Simplify joined path caching.

Lib/pathlib/_abc.py

@@ -116,10 +116,6 @@ class PurePathBase:
         # 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
         # work from occurring when `resolve()` calls `stat()` or `readlink()`.
@@ -145,25 +141,22 @@ 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."""
-        try:
-            return self._str
-        except AttributeError:
-            self._str = self._raw_path
-            return self._str
+        paths = self._raw_paths
+        if len(paths) == 1:
+            return paths[0]
+        elif paths:
+            # Join path segments from the initializer.
+            path = self.parser.join(*paths)
+            # Cache the joined path.
+            paths.clear()
+            paths.append(path)
+            return path
+        else:
+            paths.append('')
+            return ''
 
     def as_posix(self):
         """Return the string representation of the path with forward (/)

Lib/pathlib/_local.py

@@ -77,6 +77,11 @@ 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__()`
@@ -296,7 +301,8 @@ def drive(self):
         try:
             return self._drv
         except AttributeError:
-            self._drv, self._root, self._tail_cached = self._parse_path(self._raw_path)
+            raw_path = PurePathBase.__str__(self)
+            self._drv, self._root, self._tail_cached = self._parse_path(raw_path)
             return self._drv
 
     @property
@@ -305,15 +311,17 @@ def root(self):
         try:
             return self._root
         except AttributeError:
-            self._drv, self._root, self._tail_cached = self._parse_path(self._raw_path)
+            raw_path = PurePathBase.__str__(self)
+            self._drv, self._root, self._tail_cached = self._parse_path(raw_path)
             return self._root
 
     @property
     def _tail(self):
         try:
             return self._tail_cached
         except AttributeError:
-            self._drv, self._root, self._tail_cached = self._parse_path(self._raw_path)
+            raw_path = PurePathBase.__str__(self)
+            self._drv, self._root, self._tail_cached = self._parse_path(raw_path)
             return self._tail_cached
 
     @property