pathlib ABCs: defer path joining

Defer joining of path segments in the private `PurePathBase` ABC. The new
behaviour matches how the public `PurePath` class handles path segments.

This slightly reduces the size of `PurePath` objects by eliminating a
`_raw_path` slot.

Lib/pathlib/_abc.py

@@ -99,7 +99,7 @@ class PathGlobber(_GlobberBase):
     @staticmethod
     def concat_path(path, text):
         """Appends text to the given path."""
-        return path.with_segments(path._raw_path + text)
+        return path.with_segments(str(path) + text)
 
 
 class PurePathBase:
@@ -112,9 +112,13 @@ class PurePathBase:
     """
 
     __slots__ = (
-        # The `_raw_path` slot store a joined string path. This is set in the
-        # `__init__()` method.
-        '_raw_path',
+        # The `_raw_paths` slot stores unjoined string paths. This is set in
+        # the `__init__()` method.
+        '_raw_paths',
+
+        # The `_str` slot stores the string representation of the path,
+        # computed when `__str__()` is called for the first time.
+        '_str',
 
         # The '_resolving' slot stores a boolean indicating whether the path
         # is being processed by `PathBase.resolve()`. This prevents duplicate
@@ -124,11 +128,14 @@ class PurePathBase:
     parser = ParserBase()
     _globber = PathGlobber
 
-    def __init__(self, path, *paths):
-        self._raw_path = self.parser.join(path, *paths) if paths else path
-        if not isinstance(self._raw_path, str):
-            raise TypeError(
-                f"path should be a str, not {type(self._raw_path).__name__!r}")
+    def __init__(self, arg, *args):
+        paths = [arg]
+        paths.extend(args)
+        for path in paths:
+            if not isinstance(path, str):
+                raise TypeError(
+                    f"path should be a str, not {type(path).__name__!r}")
+        self._raw_paths = paths
         self._resolving = False
 
     def with_segments(self, *pathsegments):
@@ -138,10 +145,25 @@ def with_segments(self, *pathsegments):
         """
         return type(self)(*pathsegments)
 
+    @property
+    def _raw_path(self):
+        paths = self._raw_paths
+        if len(paths) == 0:
+            path = ''
+        elif len(paths) == 1:
+            path = paths[0]
+        else:
+            path = self.parser.join(*paths)
+        return path
+
     def __str__(self):
         """Return the string representation of the path, suitable for
         passing to system calls."""
-        return self._raw_path
+        try:
+            return self._str
+        except AttributeError:
+            self._str = self._raw_path
+            return self._str
 
     def as_posix(self):
         """Return the string representation of the path with forward (/)
@@ -166,7 +188,7 @@ def anchor(self):
     @property
     def name(self):
         """The final path component, if any."""
-        return self.parser.split(self._raw_path)[1]
+        return self.parser.split(str(self))[1]
 
     @property
     def suffix(self):
@@ -202,7 +224,7 @@ def with_name(self, name):
         split = self.parser.split
         if split(name)[0]:
             raise ValueError(f"Invalid name {name!r}")
-        return self.with_segments(split(self._raw_path)[0], name)
+        return self.with_segments(split(str(self))[0], name)
 
     def with_stem(self, stem):
         """Return a new path with the stem changed."""
@@ -242,17 +264,17 @@ def relative_to(self, other, *, walk_up=False):
         anchor0, parts0 = self._stack
         anchor1, parts1 = other._stack
         if anchor0 != anchor1:
-            raise ValueError(f"{self._raw_path!r} and {other._raw_path!r} have different anchors")
+            raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
         while parts0 and parts1 and parts0[-1] == parts1[-1]:
             parts0.pop()
             parts1.pop()
         for part in parts1:
             if not part or part == '.':
                 pass
             elif not walk_up:
-                raise ValueError(f"{self._raw_path!r} is not in the subpath of {other._raw_path!r}")
+                raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")
             elif part == '..':
-                raise ValueError(f"'..' segment in {other._raw_path!r} cannot be walked")
+                raise ValueError(f"'..' segment in {str(other)!r} cannot be walked")
             else:
                 parts0.append('..')
         return self.with_segments('', *reversed(parts0))
@@ -289,17 +311,17 @@ def joinpath(self, *pathsegments):
         paths) or a totally different path (if one of the arguments is
         anchored).
         """
-        return self.with_segments(self._raw_path, *pathsegments)
+        return self.with_segments(*self._raw_paths, *pathsegments)
 
