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When Scraping Goes Off The Rails

This post was originally published by Ian Watt on 10ml.com

The art of scraping websites is one beset by difficulties, as I was reminded this week when re-testing a scraper that I built recently.

Schienenbruch

 

Railway performance

As part of my participation in 100 Days of Code I’ve been working on a few projects.

The first one that I tackled was a scraper to gather data from the PDF performance reports which are published on a four-weekly cycle Scotrail’s website. On the face of it this is a straightforward things to do.

  1. Find the link to the latest PDF on the performance page using the label “Download Monthly Performance Results”.
  2. Grab that PDF to archive it. (Scotrail don’t do that – they vanish each one and replace it with a new one every four weeks, so there is no archive).
  3. Use a service such as PDFTables which has an API, uploading the PDF and getting a CSV file in return (XSLX and XML versions are also available but less useful in this project).
  4. Parse the CSV file and extract a number of values, including headline figures, and four monthly measures for each of the 73 stations in Scotland.
  5. Store those values somewhere. I decided on clean monthly CSV output files as a failsafe, and a relational SQLite database as an additional, better solution.

Creating the scraper

So, I built the bones of the scraper in a few hours over the first couple of days of the year. I tested it on the then current PDF which was for period nine of 2016-17. That worked, first creating the clean CSV, then later adding the DB-write routines.

Boom – number 1

I then remembered that I had downloaded the previous period’s PDF. So I modified the code (to omit the downloading routine) and ran it to test the scraping routine on it – and it blew up my code. The format of the table structure in the PDF had changed with an extra blank link to the right of the first list of station names.

After creating a new version and publishing that, I sat back and waited for the publication of period 10 data. That was published in the middle of this week.

Boom – number 2

I re-ran the scraper to add that new PDF to my database – and guess what? It blew up the scraper again. What had happened? Scotrail had changed the structure of the filename of the PDF – from using dashes (as in ‘performance-display-p1617-09.pdf’) to underscores (‘performance_display_p1617_10.pdf’)

That change meant that my routine for sicking out the year and period, which is used to identify database records, broke. So I had to rewrite it. Not a major hassle – but it means that each new publication has necessitated a tweaking of the code. Hopefully in time the code will be flexible enough to accommodate minor deviations from what is expected without manual changes. We’ll see.

We’re ‘doing the wrong thing righter’ – Drucker

Of course, none of this should be necessary.

In a perfect world Scotrail would publish well structured, machine-readable open data for performance. I did email them on 26th November 2016, long before I started the scraper, both asking for past periods’ data and asking if they wanted assistance in creating Open Data. I got a customer service reply on 7th December saying that a manager would be in touch. To date (15 Jan 2017) I’ve had no further response.

The right thing

Abelio operates the Scotrail franchise under contract to the Scottish Government.

Should the terms of such contracts not put an obligation on the companies not only to put the monthly data into the public domain, but also that it be made available as good open data – and follow the Scottish Government’s on strategy for Open Data ? Extending the government’s open data obligation to those performing contracts for governments would be a welcome step forward for Scotland.

Posted in ODIAberdeen and tagged , , , .