Why would one use both, os.path.abspath and os.path.realpath?

For your stated scenario, there is no reason to combine realpath and abspath, since os.path.realpath actually calls os.path.abspath before returning a result (I checked Python 2.5 to Python 3.6).

• os.path.abspath returns the absolute path, but does NOT resolve symlinks in its argument.
• os.path.realpath will first resolve any symbolic links in the path, and then return the absolute path.

However, if you expect your path to contain a ~, neither abspath or realpath will resolve ~ to the user's home directory, and the resulting path will be invalid. You will need to use os.path.expanduser to resolve this to the user's directory.

For the sake of a thorough explanation, here are some results which I've verified in Windows and Linux, in Python 3.4 and Python 2.6. The current directory (./) is my home directory, which looks like this:

myhome
|- data (symlink to /mnt/data)
|- subdir (extra directory, for verbose explanation)

# os.path.abspath returns the absolute path, but does NOT resolve symlinks in its argument
os.path.abspath('./')
'/home/myhome'
os.path.abspath('./subdir/../data')
'/home/myhome/data'


# os.path.realpath will resolve symlinks AND return an absolute path from a relative path
os.path.realpath('./')
'/home/myhome'
os.path.realpath('./subdir/../')
'/home/myhome'
os.path.realpath('./subdir/../data')
'/mnt/data'

# NEITHER abspath or realpath will resolve or remove ~.
os.path.abspath('~/data')
'/home/myhome/~/data'

os.path.realpath('~/data')
'/home/myhome/~/data'

# And the returned path will be invalid
os.path.exists(os.path.abspath('~/data'))
False
os.path.exists(os.path.realpath('~/data'))
False

# Use realpath + expanduser to resolve ~
os.path.realpath(os.path.expanduser('~/subdir/../data'))
'/mnt/data'

os.path.realpath derefences symbolic links on those operating systems which support them.

os.path.abspath simply removes things like . and .. from the path giving a full path from the root of the directory tree to the named file (or symlink)

For example, on Ubuntu

$ ls -l
total 0
-rw-rw-r-- 1 guest guest 0 Jun 16 08:36 a
lrwxrwxrwx 1 guest guest 1 Jun 16 08:36 b -> a

$ python
Python 2.7.11 (default, Dec 15 2015, 16:46:19) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.

>>> from os.path import abspath, realpath

>>> abspath('b')
'/home/guest/play/paths/b'

>>> realpath('b')
'/home/guest/play/paths/a'

Symlinks can contain relative paths, hence the need to use both. The inner call to realpath might return a path with embedded .. parts, which abspath then removes.