Now or Heroku.
  • Datasette Publish is a web app that lets you upload CSV files and deploy them to a new Datasette instance without installing any software. Here’s my announcement post.

We talked a lot about SQLite. Here’s a document describing How SQLite is tested and a bunch more about SQLite from my blog.

Datasette examples

A few more fun examples of datasettes that we discussed:

Tools for manipulating data

I’m a huge fan of the combination of Jupyter notebooks and Pandas. Here’s a collection of notes on jupyter and pandas from my blog.

Contributing to Datasette

The best thing you can do is use Datasette to publish your data and then give me feedback!

If you want to get involved in the code, take a look at the help wanted label on GitHub or have a look at the open issues for csvs-to-sqlite.

Even better: write plugins! Datasette’s plugin architecture is just getting started so the more experience and feedback we can get with it the better.

And finally… I mentioned US Navy Airship and Zeppelin photos. I also have a Pinterest Board.

Exploring the UK Register of Members Interests with SQL and Datasette one month ago

Ever wondered which UK Members of Parliament get gifted the most helicopter rides? How about which MPs have been given Christmas hampers by the Sultan of Brunei? (David Cameron, William Hague and Michael Howard apparently). Here’s how to dig through the Register of Members Interests using SQL and Datasette.

Gifts from the Sultan

mySociety have been building incredible civic participation applications like TheyWorkForYou and FixMyStreet for nearly 15 years now, and have accumulated all kinds of interesting data along the way.

They recently launched their own data portal at listing all of the information they have available. While exploring it I stumbled across their copy of the UK Register of Members Interests. Every UK Member of Parliament has to register their conflicts of interest and income sources, and mySociety have an ongoing project to parse that data into a more useful format.

It won’t surprise you to hear that I couldn’t resist turning their XML files into a SQLite database.

The result is—a Datasette instance running against a SQLite database containing over 1.3 million line-items registered by 1,419 MPs over the course of 18 years.

Some fun queries

A few of my favourites so far:

Understanding the data model

Most of the action takes place in the items table, where each item is a line-item from an MP’s filing. You can search that table by keyword (see helicopter example above) or apply filters to it using the standard Datasette interface. You can also execute your own SQL directly against the database.

Each item is filed against a category. There appears to have been quite a bit of churn in the way that the categories are defined over the years, plus the data is pretty untidy—there are no less than 10 ways of spelling “Remunerated employment, office, profession etc.” for example!


There are also a LOT of duplicate items in the set—it appears that MPs frequently list the same item (a rental property for example) every time they fill out the register. SQL DISTINCT clauses can help filter through these, as seen in some of the above examples.

The data also has the concepts of both members and people. As far as I can tell people are distinct, but members may contain duplicates—presumably to represent MPs who have served more than one term in office. It looks like the member field stopped being populated in March 2015 so analysis is best performed against the people table.

Once concept I have introduced myself is the record_id. In the XML documents the items are often grouped together into a related collection, like this:

<regmem personid=""
    memberid="" membername="Diane Abbott" date="2014-07-14">
    <category type="2" name="Remunerated employment, office, profession etc">
        <item>Payments from MRL Public Sector Consultants, Pepple House, 8 Broad Street, Great Cambourne, Cambridge CB23 6HJ:</item>
        <item>26 November 2013, I received a fee of £1,000 for speaking at the 1st African Legislative Summit, National Assembly, Abuja, Nigeria.  Hours: 8 hrs. The cost of my flights, transfers and hotel accommodation in Abuja were also met; estimated value £5,000. <em>(Registered 3 December 2013)</em></item>
        <item>23 July 2013, I received a fee of £5,000 for appearing as a contestant on ITV&#8217;s &#8216;The Chase Celebrity &#8211; Series 3&#8217; television programme.  Address of payer:  ITV Studios Ltd, London Television Centre, Upper Ground, London SE1 9Lt.  Hours: 12 hrs.   <em>(Registered 23 July 2013)</em></item>

While these items are presented as separate line items, their grouping carries meaning: the first line item here acts as a kind of heading to help provide context to the other items.

To model this in the simplest way possible, I’ve attempted to preserve the order of these groups using a pair of additional columns: the record_id and the sort_order. I construct the record_id using a collection of other fields—the idea is for it to be sortable, and for each line-item in the same grouping to have the same record_id:

record_id = "{date}-{category_id}-{person_id}-{record}".format(

The resulting record_id might look like this: 2018-04-16-70b64e89-24878-0

To recreate that particular sequence of line-items, you can search for all items matching that record_id and then sort them by their sort_order. Here’s that record from Diane Abbott shown with its surrounding context.

