package with no "Python-Version" can be half-installed but not removed

[Michael Vogt]

Binary package hint: python-central

A package with no "Python-Version" field causes pycentral to be in a inrecoverable state.

The half-installed package can not be removed:
$ sudo apt-get install python-ropemacs
The following NEW packages will be installed:
  python-ropemacs
0 upgraded, 1 newly installed, 0 to remove and 819 not upgraded.
Need to get 24,3kB of archives.
After this operation, 160kB of additional disk space will be used.
Get:1 http://archive.ubuntu.com jaunty/universe python-ropemacs 0.6c2-3 [24,3kB]
Fetched 24,3kB in 0s (65,3kB/s)
Selecting previously deselected package python-ropemacs.
(Reading database ... 524815 files and directories currently installed.)
Unpacking python-ropemacs (from .../python-ropemacs_0.6c2-3_all.deb) ...
Setting up python-ropemacs (0.6c2-3) ...
Traceback (most recent call last):
  File "/usr/bin/pycentral", line 2096, in <module>
    main()
  File "/usr/bin/pycentral", line 2090, in main
    rv = action.run(global_options)
  File "/usr/bin/pycentral", line 1386, in run
    pkg.read_version_info()
  File "/usr/bin/pycentral", line 804, in read_version_info
    raise PyCentralError, "package has no field Python-Version"
__main__.PyCentralError: package has no field Python-Version
dpkg: error processing python-ropemacs (--configure):
 subprocess post-installation script returned error exit status 1
Errors were encountered while processing:
 python-ropemacs
E: Sub-process /usr/bin/dpkg returned an error code (1)

$ sudo dpkg -r python-ropemacs
(Reading database ... 524830 files and directories currently installed.)
Removing python-ropemacs ...
Traceback (most recent call last):
  File "/usr/bin/pycentral", line 2096, in <module>
    main()
  File "/usr/bin/pycentral", line 2090, in main
    rv = action.run(global_options)
  File "/usr/bin/pycentral", line 1550, in run
    pkg.read_version_info()
  File "/usr/bin/pycentral", line 804, in read_version_info
    raise PyCentralError, "package has no field Python-Version"
__main__.PyCentralError: package has no field Python-Version
dpkg: error while cleaning up:
 subprocess post-installation script returned error exit status 1
Errors were encountered while processing:
 python-ropemacs

[Michael Vogt (mvo)]

It also can not be upgraded to a fixed version because the prerm script fails (check dpkg-querry -s) and the new version is not available yet so it fails here too.

[Michael Vogt (mvo)]

fixes:
1. fail early
2. allow recovery from broken installs