GH-128520: Divide pathlib ABCs into three classes

In the private pathlib ABCs, rename `PurePathBase` to `JoinablePath`, and
split `PathBase` into `ReadablePath` and `WritablePath`. This improves the
API fit for read-only virtual filesystems.

The split of `PathBase` entails a similar split of `CopyWorker` (implements
copying) and the test cases in `test_pathlib_abc`. For a couple of reasons,
this isn't quite possible yet.

In a later patch, we'll make `WritablePath` inherit directly from
`JoinablePath` rather than `ReadablePath`.

Lib/pathlib/_abc.py

@@ -7,8 +7,8 @@
 it's developed alongside pathlib. If it finds success and maturity as a PyPI
 package, it could become a public part of the standard library.
 
-Two base classes are defined here -- PurePathBase and PathBase -- that
-resemble pathlib's PurePath and Path respectively.
+Three base classes are defined here -- JoinablePath, ReadablePath and
+WritablePath.
 """
 
 import functools
@@ -56,13 +56,13 @@ def concat_path(path, text):
         return path.with_segments(str(path) + text)
 
 
-class CopyWorker:
+class CopyReader:
     """
     Class that implements copying between path objects. An instance of this
-    class is available from the PathBase.copy property; it's made callable so
-    that PathBase.copy() can be treated as a method.
+    class is available from the ReadablePath.copy property; it's made callable
+    so that ReadablePath.copy() can be treated as a method.
 
-    The target path's CopyWorker drives the process from its _create() method.
+    The target path's CopyWriter drives the process from its _create() method.
     Files and directories are exchanged by calling methods on the source and
     target paths, and metadata is exchanged by calling
     source.copy._read_metadata() and target.copy._write_metadata().
@@ -77,11 +77,15 @@ def __call__(self, target, follow_symlinks=True, dirs_exist_ok=False,
         """
         Recursively copy this file or directory tree to the given destination.
         """
-        if not isinstance(target, PathBase):
+        if not isinstance(target, ReadablePath):
             target = self._path.with_segments(target)
 
-        # Delegate to the target path's CopyWorker object.
-        return target.copy._create(self._path, follow_symlinks, dirs_exist_ok, preserve_metadata)
+        # Delegate to the target path's CopyWriter object.
+        try:
+            create = target.copy._create
+        except AttributeError:
+            raise TypeError(f"Target is not writable: {target}") from None
+        return create(self._path, follow_symlinks, dirs_exist_ok, preserve_metadata)
 
     _readable_metakeys = frozenset()
 
@@ -91,6 +95,10 @@ def _read_metadata(self, metakeys, *, follow_symlinks=True):
         """
         raise NotImplementedError
 
+
+class CopyWriter(CopyReader):
+    __slots__ = ()
+
     _writable_metakeys = frozenset()
 
     def _write_metadata(self, metadata, *, follow_symlinks=True):
@@ -182,7 +190,7 @@ def _ensure_distinct_path(self, source):
         raise err
 
 
-class PurePathBase:
+class JoinablePath:
     """Base class for pure path objects.
 
     This class *does not* provide several magic methods that are defined in
@@ -334,7 +342,7 @@ def match(self, path_pattern, *, case_sensitive=None):
         is matched. The recursive wildcard '**' is *not* supported by this
         method.
         """
-        if not isinstance(path_pattern, PurePathBase):
+        if not isinstance(path_pattern, JoinablePath):
             path_pattern = self.with_segments(path_pattern)
         if case_sensitive is None:
             case_sensitive = _is_case_sensitive(self.parser)
@@ -359,7 +367,7 @@ def full_match(self, pattern, *, case_sensitive=None):
         Return True if this path matches the given glob-style pattern. The
         pattern is matched against the entire path.
         """
-        if not isinstance(pattern, PurePathBase):
+        if not isinstance(pattern, JoinablePath):
             pattern = self.with_segments(pattern)
         if case_sensitive is None:
             case_sensitive = _is_case_sensitive(self.parser)
@@ -369,7 +377,7 @@ def full_match(self, pattern, *, case_sensitive=None):
 
 
 
-class PathBase(PurePathBase):
+class ReadablePath(JoinablePath):
     """Base class for concrete path objects.
 
     This class provides dummy implementations for many methods that derived
@@ -434,25 +442,6 @@ def read_text(self, encoding=None, errors=None, newline=None):
         with self.open(mode='r', encoding=encoding, errors=errors, newline=newline) as f:
             return f.read()
 
-    def write_bytes(self, data):
-        """
-        Open the file in bytes mode, write to it, and close the file.
-        """
-        # type-check for the buffer interface before truncating the file
-        view = memoryview(data)
-        with self.open(mode='wb') as f:
-            return f.write(view)
-
-    def write_text(self, data, encoding=None, errors=None, newline=None):
-        """
-        Open the file in text mode, write to it, and close the file.
-        """
-        if not isinstance(data, str):
-            raise TypeError('data must be str, not %s' %
-                            data.__class__.__name__)
-        with self.open(mode='w', encoding=encoding, errors=errors, newline=newline) as f:
-            return f.write(data)
-
     def _scandir(self):
         """Yield os.DirEntry-like objects of the directory contents.
 