A single record

How I built it

The short version: I downloaded all of the XML files and wrote a Python script which parsed them using ElementTree and inserted them into a SQLite database. I’ve put the code on GitHub.

A couple of fun tricks: firstly, I borrowed some code from csvs-to-sqlite to create the full-text search index and enable searching:

def create_and_populate_fts(conn):
    create_sql = """
        CREATE VIRTUAL TABLE "items_fts"
        USING {fts_version} (item, person_name, content="items")
        INSERT INTO "items_fts" (rowid, item, person_name)
        SELECT items.rowid, items.item,
        FROM items LEFT JOIN people ON items.person_id =

The best_fts_version() function implements basic feature detection against SQLite by trying operations in an in-memory database.

Secondly, I ended up writing my own tiny utility function for inserting records into SQLite. SQLite has useful INSERT OR REPLACE INTO syntax which allows you to insert a record and will automatically update an existing record if there is a match on the primary key. This meant I could write this utility function and use it for all of my data inserts:

def insert_or_replace(conn, table, record):
    pairs = record.items()
    columns = [p[0] for p in pairs]
    params = [p[1] for p in pairs]
    sql = "INSERT OR REPLACE INTO {table} ({column_list}) VALUES ({value_list});".format(
        column_list=", ".join(columns),
        value_list=", ".join(["?" for p in params]),
    conn.execute(sql, params)

# ...

        "id": person_id,
        "name": regmem_el.attrib["membername"],

What can you find?

I’ve really only scratched the surface of what’s in here with my initial queries. What can you find? Send me Datasette query links on Twitter with your discoveries!

Datasette plugins, and building a clustered map visualization two months ago

Datasette now supports plugins!

Last Saturday I asked Twitter for examples of Python projects with successful plugin ecosystems. pytest was the clear winner: the pytest plugin compatibility table (an ingenious innovation that I would love to eventually copy for Datasette) lists 457 plugins, and even the core pytest system itself is built as a collection of default plugins that can be replaced or over-ridden.

Best of all: pytest’s plugin mechanism is available as a separate package: pluggy. And pluggy was exactly what I needed for Datasette.

You can follow the ongoing development of the feature in issue #14. This morning I released Datasette 0.20 with support for a number of different plugin hooks: plugins can add custom template tags and SQL functions, and can also bundle their own static assets, JavaScript, CSS and templates. The hooks are described in some detail in the Datasette Plugins documentation.


I also released my first plugin: datasette-cluster-map. Once installed, it looks out for database tables that have a latitude and longitude column. When it finds them, it draws all of the points on an interactive map using Leaflet and Leaflet.markercluster.

Let’s try it out on some polar bears!

Polar Bears on a cluster map

The USGS Alaska Science Center have released a delightful set of data entitled Sensor and Location data from Ear Tag PTTs Deployed on Polar Bears in the Southern Beaufort Sea 2009 to 2011. It’s a collection of CSV files, which means it’s trivial to convert it to SQLite using my csvs-to-sqlite tool.

Having created the SQLite database, we can deploy it to a hosting account on Zeit Now alongside the new plugin like this:

# Make sure we have the latest datasette
pip3 install datasette --upgrade
# Deploy polar-bears.db to now with an increased default page_size
datasette publish now \
    --install=datasette-cluster-map \
    --extra-options "--page_size=500" \

The --install option is new in Datasette 0.20 (it works for datasette publish heroku as well)—it tells the publishing provider to pip install the specified package. You can use it more than once to install multiple plugins, and it accepts a path to a zip file in addition to the name of a PyPI package.

Explore the full demo at

Visualize any query on a map

Since the plugin inserts itself at the top of any Datasette table view with latitude and longitude columns, there are all sorts of neat tricks you can do with it.

I also loaded the San Francisco tree list (thanks, Department of Public Works) into the demo. Impressively, you can click “load all” on this page and Leaflet.markercluster will load in all 189,144 points and display them on the same map… and it works fine on my laptop and my phone. Computers in 2018 are pretty good!

But since it’s a Datasette table, we can filter it. Here’s a map of every New Zealand Xmas Tree in San Francisco (8,683 points). Here’s every tree where the Caretaker is Friends of the Urban Forest. Here’s every palm tree planted in 1990:

Palm trees planted in 1990

Update: This is an incorrect example: there are 21 matches on “palm avenue” because the FTS search index covers the address field—they’re not actually palm trees. Here’s a corrected query for palm trees planted in 1990.

