pythonGH-128520: Subclass abc.ABC in pathlib._abc

Convert `JoinablePath`, `ReadablePath` and `WritablePath` to real ABCs
derived from `abc.ABC`.

Make `JoinablePath.parser` abstract, rather than defaulting to `posixpath`.

Register `PurePath` and `Path` as virtual subclasses of the ABCs rather
than deriving. This avoids a hit to path object instantiation performance.

Lib/pathlib/_abc.py

@@ -13,7 +13,7 @@
 
 import functools
 import operator
-import posixpath
+from abc import ABC, abstractmethod
 from errno import EINVAL
 from glob import _GlobberBase, _no_recurse_symlinks
 from pathlib._os import copyfileobj
@@ -190,24 +190,32 @@ def _ensure_distinct_path(self, source):
         raise err
 
 
-class JoinablePath:
-    """Base class for pure path objects.
+class JoinablePath(ABC):
+    """Abstract base class for pure path objects.
 
     This class *does not* provide several magic methods that are defined in
-    its subclass PurePath. They are: __init__, __fspath__, __bytes__,
+    its implementation PurePath. They are: __init__, __fspath__, __bytes__,
     __reduce__, __hash__, __eq__, __lt__, __le__, __gt__, __ge__.
     """
-
     __slots__ = ()
-    parser = posixpath
 
+    @property
+    @abstractmethod
+    def parser(self):
+        """Implementation of pathlib._types.Parser used for low-level path
+        parsing and manipulation.
+        """
+        raise NotImplementedError
+
+    @abstractmethod
     def with_segments(self, *pathsegments):
         """Construct a new path object from any number of path-like objects.
         Subclasses may override this method to customize how new path objects
         are created from methods like `iterdir()`.
         """
         raise NotImplementedError
 
+    @abstractmethod
     def __str__(self):
         """Return the string representation of the path, suitable for
         passing to system calls."""
@@ -378,20 +386,15 @@ def full_match(self, pattern, *, case_sensitive=None):
 
 
 class ReadablePath(JoinablePath):
-    """Base class for concrete path objects.
-
-    This class provides dummy implementations for many methods that derived
-    classes can override selectively; the default implementations raise
-    NotImplementedError. The most basic methods, such as stat() and open(),
-    directly raise NotImplementedError; these basic methods are called by
-    other methods such as is_dir() and read_text().
+    """Abstract base class for readable path objects.
 
-    The Path class derives this class to implement local filesystem paths.
-    Users may derive their own classes to implement virtual filesystem paths,
-    such as paths in archive files or on remote storage systems.
+    The Path class implements this ABC for local filesystem paths. Users may
+    create subclasses to implement readable virtual filesystem paths, such as
+    paths in archive files or on remote storage systems.
     """
     __slots__ = ()
 
+    @abstractmethod
     def exists(self, *, follow_symlinks=True):
         """
         Whether this path exists.
@@ -401,25 +404,29 @@ def exists(self, *, follow_symlinks=True):
         """
         raise NotImplementedError
 
+    @abstractmethod
     def is_dir(self, *, follow_symlinks=True):
         """
         Whether this path is a directory.
         """
         raise NotImplementedError
 
+    @abstractmethod
     def is_file(self, *, follow_symlinks=True):
         """
         Whether this path is a regular file (also True for symlinks pointing
         to regular files).
         """
         raise NotImplementedError
 
+    @abstractmethod
     def is_symlink(self):
         """
         Whether this path is a symbolic link.
         """
         raise NotImplementedError
 
+    @abstractmethod
     def open(self, mode='r', buffering=-1, encoding=None,
              errors=None, newline=None):
         """
@@ -451,6 +458,7 @@ def _scandir(self):
         import contextlib
         return contextlib.nullcontext(self.iterdir())
 
+    @abstractmethod
     def iterdir(self):
         """Yield path objects of the directory contents.
 
@@ -526,6 +534,7 @@ def walk(self, top_down=True, on_error=None, follow_symlinks=False):
                 yield path, dirnames, filenames
                 paths += [path.joinpath(d) for d in reversed(dirnames)]
 
+    @abstractmethod
     def readlink(self):
         """
         Return the path to which the symbolic link points.
@@ -552,15 +561,23 @@ def copy_into(self, target_dir, *, follow_symlinks=True,
 
 
 class WritablePath(ReadablePath):
+    """Abstract base class for writable path objects.
+
+    The Path class implements this ABC for local filesystem paths. Users may
+    create subclasses to implement writable virtual filesystem paths, such as
+    paths in archive files or on remote storage systems.
+    """
     __slots__ = ()
 
+    @abstractmethod
     def symlink_to(self, target, target_is_directory=False):
         """
         Make this path a symlink pointing to the target path.
         Note the order of arguments (link, target) is the reverse of os.symlink.
         """
         raise NotImplementedError
 
+    @abstractmethod
     def mkdir(self, mode=0o777, parents=False, exist_ok=False):
         """
         Create a new directory at this given path.

