# Parallel geocoding in Python with geopy

September 10, 2015

Geocoding: the process of taking an address as a piece of semi-structured text, and returning a coordinate somewhere on the surface of the Earth. It’s very unlikely that you’re not going to use an online service to do this. This means a few things. Firstly, that you have to decide on a geocoding provider. Secondly, you’re going to have to sit tight while the geocoding takes place. Python can address both of these issues for you.

## GeoPy

GeoPy is a Python 2/3 module for accessing a whole suite of geocoding web services. As of version 1.10.0, these include: ArcGIS; Baidu; Bing; DataBC; GeocodeFarm; GoecoderDotUS; GeoNames; Google; IGN France; LiveAddress; NaviData; Nominatim (OSM); OpenCage; OpenMapQuest; Yahoo! BOSS Place Finder; What3Words; and Yandex.

Of these, I would recommend Google if you are OK with a reasonably low limit but high tolerance to poorly-formatted addresses, or otherwise Nominatim. Nominatim is free, requires no API token, and only uses OpenStreetMap data. However, OpenMapQuest is even better, as it is also based on OSM, but doesn’t restrict you to geocoding in multiple threads.

## Basic usage

Using GeoPy is great, because it abstracts away all the details of which provider you’re connecting to. Although I’m yet to try it, it would be trivial to iterate over a set of providers and actually produce multiple geocoded locations per address, perhaps as part of a process to determine error, or to average the returned locations.

Here’s a short sample demonstrating the simplicity of the module:

If you want to use OpenMapQuest (for example), you just need to swap it out for Nominatim, and also remove the country bias (which OpenMapQuest doesn’t support).

## Multiprocessing

There’s still a problem, despite all this. Geocoding takes forever. All web services take some time, really. But geocoding is special, given what is actually going on behind the scenes. A list of roughly 500 addresses, most of which needed the recursion I alluded to above, took me about 15 minutes to resolve. Duane Wilkins posted on LinkedIn recently that using the Google geocoding service via Google Sheets resulted in about 800 addresses done in about 5 minutes. I suspect that they were clean, but Google is damn fast at geocoding. You’d expect that relative speed difference though: Nominatim is sitting on donated hardware.

However both of these are very slow, in my opinion. Agonisingly so.

A good Python free lunch to speed this up is to launch mutliple geoprocessing calls asynchronously. In my case, I had a small number of very long and poorly-formatted adddresses that took forever to resolve because the geocoder had to be performed multiple times, most of which resulted in nothing as the string was progressively cleaned, or indeed stripped of information that the geocoder simply could not understand (“5 nm out to sea east of Tauranga” is the best example). What if I could get these slow processes running, and run others alongside, in parallel? This is totally possible! And because Python is batteries-included, it is very, very easy!

In my case, I had a list of objects (sightings) which are members of a class that has a method called geocode that essentially launches the geocoding routine I showed you above. (In my case, the function doesn’t return anything, but has the side-effect of setting each Address object’s latitude and longitude.)

A quick gotcha is that the worker function has to be able to be pickled. In simple terms, you have to have the worker function at the top level. The above snippet would not work if geocode_worker were defined within main. In that case, you’d see this error:

Don’t use Nominatim for this! I made the mistake of not reading the usage restrictions of the geocoding service, and forgetting a debug switch, and then launched tens of requests at the same time, repeatedly, hundreds of times, and then had my IP address blocked. Currently I can’t even visit http://nominatim.openstreetmap.org/, let alone use the geocoder.

The restriction is that you must use a single thread. That is, you must not do what I just showed you using Nominatim as your geocoding service, or you will end-up thrashing their limited resources. You also should really cache the results as they come in.

However, OpenMapQuest uses Nominatim but does not have usage restrictions. You can launch some nice parallel processes with this geocoder… just be respectful.

## Results

I achieved a speed-up of around 20x over the full set. The small number of dirty addresses that earlier caused the geocoder to hang still did so, but real work was going on around them; they were no longer blocking.

Looking at the logs, here’s what the difference in processing looks like for a sample of my addresses. When you see fail, that is when my recursion kicks in that applies an algorithm to clean the address string before trying to geocode again.

Notice the difference? In the latter case, there are multiple processes: 8 addresses get geocoded at once. They don’t return in order because they’re all happening at the same time and take different lengths of time to complete depending on their complexity. Failing addresses don’t prevent others from getting underway, and the whole process is so much smoother.

The blocking geocoder ran in 16 seconds, while the second took only 4 seconds. Extrapolate that over hundreds of addresses, and before you declare that that is still not worth the effort, look how little code I had to write. It pays off even for very small, one-off jobs.

There is much more that can be done with Python’s builtin multiprocessing, particularly using queues. There are a bunch of third-party modules that also implement the idea. I encourage you to explore, perhaps by looking at tutorials for how to write a daemon, and then make something that polls mutliple related APIs at the same time, populating a common database in near real time.

Parallel geocoding in Python with geopy - September 10, 2015 - Richard Law