On Sun, Jan 7, 2018 at 10:51 AM, Albert-Jan Roskam
<sjeik_ap...@hotmail.com> wrote:
>
> On Jan 7, 2018 09:08, Steven D'Aprano <st...@pearwood.info> wrote:
>>
>> realpath() returns the canonical path of the given filename. It doesn't
>> try to locate some actual existing file.
>
> I always thought that os.path.realpath is the Python equivalent of Linux
> realpath/readlink
In POSIX, os.path.realpath resolves a path that contains symlinks by
calling os.readlink in a loop until all links are resolved.
In Windows, os.path.realpath is an alias for os.path.abspath. This
decision predates NTFS support for junctions (2000) and symbolic links
(Vista). It's simply the case that no one has gotten around to
implementing realpath for Windows.
In Windows, os.path.abspath calls os.path._getfullpathname (WinAPI
GetFullPathName). Other than resolving the working directory, this is
a string transformation to return a canonical, fully-qualified path.
It doesn't touch the filesystem. Note that in the case of classic DOS
devices, the canonical path is a local-device path (i.e. "\\.\"
prefix). For example:
>>> print(os.path.abspath('C:/path/to/nul'))
\\.\nul
> maybe also when it's a hard link - in this case the function actually also
> does
> something in Windows
A hard link shouldn't be resolved to another path by os.path.realpath.
It's already a real path.
Cross-platform Python 3 code that needs to resolve symbolic links (or
junctions) can use pathlib.Path.resolve(). For example:
>>> pathlib.Path('C:/Documents and Settings').resolve()
WindowsPath('C:/Users')
On Windows this relies on os.path._getfinalpathname (WinAPI
GetFinalPathNameByHandle). This function returns the final opened
filename, so the OS does all of the work to resolve the final path. In
non-strict mode pathlib uses a loop to reduce the path when it can't
be resolved to an existing file. (There are open issues for edge cases
in this code, but it should work as intended with regular files in
accessible directories.)
For those who are curious, WinAPI GetFinalPathNameByHandle is called
with a handle for a File object that's opened via CreateFile. It gets
the NT name of the volume (e.g, "\Device\HarddiskVolume1") and the
filesystem path (e.g. "\Users"). Then it asks the mount-point manager
to map this NT name back to a DOS name such as drive "C:"; or an NTFS
volume mountpoint such as "C:\Mount\VolumeName"; or a "Volume{GUID}"
name. (In the latter case, Python's _getfinalpathname and pathlib have
bugs and need improvement.)
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