Lib/pathlib/_local.py

@@ -20,7 +20,7 @@
     grp = None
 
 from pathlib._os import copyfile
-from pathlib._abc import CopyWriter, JoinablePath, WritablePath
+from pathlib._abc import CopyWriter, JoinablePath, ReadablePath, WritablePath
 
 
 __all__ = [
@@ -190,7 +190,7 @@ def _ensure_different_file(self, source):
         raise err
 
 
-class PurePath(JoinablePath):
+class PurePath:
     """Base class for manipulating paths without I/O.
 
     PurePath represents a filesystem path and offers operations which
@@ -534,6 +534,9 @@ def with_name(self, name):
         tail[-1] = name
         return self._from_parsed_parts(self.drive, self.root, tail)
 
+    with_stem = JoinablePath.with_stem
+    with_suffix = JoinablePath.with_suffix
+
     @property
     def stem(self):
         """The final path component, minus its last suffix."""
@@ -641,6 +644,8 @@ def as_uri(self):
         from urllib.parse import quote_from_bytes
         return prefix + quote_from_bytes(os.fsencode(path))
 
+    match = JoinablePath.match
+
     def full_match(self, pattern, *, case_sensitive=None):
         """
         Return True if this path matches the given glob-style pattern. The
@@ -658,9 +663,10 @@ def full_match(self, pattern, *, case_sensitive=None):
         globber = _StringGlobber(self.parser.sep, case_sensitive, recursive=True)
         return globber.compile(pattern)(path) is not None
 
-# Subclassing os.PathLike makes isinstance() checks slower,
-# which in turn makes Path construction slower. Register instead!
+# Subclassing abc.ABC makes isinstance() checks slower,
+# which in turn makes path construction slower. Register instead!
 os.PathLike.register(PurePath)
+JoinablePath.register(PurePath)
 
 
 class PurePosixPath(PurePath):
@@ -683,7 +689,7 @@ class PureWindowsPath(PurePath):
     __slots__ = ()
 
 
-class Path(WritablePath, PurePath):
+class Path(PurePath):
     """PurePath subclass that can make system calls.
 
     Path represents a filesystem path but unlike PurePath, also offers
@@ -823,14 +829,18 @@ def open(self, mode='r', buffering=-1, encoding=None,
             encoding = io.text_encoding(encoding)
         return io.open(self, mode, buffering, encoding, errors, newline)
 
+    read_bytes = ReadablePath.read_bytes
+
     def read_text(self, encoding=None, errors=None, newline=None):
         """
         Open the file in text mode, read it, and close the file.
         """
         # Call io.text_encoding() here to ensure any warning is raised at an
         # appropriate stack level.
         encoding = io.text_encoding(encoding)
-        return super().read_text(encoding, errors, newline)
+        return ReadablePath.read_text(self, encoding, errors, newline)
+
+    write_bytes = WritablePath.write_bytes
 
     def write_text(self, data, encoding=None, errors=None, newline=None):
         """
@@ -839,7 +849,7 @@ def write_text(self, data, encoding=None, errors=None, newline=None):
         # Call io.text_encoding() here to ensure any warning is raised at an
         # appropriate stack level.
         encoding = io.text_encoding(encoding)
-        return super().write_text(data, encoding, errors, newline)
+        return WritablePath.write_text(self, data, encoding, errors, newline)
 
     _remove_leading_dot = operator.itemgetter(slice(2, None))
     _remove_trailing_slash = operator.itemgetter(slice(-1))
@@ -1124,6 +1134,8 @@ def replace(self, target):
 
     copy = property(_LocalCopyWriter, doc=_LocalCopyWriter.__call__.__doc__)
 
+    copy_into = ReadablePath.copy_into
+
     def move(self, target):
         """
         Recursively move this file or directory tree to the given destination.
@@ -1242,6 +1254,11 @@ def from_uri(cls, uri):
             raise ValueError(f"URI is not absolute: {uri!r}")
         return path
 
+# Subclassing abc.ABC makes isinstance() checks slower,
+# which in turn makes path construction slower. Register instead!
+ReadablePath.register(Path)
+WritablePath.register(Path)
+
 
 class PosixPath(Path, PurePosixPath):
     """Path subclass for non-Windows systems.

