Why is this OR dependency not working as I expected

Question

I tried to remove python2.6 from my Debian system:

$ sudo apt-get purge python2.6
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be REMOVED:
  fabric* python-nose* python-pip* python2.6*
0 upgraded, 0 newly installed, 4 to remove and 0 not upgraded.
After this operation, 11.0 MB disk space will be freed.
Do you want to continue [Y/n]? n
Abort.

I did not expect fabric to be removed because it has an OR dependency on python2.6 and python2.7 (and I got the latter installed):

$ apt-cache show fabric | grep Depends
Depends: python2.7 | python2.6, python (>= 2.6.6-7~), python (<< 2.8), python-paramiko (>= 1.6), python-pkg-resources, python-nose

Answer 1

Checking the reverse dependencies on the other packages reveals the cause:

$ apt-cache show python-nose | grep Depends
Depends: python-pkg-resources, python2.7, python (>= 2.6.6-7~), python (<< 2.8), python2.6

There is already a bug reported about the issue.