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

Question

In multiple open source projects, I have seen people do os.path.abspath(os.path.realpath(__file__)) to get the absolute path to the current file.

However, I find that os.path.abspath(__file__) and os.path.realpath(__file__) produce the same result. os.path.abspath(os.path.realpath(__file__)) seems to be a bit redundant.

Is there a reason people are using that?

Answer 1

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'