Updates and Errata - Ninth printing

This page is broken into two parts, Updates and Errata. Updates address issues that affect whether your code will run or not. Errata refer to minor issues such as typos, and errors in grayed-out code that probably won’t affect the code you’re entering.

If you find an error in the book or can’t get something to work, please let me know. You can reach me through email at ehmatthes@gmail.com, or on Twitter at @ehmatthes.


Chapter 17

Running hn_submissions.py sometimes results in a KeyError (page 373)

The program hn_submissions.py makes a series of API calls to get information about each of the articles on the front page of Hacker News. When processing the data associated with each article, the code looks for the 'descendants' key, which tells us how many comments the article has associated with it. Hacker News is maintained partially as a promotional tool for the startup accelerator YCombinator, and YC companies can make special posts on Hacker News that are exempt from comments. For example, YC companies can make hiring posts that sit on the front page of HN for a while, with comments disabled.

If you run hn_submissions.py when one of these posts is on the front page, you’ll get a KeyError because there is no 'descendants' key for these posts. This doesn’t happen all that often, but if you run into this issue you can catch the KeyError and continue the loop when one of these posts is present:

-- snip --
for submission_id in submission_ids[:30]:
    -- snip --
    # Build a dictionary for each article.
        submission_dict = {
            'title': response_dict['title'],
            'hn_link': f"http://news.ycombinator.com/item?id={submission_id}",
            'comments': response_dict['descendants'],
    except KeyError:
        # This is a special YC post with comments disabled.
-- snip --


No errata have been reported for the ninth printing at this time.