Lib/test/test_pathlib/test_pathlib.py

@@ -75,7 +75,7 @@ def test_is_notimplemented(self):
 # Tests for the pure classes.
 #
 
-class PurePathTest(test_pathlib_abc.DummyJoinablePathTest):
+class PurePathTest(test_pathlib_abc.JoinablePathTest):
     cls = pathlib.PurePath
 
     # Make sure any symbolic links in the base test path are resolved.
@@ -924,7 +924,7 @@ class cls(pathlib.PurePath):
 # Tests for the concrete classes.
 #
 
-class PathTest(test_pathlib_abc.DummyWritablePathTest, PurePathTest):
+class PathTest(test_pathlib_abc.WritablePathTest, PurePathTest):
     """Tests for the FS-accessing functionalities of the Path classes."""
     cls = pathlib.Path
     can_symlink = os_helper.can_symlink()
@@ -3019,7 +3019,7 @@ def test_group_windows(self):
             P('c:/').group()
 
 
-class PathWalkTest(test_pathlib_abc.DummyReadablePathWalkTest):
+class PathWalkTest(test_pathlib_abc.ReadablePathWalkTest):
     cls = pathlib.Path
     base = PathTest.base
     can_symlink = PathTest.can_symlink

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -31,29 +31,11 @@ def needs_windows(fn):
 #
 
 
-class JoinablePathTest(unittest.TestCase):
-    cls = JoinablePath
-
-    def test_magic_methods(self):
-        P = self.cls
-        self.assertFalse(hasattr(P, '__fspath__'))
-        self.assertFalse(hasattr(P, '__bytes__'))
-        self.assertIs(P.__reduce__, object.__reduce__)
-        self.assertIs(P.__repr__, object.__repr__)
-        self.assertIs(P.__hash__, object.__hash__)
-        self.assertIs(P.__eq__, object.__eq__)
-        self.assertIs(P.__lt__, object.__lt__)
-        self.assertIs(P.__le__, object.__le__)
-        self.assertIs(P.__gt__, object.__gt__)
-        self.assertIs(P.__ge__, object.__ge__)
-
-    def test_parser(self):
-        self.assertIs(self.cls.parser, posixpath)
-
-
 class DummyJoinablePath(JoinablePath):
     __slots__ = ('_segments',)
 
+    parser = posixpath
+
     def __init__(self, *segments):
         self._segments = segments
 
@@ -77,7 +59,7 @@ def with_segments(self, *pathsegments):
         return type(self)(*pathsegments)
 
 
-class DummyJoinablePathTest(unittest.TestCase):
+class JoinablePathTest(unittest.TestCase):
     cls = DummyJoinablePath
 
     # Use a base path that's unrelated to any real filesystem path.
@@ -94,6 +76,10 @@ def setUp(self):
         self.sep = self.parser.sep
         self.altsep = self.parser.altsep
 
+    def test_is_joinable(self):
+        p = self.cls(self.base)
+        self.assertIsInstance(p, JoinablePath)
+
     def test_parser(self):
         self.assertIsInstance(self.cls.parser, Parser)
 
@@ -940,6 +926,7 @@ class DummyReadablePath(ReadablePath):
 
     _files = {}
     _directories = {}
+    parser = posixpath
 
     def __init__(self, *segments):
         self._segments = segments
@@ -1012,6 +999,9 @@ def iterdir(self):
         else:
             raise FileNotFoundError(errno.ENOENT, "File not found", path)
 
+    def readlink(self):
+        raise NotImplementedError
+
 
 class DummyWritablePath(DummyReadablePath, WritablePath):
     __slots__ = ()
@@ -1034,8 +1024,11 @@ def mkdir(self, mode=0o777, parents=False, exist_ok=False):
             self.parent.mkdir(parents=True, exist_ok=True)
             self.mkdir(mode, parents=False, exist_ok=exist_ok)
 
+    def symlink_to(self, target, target_is_directory=False):
+        raise NotImplementedError
+
 
-class DummyReadablePathTest(DummyJoinablePathTest):
+class ReadablePathTest(JoinablePathTest):
     """Tests for ReadablePathTest methods that use stat(), open() and iterdir()."""
 
     cls = DummyReadablePath
@@ -1102,6 +1095,10 @@ def assertEqualNormCase(self, path_a, path_b):
         normcase = self.parser.normcase
         self.assertEqual(normcase(path_a), normcase(path_b))
 
+    def test_is_readable(self):
+        p = self.cls(self.base)
+        self.assertIsInstance(p, ReadablePath)
+
     def test_exists(self):
         P = self.cls
         p = P(self.base)
@@ -1359,9 +1356,13 @@ def test_is_symlink(self):
             self.assertIs((P / 'linkA\x00').is_file(), False)
 
 
-class DummyWritablePathTest(DummyReadablePathTest):
+class WritablePathTest(ReadablePathTest):
     cls = DummyWritablePath
 
+    def test_is_writable(self):
+        p = self.cls(self.base)
+        self.assertIsInstance(p, WritablePath)
+
     def test_read_write_bytes(self):
         p = self.cls(self.base)
         (p / 'fileA').write_bytes(b'abcdefg')
@@ -1570,9 +1571,9 @@ def test_copy_into_empty_name(self):
         self.assertRaises(ValueError, source.copy_into, target_dir)
 
 
-class DummyReadablePathWalkTest(unittest.TestCase):
+class ReadablePathWalkTest(unittest.TestCase):
     cls = DummyReadablePath
-    base = DummyReadablePathTest.base
+    base = ReadablePathTest.base
     can_symlink = False
 
     def setUp(self):