@@ -474,7 +463,7 @@ def glob(self, pattern, *, case_sensitive=None, recurse_symlinks=True):
         """Iterate over this subtree and yield all existing files (of any
         kind, including directories) matching the given relative pattern.
         """
-        if not isinstance(pattern, PurePathBase):
+        if not isinstance(pattern, JoinablePath):
             pattern = self.with_segments(pattern)
         anchor, parts = _explode_path(pattern)
         if anchor:
@@ -496,7 +485,7 @@ def rglob(self, pattern, *, case_sensitive=None, recurse_symlinks=True):
         directories) matching the given relative pattern, anywhere in
         this subtree.
         """
-        if not isinstance(pattern, PurePathBase):
+        if not isinstance(pattern, JoinablePath):
             pattern = self.with_segments(pattern)
         pattern = '**' / pattern
         return self.glob(pattern, case_sensitive=case_sensitive, recurse_symlinks=recurse_symlinks)
@@ -543,6 +532,28 @@ def readlink(self):
         """
         raise NotImplementedError
 
+    copy = property(CopyReader, doc=CopyReader.__call__.__doc__)
+
+    def copy_into(self, target_dir, *, follow_symlinks=True,
+                  dirs_exist_ok=False, preserve_metadata=False):
+        """
+        Copy this file or directory tree into the given existing directory.
+        """
+        name = self.name
+        if not name:
+            raise ValueError(f"{self!r} has an empty name")
+        elif isinstance(target_dir, ReadablePath):
+            target = target_dir / name
+        else:
+            target = self.with_segments(target_dir, name)
+        return self.copy(target, follow_symlinks=follow_symlinks,
+                         dirs_exist_ok=dirs_exist_ok,
+                         preserve_metadata=preserve_metadata)
+
+
+class WritablePath(ReadablePath):
+    __slots__ = ()
+
     def symlink_to(self, target, target_is_directory=False):
         """
         Make this path a symlink pointing to the target path.
@@ -556,20 +567,23 @@ def mkdir(self, mode=0o777, parents=False, exist_ok=False):
         """
         raise NotImplementedError
 
-    copy = property(CopyWorker, doc=CopyWorker.__call__.__doc__)
+    def write_bytes(self, data):
+        """
+        Open the file in bytes mode, write to it, and close the file.
+        """
+        # type-check for the buffer interface before truncating the file
+        view = memoryview(data)
+        with self.open(mode='wb') as f:
+            return f.write(view)
 
-    def copy_into(self, target_dir, *, follow_symlinks=True,
-                  dirs_exist_ok=False, preserve_metadata=False):
+    def write_text(self, data, encoding=None, errors=None, newline=None):
         """
-        Copy this file or directory tree into the given existing directory.
+        Open the file in text mode, write to it, and close the file.
         """
-        name = self.name
-        if not name:
-            raise ValueError(f"{self!r} has an empty name")
-        elif isinstance(target_dir, PathBase):
-            target = target_dir / name
-        else:
-            target = self.with_segments(target_dir, name)
-        return self.copy(target, follow_symlinks=follow_symlinks,
-                         dirs_exist_ok=dirs_exist_ok,
-                         preserve_metadata=preserve_metadata)
+        if not isinstance(data, str):
+            raise TypeError('data must be str, not %s' %
+                            data.__class__.__name__)
+        with self.open(mode='w', encoding=encoding, errors=errors, newline=newline) as f:
+            return f.write(data)
+
+    copy = property(CopyWriter, doc=CopyWriter.__call__.__doc__)

Lib/pathlib/_local.py

@@ -20,7 +20,7 @@
     grp = None
 
 from pathlib._os import copyfile
-from pathlib._abc import CopyWorker, PurePathBase, PathBase
+from pathlib._abc import CopyWriter, JoinablePath, WritablePath
 
 
 __all__ = [
@@ -65,7 +65,7 @@ def __repr__(self):
         return "<{}.parents>".format(type(self._path).__name__)
 
 
-class _LocalCopyWorker(CopyWorker):
+class _LocalCopyWriter(CopyWriter):
     """This object implements the Path.copy callable.  Don't try to construct
     it yourself."""
     __slots__ = ()
@@ -158,7 +158,7 @@ def _create_file(self, source, metakeys):
             try:
                 source = os.fspath(source)
             except TypeError:
-                if not isinstance(source, PathBase):
+                if not isinstance(source, WritablePath):
                     raise
                 super()._create_file(source, metakeys)
             else:
@@ -190,7 +190,7 @@ def _ensure_different_file(self, source):
         raise err
 
 
-class PurePath(PurePathBase):
+class PurePath(JoinablePath):
     """Base class for manipulating paths without I/O.
 
