pattern.web cache access error — IOError: [Errno 13] Permission denied

It appears pattern.web can't write to the directory the cache is kept in. When I try to download text from a URL object, I get a permission error:

Python 2.7.3 (default, Dec 18 2014, 19:10:20) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pattern.web
>>> urlObject=pattern.web.URL("http://google.com")
>>> urlResponse = urlObject.download()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/pattern/web/__init__.py", line 426, in download
    cache[id] = data
  File "/usr/local/lib/python2.7/dist-packages/pattern/web/cache/__init__.py", line 96, in __setitem__
    f = open(self._hash(k), "wb")
IOError: [Errno 13] Permission denied: '/usr/local/lib/python2.7/dist-packages/pattern/web/cache/tmp/c7b920f57e553df2bb68272f61570210'

The permissions on /usr/local/lib/python-2.7/dist-packages/pattern/web/cache, subdir tmp and all files there were rwxr-sr-x, owner root and group "staff".

I had no problem with this when running python with sudo. And changing the owner and group to my user also made the error disappear. However neither of those workarounds seem like a good solution. Why should I need to give my user write access to /usr/local/lib? That seems to violate convention.

Running Ubuntu 12.04, python 2.7.3, pattern version 2.6. I installed pattern via pip, nothing fancy.

As seen on stackoverflow

Start with pattern/web/cache/init.py, especially setitem, and trace its caller in pattern/web/init.py at URL.download. Reproduce the download without sudo and verify that the cache can be used by a normal pip-installed user without changing permissions on /usr/local/lib.