Allow changing the installation root.

src/installer/destinations.py

@@ -2,6 +2,7 @@
 
 import io
 import os
+import sysconfig
 from typing import TYPE_CHECKING, BinaryIO, Dict, Iterable, Optional, Tuple, Union
 
 from installer.records import Hash, RecordEntry
@@ -12,6 +13,7 @@
     copyfileobj_with_hashing,
     fix_shebang,
 )
+from src.installer.utils import SCHEME_NAMES, change_root
 
 if TYPE_CHECKING:
     from installer.scripts import LauncherKind, ScriptSection
@@ -89,10 +91,11 @@ class SchemeDictionaryDestination(WheelDestination):
 
     def __init__(
         self,
-        scheme_dict: Dict[str, str],
+        scheme_dict: Optional[Dict[str, str]],
         interpreter: str,
         script_kind: "LauncherKind",
         hash_algorithm: str = "sha256",
+        root: Optional["os.PathLike[str]"] = None,
     ) -> None:
         """Construct a ``SchemeDictionaryDestination`` object.
 
@@ -103,10 +106,16 @@ def __init__(
             of :any:`hashlib.algorithms_available` (ideally from
             :any:`hashlib.algorithms_guaranteed`).
         """
-        self.scheme_dict = scheme_dict
+        if scheme_dict is not None:
+            self.scheme_dict = scheme_dict
+        else:
+            self.scheme_dict = sysconfig.get_paths()
         self.interpreter = interpreter
         self.script_kind = script_kind
         self.hash_algorithm = hash_algorithm
+        if root is not None:
+            for scheme in SCHEME_NAMES:
+                self.scheme_dict[scheme] = change_root(root, self.scheme_dict[scheme])
 
     def write_to_fs(self, scheme: Scheme, path: str, stream: BinaryIO) -> RecordEntry:
         """Write contents of ``stream`` to the correct location on the filesystem.

src/installer/utils.py

@@ -229,3 +229,26 @@ def parse_entrypoints(text: str) -> Iterable[Tuple[str, str, str, "ScriptSection
             script_section = cast("ScriptSection", section[: -len("_scripts")])
 
             yield name, module, attrs, script_section
+
+
+def change_root(new_root: os.PathLike[str], pathname) -> str:
+    """Return 'pathname' with 'new_root' prepended.
+
+    If 'pathname' is relative, this is equivalent to "os.path.join(new_root,pathname)".
+    Otherwise, it requires making 'pathname' relative and then joining the
+    two, which is tricky on DOS/Windows and Mac OS.
+    """
+    if os.name == "posix":
+        if not os.path.isabs(pathname):
+            return os.path.join(new_root, pathname)
+        else:
+            return os.path.join(new_root, pathname[1:])
+
+    elif os.name == "nt":
+        (drive, path) = os.path.splitdrive(pathname)
+        if path[0] == "\\":
+            path = path[1:]
+        return os.path.join(new_root, path)
+
+    else:
+        raise NotImplementedError("nothing known about platform '%s'" % os.name)