gh-101000: Add os.path.splitroot() (#101002)

Co-authored-by: Eryk Sun <eryksun@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Doc/library/os.path.rst

@@ -488,6 +488,39 @@ the :mod:`glob` module.)
       Accepts a :term:`path-like object`.
 
 
+.. function:: splitroot(path)
+
+   Split the pathname *path* into a 3-item tuple ``(drive, root, tail)`` where
+   *drive* is a device name or mount point, *root* is a string of separators
+   after the drive, and *tail* is everything after the root. Any of these
+   items may be the empty string. In all cases, ``drive + root + tail`` will
+   be the same as *path*.
+
+   On POSIX systems, *drive* is always empty. The *root* may be empty (if *path* is
+   relative), a single forward slash (if *path* is absolute), or two forward slashes
+   (implementation-defined per `IEEE Std 1003.1-2017; 4.13 Pathname Resolution
+   <https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13>`_.)
+   For example::
+
+      >>> splitroot('/home/sam')
+      ('', '/', 'home/sam')
+      >>> splitroot('//home/sam')
+      ('', '//', 'home/sam')
+      >>> splitroot('///home/sam')
+      ('', '/', '//home/sam')
+
+   On Windows, *drive* may be empty, a drive-letter name, a UNC share, or a device
+   name. The *root* may be empty, a forward slash, or a backward slash. For
+   example::
+
+      >>> splitroot('C:/Users/Sam')
+      ('C:', '/', 'Users/Sam')
+      >>> splitroot('//Server/Share/Users/Sam')
+      ('//Server/Share', '/', 'Users/Sam')
+
+   .. versionadded:: 3.12
+
+
 .. function:: splitext(path)
 
    Split the pathname *path* into a pair ``(root, ext)``  such that ``root + ext ==

Doc/whatsnew/3.12.rst

@@ -288,13 +288,18 @@ os
   for a process with :func:`os.pidfd_open` in non-blocking mode.
   (Contributed by Kumar Aditya in :gh:`93312`.)
 
-* Add :func:`os.path.isjunction` to check if a given path is a junction.
-  (Contributed by Charles Machalow in :gh:`99547`.)
-
 * :class:`os.DirEntry` now includes an :meth:`os.DirEntry.is_junction`
   method to check if the entry is a junction.
   (Contributed by Charles Machalow in :gh:`99547`.)
 
+os.path
+-------
+
+* Add :func:`os.path.isjunction` to check if a given path is a junction.
+  (Contributed by Charles Machalow in :gh:`99547`.)
+
+* Add :func:`os.path.splitroot` to split a path into a triad
+  ``(drive, root, tail)``. (Contributed by Barney Gale in :gh:`101000`.)
 
 shutil
 ------

Lib/ntpath.py

@@ -24,7 +24,7 @@
 from genericpath import *
 
 
-__all__ = ["normcase","isabs","join","splitdrive","split","splitext",
+__all__ = ["normcase","isabs","join","splitdrive","splitroot","split","splitext",
            "basename","dirname","commonprefix","getsize","getmtime",
            "getatime","getctime", "islink","exists","lexists","isdir","isfile",
            "ismount", "expanduser","expandvars","normpath","abspath",
@@ -117,19 +117,21 @@ def join(path, *paths):
     try:
         if not paths:
             path[:0] + sep  #23780: Ensure compatible data type even if p is null.
-        result_drive, result_path = splitdrive(path)
+        result_drive, result_root, result_path = splitroot(path)
         for p in map(os.fspath, paths):
-            p_drive, p_path = splitdrive(p)
-            if p_path and p_path[0] in seps:
+            p_drive, p_root, p_path = splitroot(p)
+            if p_root:
                 # Second path is absolute
                 if p_drive or not result_drive:
                     result_drive = p_drive
+                result_root = p_root
                 result_path = p_path
                 continue
             elif p_drive and p_drive != result_drive:
                 if p_drive.lower() != result_drive.lower():
                     # Different drives => ignore the first path entirely
                     result_drive = p_drive
+                    result_root = p_root
                     result_path = p_path
                     continue
                 # Same drive in different case
@@ -139,10 +141,10 @@ def join(path, *paths):
                 result_path = result_path + sep
             result_path = result_path + p_path
         ## add separator between UNC and non-absolute path
-        if (result_path and result_path[0] not in seps and
+        if (result_path and not result_root and
             result_drive and result_drive[-1:] != colon):
             return result_drive + sep + result_path
-        return result_drive + result_path
+        return result_drive + result_root + result_path
     except (TypeError, AttributeError, BytesWarning):
         genericpath._check_arg_types('join', path, *paths)
         raise
@@ -169,35 +171,61 @@ def splitdrive(p):
 
     Paths cannot contain both a drive letter and a UNC path.
 