The plugin currently only works against columns called latitude and longitude… but if your columns are called something else, don’t worry: you can craft a custom SQL query that aliases your columns and everything will work as intended. Here’s an example against some more polar bear data:

select *, "Capture Latitude" as latitude, "Capture Longitude" as longitude
from [USGS_WC_eartag_deployments_2009-2011]

Writing your own plugins

I’m really excited to see what people invent. If you want to have a go, your first stop should be the Plugins documentation. If you want an example of a simple plugin (including the all-important mechanism for packaging it up using take a look at datasette-cluster-map on GitHub.

And if you have any thoughts, ideas or suggestions on how the plugin mechanism can be further employed please join the conversation on issue #14. I’ve literally just got started with Datasette’s plugin hooks, and I’m very keen to hear about things people want to build that aren’t yet supported.


22nd June 2018

  • The Four Golden Signals. “The four golden signals of monitoring are latency, traffic, errors, and saturation. If you can only measure four metrics of your user-facing system, focus on these four.”—from the excellent (and free) Google Site Reliability Engineering book. #
  • lemongraph. An open-source “log-based transactional graph engine”. Written by the NSA. In Python. It runs on top of LMDB, which is the fast memory-mapped transactional key-value store that was developed by the OpenLDAP project as a replacement for BerkeleyDB. #

20th June 2018

  • MySQL High Availability at GitHub. Cutting edge high availability case-study: GitHub are now using Consul, raft, their own custom load balancer and their own custom orchestrator replication management toolkit to achieve cross-datacenter failover for their MySQL master/replica clusters. #
  • Notebook: How to build a Teachable Machine with TensorFlow.js (via) This is a really cool Observable notebook. It explains how to build image classification that runs in the browser on top of Tensorflow.js, and includes interactive demos that hook into your webcam and let you hold up items and use them to train a classifier. Since it’s built on Observable every single underlying line of source code is available to browse as part of the essay. #

19th June 2018

  • Sunsetting React Native at Airbnb. “Due to a variety of technical and organizational issues, we will be sunsetting React Native and putting all of our efforts into making native amazing.” Fascinating write-up from Airbnb (part of a series) based on two years of working with React Native. It’s worth reading this in full: 63% of their engineers they surveyed would have chosen React Native again given the chance and 74% would consider it for a new project—but the larger technical and organizational challenges (in particular the fact that React Native remains a polarizing choice in the mobile world, making it harder to hire great native engineers) mean that Airbnb are migrating back to pure-native for their iOS and Android apps. #
  • github/gh-ost: Thoughts on Foreign Keys? The biggest challenge I’ve seen with foreign key constraints at scale (at least with MySQL) is how they conflict with online schema migrations using tools like pt-online-schema-change or GitHub’s gh-ost. This is a good explanation of the issue by Shlomi Noach, one of the gh-ost maintainers. #

18th June 2018

  • Datasette 0.23: CSV, SpatiaLite and more (via) The big new feature in 0.23 is CSV export: any Datasette table or query can now be exported as CSV, including the option to get all matching rows in one giant CSV file taking advantage of Python 3 async and Datasette’s efficient keyset pagination. Also in this release: improved support for SpatiaLite and various JSON API improvements including the ability to expand foreign key labels in JSON and CSV responses. #

16th June 2018

  • Django Bakery (via) “A set of helpers for baking your Django site out as flat files”. Released by the LA Times Data Desk, who use it for a large number of projects from election results to data journalism interactives. Statically publishing these projects to S3 lets them handle huge traffic spikes at a very low cost. #

15th June 2018

  • Raccoons don’t think ahead very much, so raccoons don’t have very good impulse control. I don’t think the raccoon realized when it started climbing what it was in for.

    Suzanne MacDonald, raccoon behavior expert #

14th June 2018

  • One of the ways the internet has changed around us over the years is the blog-o-sphere of MetaFilter’s early years has all but disappeared, and so has the kind of link-sharing culture that went with it.

    Josh Millard #

  • Metafilter financial update and future directions. Recent drops in revenue from Google AdSense and Amazon Affiliates have left MetaFilter (19th birthday coming up next month) with a $8,000/month shortfall. They have an optional monthly subscription which currently brings in $7,500/month (monthly expenses are $38,000) so I’ve opted in and thankfully it looks like a lot of other people are subscribing or upping their subscription. I joined the site nearly 14 years ago and it’s been an important part of my online world ever since. #

13th June 2018

  • Changelog 2018-06-12 / Observable. The ability to download an Observable notebook as a stand-alone ES module and run it anywhere using their open source runtime is fascinating, but it’s also worth reading the changelog for some of the new clever tricks they are pulling using await—“await visibility();” in a notebook cell will cause execution to pause until the cell scrolls into view for example. #

12th June 2018

11th June 2018

  • mycli. Really neat auto-complete enabled MySQL terminal client, built using the excellent python-prompt-toolkit. Has a sister-project for PostgreSQL called pgcli. #