为什么要同时使用os.path.abspath和os.path.realpath？

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?

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.

For your stated scenario, there is no reason to combine realpath and abspath, since 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'

In the layman terms, if you are trying to get the path of a shortcut file, absolute path gives the complete path of the file present in the shortcut location, while realpath gives the original location path of the file.

Absolute path, os.path.abspath(), gives the complete path of the file which is located in the current working directory or the directory you mentioned.

Real path, os.path.realpath(), gives the complete path of the file which is being referred.

Eg:

file = "shortcut_folder/filename"
os.path.abspath(file) = "C:/Desktop/shortcut_folder/filename"
os.path.realpath(file) = "D:/PyCharmProjects/Python1stClass/filename"