+    """
+    drive, root, tail = splitroot(p)
+    return drive, root + tail
+
+
+def splitroot(p):
+    """Split a pathname into drive, root and tail. The drive is defined
+    exactly as in splitdrive(). On Windows, the root may be a single path
+    separator or an empty string. The tail contains anything after the root.
+    For example:
+
+        splitroot('//server/share/') == ('//server/share', '/', '')
+        splitroot('C:/Users/Barney') == ('C:', '/', 'Users/Barney')
+        splitroot('C:///spam///ham') == ('C:', '/', '//spam///ham')
+        splitroot('Windows/notepad') == ('', '', 'Windows/notepad')
     """
     p = os.fspath(p)
-    if len(p) >= 2:
-        if isinstance(p, bytes):
-            sep = b'\\'
-            altsep = b'/'
-            colon = b':'
-            unc_prefix = b'\\\\?\\UNC\\'
-        else:
-            sep = '\\'
-            altsep = '/'
-            colon = ':'
-            unc_prefix = '\\\\?\\UNC\\'
-        normp = p.replace(altsep, sep)
-        if normp[0:2] == sep * 2:
+    if isinstance(p, bytes):
+        sep = b'\\'
+        altsep = b'/'
+        colon = b':'
+        unc_prefix = b'\\\\?\\UNC\\'
+        empty = b''
+    else:
+        sep = '\\'
+        altsep = '/'
+        colon = ':'
+        unc_prefix = '\\\\?\\UNC\\'
+        empty = ''
+    normp = p.replace(altsep, sep)
+    if normp[:1] == sep:
+        if normp[1:2] == sep:
             # UNC drives, e.g. \\server\share or \\?\UNC\server\share
             # Device drives, e.g. \\.\device or \\?\device
             start = 8 if normp[:8].upper() == unc_prefix else 2
             index = normp.find(sep, start)
             if index == -1:
-                return p, p[:0]
+                return p, empty, empty
             index2 = normp.find(sep, index + 1)
             if index2 == -1:
-                return p, p[:0]
-            return p[:index2], p[index2:]
-        if normp[1:2] == colon:
-            # Drive-letter drives, e.g. X:
-            return p[:2], p[2:]
-    return p[:0], p
+                return p, empty, empty
+            return p[:index2], p[index2:index2 + 1], p[index2 + 1:]
+        else:
+            # Relative path with root, e.g. \Windows
+            return empty, p[:1], p[1:]
+    elif normp[1:2] == colon:
+        if normp[2:3] == sep:
+            # Absolute drive-letter path, e.g. X:\Windows
+            return p[:2], p[2:3], p[3:]
+        else:
+            # Relative path with drive, e.g. X:Windows
+            return p[:2], empty, p[2:]
+    else:
+        # Relative path, e.g. Windows
+        return empty, empty, p
 
 
 # Split a path in head (everything up to the last '/') and tail (the
@@ -212,15 +240,13 @@ def split(p):
     Either part may be empty."""
     p = os.fspath(p)
     seps = _get_bothseps(p)
-    d, p = splitdrive(p)
+    d, r, p = splitroot(p)
     # set i to index beyond p's last slash
     i = len(p)
     while i and p[i-1] not in seps:
         i -= 1
     head, tail = p[:i], p[i:]  # now tail has no slashes
-    # remove trailing slashes from head, unless it's all slashes
-    head = head.rstrip(seps) or head
-    return d + head, tail
+    return d + r + head.rstrip(seps), tail
 
 
 # Split a path in root and extension.
@@ -311,10 +337,10 @@ def ismount(path):
     path = os.fspath(path)
     seps = _get_bothseps(path)
     path = abspath(path)
-    root, rest = splitdrive(path)
-    if root and root[0] in seps:
-        return (not rest) or (rest in seps)
-    if rest and rest in seps:
+    drive, root, rest = splitroot(path)
+    if drive and drive[0] in seps:
+        return not rest
+    if root and not rest:
         return True
 
