Parsing the Infamous Japanese Postal CSV
The author discusses the difficulties of parsing the Japanese postal code CSV file provided by Japan Post. The file's non-standard formatting, including arbitrary line breaks and parenthetical notes, makes it notoriously difficult for developers to process.
Why it matters
This highlights the real-world friction caused by poor data governance and legacy file formats in public sector digital infrastructure.
Late last year I released posuto , a package presenting Japanese postal code data in an easy-to-use format. It's based on data released by Japan Post , which is infamous for being widely used but hard to parse.
This adorable character by Irasutoya is cute, but the raw postal CSV data is not.
I first became aware of the postal data when I entered my postal code in an online form and it auto-completed my address as "XXX-borough (except the following buildings)". I had no idea what that parenthetical was referring to, so I looked for a common source of postal data, found the CSV, and found the issue. It turns out the CSV file contains parenthetical notes for anyone reading the CSV file and makes reference to the order of the rows.
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