     def __truediv__(self, key):
         try:
-            return self.with_segments(self._raw_path, key)
+            return self.with_segments(*self._raw_paths, key)
         except TypeError:
             return NotImplemented
 
     def __rtruediv__(self, key):
         try:
-            return self.with_segments(key, self._raw_path)
+            return self.with_segments(key, *self._raw_paths)
         except TypeError:
             return NotImplemented
 
@@ -311,7 +333,7 @@ def _stack(self):
         *parts* is a reversed list of parts following the anchor.
         """
         split = self.parser.split
-        path = self._raw_path
+        path = str(self)
         parent, name = split(path)
         names = []
         while path != parent:
@@ -323,7 +345,7 @@ def _stack(self):
     @property
     def parent(self):
         """The logical parent of the path."""
-        path = self._raw_path
+        path = str(self)
         parent = self.parser.split(path)[0]
         if path != parent:
             parent = self.with_segments(parent)
@@ -335,7 +357,7 @@ def parent(self):
     def parents(self):
         """A sequence of this path's logical parents."""
         split = self.parser.split
-        path = self._raw_path
+        path = str(self)
         parent = split(path)[0]
         parents = []
         while path != parent:
@@ -347,7 +369,7 @@ def parents(self):
     def is_absolute(self):
         """True if the path is absolute (has both a root and, if applicable,
         a drive)."""
-        return self.parser.isabs(self._raw_path)
+        return self.parser.isabs(str(self))
 
     @property
     def _pattern_str(self):

Lib/pathlib/_local.py

@@ -68,10 +68,6 @@ class PurePath(PurePathBase):
     """
 
     __slots__ = (
-        # The `_raw_paths` slot stores unnormalized string paths. This is set
-        # in the `__init__()` method.
-        '_raw_paths',
-
         # The `_drv`, `_root` and `_tail_cached` slots store parsed and
         # normalized parts of the path. They are set when any of the `drive`,
         # `root` or `_tail` properties are accessed for the first time. The
@@ -81,11 +77,6 @@ class PurePath(PurePathBase):
         # tail are normalized.
         '_drv', '_root', '_tail_cached',
 
-        # The `_str` slot stores the string representation of the path,
-        # computed from the drive, root and tail when `__str__()` is called
-        # for the first time. It's used to implement `_str_normcase`
-        '_str',
-
         # The `_str_normcase_cached` slot stores the string path with
         # normalized case. It is set when the `_str_normcase` property is
         # accessed for the first time. It's used to implement `__eq__()`
@@ -299,18 +290,6 @@ def _parse_pattern(cls, pattern):
             parts.append('')
         return parts
 
-    @property
-    def _raw_path(self):
-        """The joined but unnormalized path."""
-        paths = self._raw_paths
-        if len(paths) == 0:
-            path = ''
-        elif len(paths) == 1:
-            path = paths[0]
-        else:
-            path = self.parser.join(*paths)
-        return path
-
     @property
     def drive(self):
         """The drive prefix (letter or UNC path), if any."""

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -86,11 +86,6 @@ def test_unsupported_operation_pure(self):
             p.suffix
         with self.assertRaises(e):
             p.suffixes
-        with self.assertRaises(e):
-            p / 'bar'
-        with self.assertRaises(e):
-            'bar' / p
-        self.assertRaises(e, p.joinpath, 'bar')
         self.assertRaises(e, p.with_name, 'bar')
         self.assertRaises(e, p.with_stem, 'bar')
         self.assertRaises(e, p.with_suffix, '.txt')