Create NTFS junction point in Python

Question

Is there a way to create an NTFS junction point in Python? I know I can call the junction utility, but it would be better not to rely on external tools.

Answer 1

Based on the accepted answer by Charles, here improved (and cross-platform) versions of the functions (Python 2.7 and 3.5+).

• islink() now also detects file symbolic links under Windows (just like the POSIX equivalent)
• parse_reparse_buffer() and readlink() now actually detect the type of reparse point (NTFS Junction, symlink or generic) which is needed to correctly decode the path
• readlink() no longer fails with access denied on NTFS Junctions or directory symlinks (unless you really have no permission to read attributes)

---

import os
import struct
import sys

if sys.platform == "win32":
    from win32file import *
    from winioctlcon import FSCTL_GET_REPARSE_POINT

__all__ = ['islink', 'readlink']

# Win32file doesn't seem to have this attribute.
FILE_ATTRIBUTE_REPARSE_POINT = 1024

# These are defined in win32\lib\winnt.py, but with wrong values
IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003  # Junction
IO_REPARSE_TAG_SYMLINK = 0xA000000C

def islink(path):
    """
    Cross-platform islink implementation.

    Supports Windows NT symbolic links and reparse points.

    """
    if sys.platform != "win32" or sys.getwindowsversion()[0] < 6:
        return os.path.islink(path)
    return bool(os.path.exists(path) and GetFileAttributes(path) &
                FILE_ATTRIBUTE_REPARSE_POINT == FILE_ATTRIBUTE_REPARSE_POINT)


def parse_reparse_buffer(buf):
    """ Implementing the below in Python:

    typedef struct _REPARSE_DATA_BUFFER {
        ULONG  ReparseTag;
        USHORT ReparseDataLength;
        USHORT Reserved;
        union {
            struct {
                USHORT SubstituteNameOffset;
                USHORT SubstituteNameLength;
                USHORT PrintNameOffset;
                USHORT PrintNameLength;
                ULONG Flags;
                WCHAR PathBuffer[1];
            } SymbolicLinkReparseBuffer;
            struct {
                USHORT SubstituteNameOffset;
                USHORT SubstituteNameLength;
                USHORT PrintNameOffset;
                USHORT PrintNameLength;
                WCHAR PathBuffer[1];
            } MountPointReparseBuffer;
            struct {
                UCHAR  DataBuffer[1];
            } GenericReparseBuffer;
        } DUMMYUNIONNAME;
    } REPARSE_DATA_BUFFER, *PREPARSE_DATA_BUFFER;

    """
    # See https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/content/ntifs/ns-ntifs-_reparse_data_buffer

    data = {'tag': struct.unpack(' 4 and rpath[0:4] == '\\??\\':
        rpath = rpath[4:]
    return rpath