     if _getvolumepathname:
@@ -525,13 +551,8 @@ def normpath(path):
             curdir = '.'
             pardir = '..'
         path = path.replace(altsep, sep)
-        prefix, path = splitdrive(path)
-
-        # collapse initial backslashes
-        if path.startswith(sep):
-            prefix += sep
-            path = path.lstrip(sep)
-
+        drive, root, path = splitroot(path)
+        prefix = drive + root
         comps = path.split(sep)
         i = 0
         while i < len(comps):
@@ -541,7 +562,7 @@ def normpath(path):
                 if i > 0 and comps[i-1] != pardir:
                     del comps[i-1:i+1]
                     i -= 1
-                elif i == 0 and prefix.endswith(sep):
+                elif i == 0 and root:
                     del comps[i]
                 else:
                     i += 1
@@ -765,8 +786,8 @@ def relpath(path, start=None):
     try:
         start_abs = abspath(normpath(start))
         path_abs = abspath(normpath(path))
-        start_drive, start_rest = splitdrive(start_abs)
-        path_drive, path_rest = splitdrive(path_abs)
+        start_drive, _, start_rest = splitroot(start_abs)
+        path_drive, _, path_rest = splitroot(path_abs)
         if normcase(start_drive) != normcase(path_drive):
             raise ValueError("path is on mount %r, start on mount %r" % (
                 path_drive, start_drive))
@@ -816,21 +837,19 @@ def commonpath(paths):
         curdir = '.'
 
     try:
-        drivesplits = [splitdrive(p.replace(altsep, sep).lower()) for p in paths]
-        split_paths = [p.split(sep) for d, p in drivesplits]
+        drivesplits = [splitroot(p.replace(altsep, sep).lower()) for p in paths]
+        split_paths = [p.split(sep) for d, r, p in drivesplits]
 
-        try:
-            isabs, = set(p[:1] == sep for d, p in drivesplits)
-        except ValueError:
-            raise ValueError("Can't mix absolute and relative paths") from None
+        if len({r for d, r, p in drivesplits}) != 1:
+            raise ValueError("Can't mix absolute and relative paths")
 
         # Check that all drive letters or UNC paths match. The check is made only
         # now otherwise type errors for mixing strings and bytes would not be
         # caught.
-        if len(set(d for d, p in drivesplits)) != 1:
+        if len({d for d, r, p in drivesplits}) != 1:
             raise ValueError("Paths don't have the same drive")
 
-        drive, path = splitdrive(paths[0].replace(altsep, sep))
+        drive, root, path = splitroot(paths[0].replace(altsep, sep))
         common = path.split(sep)
         common = [c for c in common if c and c != curdir]
 
@@ -844,8 +863,7 @@ def commonpath(paths):
         else:
             common = common[:len(s1)]
 
-        prefix = drive + sep if isabs else drive
-        return prefix + sep.join(common)
+        return drive + root + sep.join(common)
     except (TypeError, AttributeError):
         genericpath._check_arg_types('commonpath', *paths)
         raise

Lib/pathlib.py

@@ -271,19 +271,6 @@ def __reduce__(self):
         # when pickling related paths.
         return (self.__class__, tuple(self._parts))
 
-    @classmethod
-    def _split_root(cls, part):
-        sep = cls._flavour.sep
-        rel = cls._flavour.splitdrive(part)[1].lstrip(sep)
-        anchor = part.removesuffix(rel)
-        if anchor:
-            anchor = cls._flavour.normpath(anchor)
-        drv, root = cls._flavour.splitdrive(anchor)
-        if drv.startswith(sep):
-            # UNC paths always have a root.
-            root = sep
-        return drv, root, rel
-
     @classmethod
     def _parse_parts(cls, parts):
         if not parts:
@@ -293,7 +280,10 @@ def _parse_parts(cls, parts):
         path = cls._flavour.join(*parts)
         if altsep:
             path = path.replace(altsep, sep)
-        drv, root, rel = cls._split_root(path)
+        drv, root, rel = cls._flavour.splitroot(path)
+        if drv.startswith(sep):
+            # pathlib assumes that UNC paths always have a root.
+            root = sep
         unfiltered_parsed = [drv + root] + rel.split(sep)
         parsed = [sys.intern(x) for x in unfiltered_parsed if x and x != '.']
         return drv, root, parsed
@@ -493,9 +483,9 @@ def with_name(self, name):
         """Return a new path with the file name changed."""
         if not self.name:
             raise ValueError("%r has an empty name" % (self,))
-        drv, root, parts = self._parse_parts((name,))
-        if (not name or name[-1] in [self._flavour.sep, self._flavour.altsep]
-            or drv or root or len(parts) != 1):
+        f = self._flavour
+        drv, root, tail = f.splitroot(name)
+        if drv or root or not tail or f.sep in tail or (f.altsep and f.altsep in tail):
             raise ValueError("Invalid name %r" % (name))
         return self._from_parsed_parts(self._drv, self._root,
                                        self._parts[:-1] + [name])