July 28, 2011 at 11:31 PM by Dr. Drang
The upgrade of my MacBook Air to Lion went gone pretty smoothly, but I had problems with Python,1 some of which were my fault, some of which were Apple’s.
Unsurprisingly, the default version of Python on Lion is from the 2.7 branch:
$ python --version Python 2.7.1
You can, if you wish, run earlier versions instead:
$ python2.6 --version Python 2.6.6 $ python2.5 --version Python 2.5.5
Versions earlier than 2.5 aren’t available. The versions available in Snow Leopard were 2.6.1 and 2.5.4.
Dr. Twoot failed for what turned out to be a variety of reasons. First, Apache wasn’t running, so I guess the Lion updater didn’t maintain the Web Sharing setting.
Dr. Twoot continued to fail after I turned Web Sharing on, apparently because of some setting in Lion’s
/etc/apache/httpd.conf file. I couldn’t access anything under the DocumentRoot, including the necessary CGI script. Fortunately, the Lion updater had preserved my old
http.conf under a different name, and when I changed the names and restarted Apache, I was able to access the DocumentRoot again.
But that didn’t mean Dr. Twoot was working. The CGI script was throwing an error, which was a Python problem. The error was a missing third-party library,
oauth2. This error made sense to me:
- The CGI script is run by the default Python,
- The default Python is version 2.7.
- Version 2.7 looks for third-party libraries in
outh2library can’t be in the 2.7
site-packagesdirectory because I installed it when Python 2.6 was the default.
The long-term solution would be to install
oauth2—and all the other third-party libraries I use—in
/Library/Python/2.7/site-packages, but I wanted to get Dr. Twoot up and running quickly, so I changed the shebang line of the CGI script from
because I knew I had
oauth2 installed the 2.6
When that failed, and the error message still said the
oauth2 library couldn’t be found, I navigated to
/Library/Python/2.6/site-packages to see what was wrong. Here’s what I found:
$ ls README
There were no libraries in
/Library/Python/2.6/site-packages, just a worthless
README. Which means that the Lion updater had overwritten my entire
I’m pretty sure this isn’t kosher. The whole point of a
site-packages directory is to keep system updates and third-party libraries separated and unaffected by each other. I checked my office computer, which is still running Snow Leopard. It has libraries in both
/Library/Python/2.6/site-packages. The former were installed under Leopard (when the default Python was version 2.5) and the latter were installed under Snow Leopard; the Snow Leopard update didn’t blow away all libraries installed under Leopard. Lion, unfortunately, wasn’t as well-behaved.
So I set about installing the missing libraries. At first,
easy_install failed with every library I tried. Eventually, I realized that I was using a mismatched version of
/usr/local/bin instead of the Lion-supplied one in
/usr/bin. I don’t know why there was an
/usr/local/bin, but after getting rid of it, the troubles went away.
Until I tried to install
appscript library, which I use in several scripts to do AppleScript-like things in Python, requires the compilation of some C source. The compilation failed with all sorts of nasty error messages referring to
llvm-gcc-4.2. I knew that Apple had switched from the old
gcc compiler, and wondered whether that was the problem.
Several Python libraries won’t compile with llvm-gcc-4.2. Is there a way to force easy_install/setuptools to use plain ol’ gcc?
Almost as soon as I tweeted this, I realized what the problem was: I’d forgotten to update Xcode after updating to Lion. As I began the download of Xcode from the Mac App Store, I got this confirmation from @mr_lightwolf:
@drdrang If you didn’t reinstall Xcode after upgrading Lion, tons of stuff will break.
Xcode 4.1 took over an hour to download and install, but when it was done
easy_install worked perfectly for all my libraries.
To summarize, if you haven’t updated to Lion yet, and you want to avoid the problems I had, be sure to
- Make a list of all the third-party libraries you currently have installed under Python 2.6, because you’ll have to reinstall all of them for 2.7 and you won’t have the 2.6
site-packagesdirectory to refer to.
- Update to Lion.
- Update to Xcode 4.1. It’s free in the App Store.
- Make sure the
easy_installyou use is the one in
On the non-Python front, if you run a local web server, you might want to put a copy of all your Apache configuration files into your home directory (or Dropbox) before updating to Lion. My configuration files were preserved by the updater, but a belt-and-suspenders approach is safer.
I suppose I’ll go ahead and install Python 3.2 in the next few weeks. None of my workaday scripts use it, but now that it’s 2½ years and 2 point releases old, it’s probably time to get with the program.