This is a technical, nerdy post, mostly so I can find the information if I need it later, but possibly of interest to others using a Mac with the Python programming language, and also since I am looking for excuses to write more here. (See also updates below.)

It seems that there is a bug in the latest (mid-May 2017) release of Apple’s macOS Sierra 10.12.5 (ok, there are plenty of bugs, as there in any sufficiently complex piece of software).

It first manifested itself (to me) as an error when I tried to load the jupyter notebook, a web-based graphical front end to Python (and other languages). When the command is run, it opens up a browser window. However, after updating macOS from 10.12.4 to 10.12.5, the browser didn’t open. Instead, I saw an error message:

0:97: execution error: "http://localhost:8888/tree?token=<removed>" doesn't understand the "open location" message. (-1708)

A little googling found that other people had seen this error, too. I was able to figure out a workaround pretty quickly: this behaviour only happens when I wanted to use the “default” browser, which is set in the “General” tab of the “System Preferences” app on the Mac (I have it set to Apple’s own “Safari” browser, but you can use Firefox or Chrome or something else). Instead, there’s a text file you can edit to explicitly set the browser that you want jupyter to use, located at ~/.jupyter/jupyter_notebook_config.py, by including the line

c.NotebookApp.browser = u'Safari'

(although an unrelated bug in Python means that you can’t currently use “Chrome” in this slot).

But it turns out this isn’t the real problem. I went and looked at the code in jupyter that is run here, and it uses a Python module called webbrowser. Even outside of jupyter, trying to use this module to open the default browser fails, with exactly the same error message (though I’m picking a simpler URL at http://python.org instead of the jupyter-related one above):

>>> import webbrowser
>>> br = webbrowser.get()
>>> br.open("http://python.org")
0:33: execution error: "http://python.org" doesn't understand the "open location" message. (-1708)
False

So I reported this as an error in the Python bug-reporting system, and hoped that someone with more experience would look at it.

But it nagged at me, so I went and looked at the source code for the webbrowser module. There, it turns out that the programmers use a macOS command called “osascript” (which is a command-line interface to Apple’s macOS automation language “AppleScript”) to launch the browser, with a slightly different syntax for the default browser compared to explicitly picking one. Basically, the command is osascript -e 'open location "http://www.python.org/"'. And this fails with exactly the same error message. (The similar code osascript -e 'tell application "Safari" to open location "http://www.python.org/"' which picks a specific browser runs just fine, which is why explicitly setting “Safari” back in the jupyter file works.)

But there is another way to run the exact same AppleScript command. Open the Mac app called “Script Editor”, type open location "http://python.org" into the window, and press the “run” button. From the experience with “osascript”, I expected it to fail, but it didn’t: it runs just fine.

So the bug is very specific, and very obscure: it depends on exactly how the offending command is run, so appears to be a proper bug, and not some sort of security patch from Apple (and it certainly doesn’t appear in the 10.12.5 release notes). I have filed a bug report with Apple, but these are not publicly accessible, and are purported to be something of a black hole, with little feedback from the still-secretive Apple development team.

Updates:

• The bug has been marked as a duplicate, which means I won’t get any more direct information from Apple, but at least they acknowledge that it’s a bug of some sort.
• It appears to have been corrected in the latest beta of macOS Sierra 10.12.6, and so should be fixed in the next official release.
• macOS Sierra 10.12.6 has now been released, so the bug is fixed…