Difference of output of apt-rdepends and apt-get install

Question

When I issue the command apt-rdepends -p python-epydoc | grep NotInstalled I get the below output

$ apt-rdepends -p python-epydoc | grep NotInstalled
Reading package lists... Done
Building dependency tree       
Reading state information... Done
  Depends: debconf-2.0 [NotInstalled]
  Depends: debconf-english [NotInstalled]
  PreDepends: perlapi-5.10.0 [NotInstalled]
  Depends: perlapi-5.10.0 [NotInstalled]
  Depends: perlapi-5.10.0 [NotInstalled]
  Depends: debconf-2.0 [NotInstalled]
  Depends: debconf-2.0 [NotInstalled]

When I issue sudo apt-get install python-epydoc

$ sudo apt-get install python-epydoc
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  blt graphviz lacheck latex-beamer latex-xcolor lmodern luatex pgf preview-latex-style prosper ps2eps python-docutils python-lxml python-pygments python-roman
  python-tk tcl8.5 tex-common texlive-base texlive-binaries texlive-common texlive-doc-base texlive-extra-utils texlive-font-utils texlive-fonts-recommended
  texlive-fonts-recommended-doc texlive-generic-recommended texlive-latex-base texlive-latex-base-doc texlive-latex-extra texlive-latex-extra-doc
  texlive-latex-recommended texlive-latex-recommended-doc texlive-luatex texlive-pictures texlive-pictures-doc texlive-pstricks texlive-pstricks-doc tipa tk8.5
Suggested packages:
  blt-demo graphviz-doc auctex texlive-lang-french epydoc-doc python-profiler python-lxml-dbg python-chardet tix python-tk-dbg tclreadline debhelper perl-tk dvidvi
  fragmaster latexmk xindy purifyeps psutils t1utils texpower
The following NEW packages will be installed:
  blt graphviz lacheck latex-beamer latex-xcolor lmodern luatex pgf preview-latex-style prosper ps2eps python-docutils python-epydoc python-lxml python-pygments
  python-roman python-tk tcl8.5 tex-common texlive-base texlive-binaries texlive-common texlive-doc-base texlive-extra-utils texlive-font-utils
  texlive-fonts-recommended texlive-fonts-recommended-doc texlive-generic-recommended texlive-latex-base texlive-latex-base-doc texlive-latex-extra
  texlive-latex-extra-doc texlive-latex-recommended texlive-latex-recommended-doc texlive-luatex texlive-pictures texlive-pictures-doc texlive-pstricks
  texlive-pstricks-doc tipa tk8.5
0 upgraded, 41 newly installed, 0 to remove and 68 not upgraded.
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The output of apt-get install says that I have to install packages that were not given by the command apt-rdepends -p python-epydoc | grep NotInstalled.

Shouldn't the NotInstalled packages in the former command and the packages to be installed in the latter command be the same? If so, what is the reason for the difference?

Answer 1

Since Ubuntu 8.10 apt defaults to not only install strict dependencies, but also recommended packages. If you don't want apt-get to install Recommends you can use the flag --no-install-recommends

apt-get install --no-install-recommends python-epydoc

You can also make that setting persistent by setting APT::Install-Recommends "false";. It can either be in /etc/apt/apt.conf or in a dedicated file under /etc/apt/apt.conf.d/. Also see the following part of the 8.10 Release notes.

https://wiki.ubuntu.com/IntrepidReleaseNotes#Recommended_packages_installed_by_default

You can tell apt-rdepends to also follow as well as show Recommends. This will give you a more complete output. Do note that that it will contain a fair share of duplicates, etc.

apt-rdepends -p --follow=1,2,4 --show=Depends,PreDepends,Recommends python-epydoc | grep NotInstalled

The reason I use --follow=1,2,4 instead of actually naming Depends,PreDepends and Recommends is Launchpad Bug #315567