Detecting symlinks (mklink) on Vista/7 in Python without Pywin32

Question

Currently the buildout recipe collective.recipe.omelette uses junction.exe on all versions of Windows to create symlinks. However junction.exe does not come with Windows by default and most importantly does not support creating symlinks to files (only directories) which causes a problem with quite a few Python packages.

On NT6+ (Vista and 7) there is now the mklink utility that not only comes by default but is also capable of creating symlinks to files as well as directories. I would like to update collective.recipe.omelette to use this if available and have done so except for one otherwise simple feature; detecting whether a file or folder is actually a symlink. Since this is a small buildout recipe, requiring Pywin32 in my opinion is a bit too much (unless setuptools could somehow only download it on Windows?).

Currently on Windows what omelette does is call junction.exe on the folder and then grep the response for "Substitute Name:" but I can't find anything as simple for mklink.

The only method I can think of is to call "dir" in the directory and then to go through the response line by line looking for "<SYMLINK>" and the folder/filename on the same line. Surely there is something better?

Answer 1

Could you use ctypes to access the various needed functions and structures? this patch, currently under discussion, is intended to add symlink functionality to module os under Vista and Windows 7 -- but it won't be in before Python 2.7 and 3.2, so the wait (and the requirement for the very latest versions when they do eventually come) will likely be too long; a ctypes-based solution might tide you over and the code in the patch shows what it takes in C to do it (and ctypes-based programmed in Python is only a bit harder than the same programming in C).

Unless somebody's already released some other stand-alone utility like junction.exe for this purpose, I don't see other workable approaches (until that patch finally makes it into a future Python's stdlib) beyond using ctypes, Pywin32, or dir, and you've already ruled out the last two of these three options...!-)

Answer 2

See jaraco.windows.filesystem (part of the jaraco.windows package) for extensive examples on symlink operations in Windows without pywin32.

Answer 3

On windows junctions and symbolic links have the attribute FILE_ATTRIBUTE_REPARSE_POINT (0x400) for reparse points. If you get the file's attributes, then detect this on?

You could use ctypes (as stated in the other answer) to access Kernel32.dll and GetFileAttributes, and detect this value.

Answer 4

You could leverage the Tcl you have available with Tkinter, as that has a 'file link' command that knows about junctions, unlike Pythons os module.

Answer 5

I have searched widely for a better solution, however python 2.7 just does not have a good solution for this. So eventually, I ended up with this (code below), its admittedly ugly, but it's downright pretty compared to all the ctypes hacks I've seen.

import os, subprocess

def realpath(path):
    # not a folder path, ignore
    if (not os.path.isdir(path)):
        return path

    rootpath = os.path.abspath(path)
    oneup, foldername = os.path.split(rootpath)
    output = subprocess.check_output("dir " + oneup, shell=True)

    links = {}
    for line in output.splitlines():
        pos = line.find("<SYMLINKD>")
        if not pos == -1:
            link = line[pos+15:].split()
            links[link[0]] = link[1].strip("[]")
    return links[foldername] if links.has_key(foldername) else rootpath