     PurePath represents a filesystem path and offers operations which
@@ -646,7 +646,7 @@ def full_match(self, pattern, *, case_sensitive=None):
         Return True if this path matches the given glob-style pattern. The
         pattern is matched against the entire path.
         """
-        if not isinstance(pattern, PurePathBase):
+        if not isinstance(pattern, PurePath):
             pattern = self.with_segments(pattern)
         if case_sensitive is None:
             case_sensitive = self.parser is posixpath
@@ -683,7 +683,7 @@ class PureWindowsPath(PurePath):
     __slots__ = ()
 
 
-class Path(PathBase, PurePath):
+class Path(WritablePath, PurePath):
     """PurePath subclass that can make system calls.
 
     Path represents a filesystem path but unlike PurePath, also offers
@@ -830,7 +830,7 @@ def read_text(self, 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 PathBase.read_text(self, encoding, errors, newline)
+        return super().read_text(encoding, errors, newline)
 
     def write_text(self, data, encoding=None, errors=None, newline=None):
         """
@@ -839,7 +839,7 @@ 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 PathBase.write_text(self, data, encoding, errors, newline)
+        return super().write_text(data, encoding, errors, newline)
 
     _remove_leading_dot = operator.itemgetter(slice(2, None))
     _remove_trailing_slash = operator.itemgetter(slice(-1))
@@ -1122,7 +1122,7 @@ def replace(self, target):
         os.replace(self, target)
         return self.with_segments(target)
 
-    copy = property(_LocalCopyWorker, doc=_LocalCopyWorker.__call__.__doc__)
+    copy = property(_LocalCopyWriter, doc=_LocalCopyWriter.__call__.__doc__)
 
     def move(self, target):
         """
@@ -1134,7 +1134,7 @@ def move(self, target):
         except TypeError:
             pass
         else:
-            if not isinstance(target, PathBase):
+            if not isinstance(target, WritablePath):
                 target = self.with_segments(target_str)
             target.copy._ensure_different_file(self)
             try:
@@ -1155,7 +1155,7 @@ def move_into(self, target_dir):
         name = self.name
         if not name:
             raise ValueError(f"{self!r} has an empty name")
-        elif isinstance(target_dir, PathBase):
+        elif isinstance(target_dir, WritablePath):
             target = target_dir / name
         else:
             target = self.with_segments(target_dir, name)

Lib/pathlib/_types.py

@@ -9,7 +9,7 @@ class Parser(Protocol):
     """Protocol for path parsers, which do low-level path manipulation.
 
     Path parsers provide a subset of the os.path API, specifically those
-    functions needed to provide PurePathBase functionality. Each PurePathBase
+    functions needed to provide JoinablePath functionality. Each JoinablePath
     subclass references its path parser via a 'parser' class attribute.
     """
 

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.DummyPurePathTest):
+class PurePathTest(test_pathlib_abc.DummyJoinablePathTest):
     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.DummyPathTest, PurePathTest):
+class PathTest(test_pathlib_abc.DummyWritablePathTest, PurePathTest):
     """Tests for the FS-accessing functionalities of the Path classes."""
     cls = pathlib.Path
     can_symlink = os_helper.can_symlink()
@@ -980,15 +980,15 @@ def tempdir(self):
         self.addCleanup(os_helper.rmtree, d)
         return d
 
-    def test_matches_pathbase_docstrings(self):
-        path_names = {name for name in dir(pathlib._abc.PathBase) if name[0] != '_'}
+    def test_matches_writablepath_docstrings(self):
+        path_names = {name for name in dir(pathlib._abc.WritablePath) if name[0] != '_'}
         for attr_name in path_names:
             if attr_name == 'parser':
-                # On Windows, Path.parser is ntpath, but PathBase.parser is
+                # On Windows, Path.parser is ntpath, but WritablePath.parser is
                 # posixpath, and so their docstrings differ.
                 continue
             our_attr = getattr(self.cls, attr_name)
-            path_attr = getattr(pathlib._abc.PathBase, attr_name)
+            path_attr = getattr(pathlib._abc.WritablePath, attr_name)
             self.assertEqual(our_attr.__doc__, path_attr.__doc__)
 
     def test_concrete_class(self):
@@ -3019,7 +3019,7 @@ def test_group_windows(self):
             P('c:/').group()
 
 
-class PathWalkTest(test_pathlib_abc.DummyPathWalkTest):
+class PathWalkTest(test_pathlib_abc.DummyReadablePathWalkTest):
     cls = pathlib.Path
     base = PathTest.base
     can_symlink = PathTest.can_symlink