Every so often, one of our Bamboo builds would break thus:

pkg_resources.ExtractionError: Can't extract file(s) to egg cache

The following error occurred while trying to extract file(s) to the Python egg
cache:

  [Errno 17] File exists: '/home/bamboo/.python-eggs'

The Python egg cache directory is currently set to:

  /home/bamboo/.python-eggs

Perhaps your account does not have write access to this directory?  You can
change the cache directory by setting the PYTHON_EGG_CACHE environment
variable to point to an accessible directory.

This occurred while trying to make use of PyCrypto.

After a little research, I decided that instead of installing PyCrypto as a zipped egg (as it does by default) into the build’s virtual en­vi­ron­ment, to instead force it to unzip itself: easy_in­stall --always-unzip pycrypto. This seems to have fixed the problem, as pkg_re­sources no longer needs to unpack anything. Pillow was also in the Egg Cache and probably needs similar treatment.