Comment 3 for bug 1804422

this seems to be a gdebi bug. bad package? maybe re-installation helps

Setting up python3 (3.6.7-1~18.04) ...
running python rtupdate hooks for python3.6...
dpkg-query: package 'gdebi-core' is not installed
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
Traceback (most recent call last):
  File "/usr/bin/py3clean", line 210, in <module>
    main()
  File "/usr/bin/py3clean", line 196, in main
    pfiles = set(dpf.from_package(options.package))
  File "/usr/share/python3/debpython/files.py", line 53, in from_package
    raise Exception("cannot get content of %s" % package_name)
Exception: cannot get content of gdebi-core
error running python rtupdate hook gdebi-core
dpkg: error processing package python3 (--configure):
 installed python3 package post-installation script subprocess returned error exit status 4