Why does 'aptitude markauto' uninstall my package?

Question

From the aptitude help:

aptitude 0.6.8.2
Usage: aptitude [-S fname] [-u|-i]

.. snip ..

markauto     - Mark packages as having been automatically installed.
unmarkauto   - Mark packages as having been manually installed.

.. snip ..

So why when I mark my package as being automatically installed, does aptitude attempt to uninstall it?!

# aptitude -s -y markauto python-pip
The following packages will be REMOVED:
  python-pip{u}
0 packages upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 468 kB will be freed.
Would download/install/remove packages.

Answer 1

The point of marking packages as having been automatically installed is so that aptitude will know to only keep them if they are required by something else. Manually-installed packages are those the user explicitly wants installed, automatically-installed packages are those required to satisfy the dependencies of the manually-installed packages (and automatically installed by tools such as apt-get or aptitude). This is useful for example with libraries: generally speaking you don't install libraries explicitly, you only want those which are needed by the programs you use; so when you install a program, all the libraries it needs are installed as well, and if you remove the program the libraries which are no longer needed are uninstalled too.

So when you mark python-pip as automatically installed, you're telling aptitude to only keep it if something else requires it; presumably nothing does, so aptitude reckons it should be removed!

See the aptitude documentation for details.