The --target option will not overwrite without --upgrade

news/10110.bugfix.rst

@@ -1,4 +1,2 @@
 Pip install's ``--target`` works more like the other 'install schemas'. This
-fixes installing namespaced packages. A caviate is that the directory must be
-in python's path to avoid un-intential upgrades. This logic now works the same
-way as ``--prefix`` when it comes to upgrading pacakges.
+fixes installing namespaced packages.

src/pip/_internal/commands/install.py

@@ -3,6 +3,7 @@
 import operator
 import os
 import site
+import sys
 from optparse import SUPPRESS_HELP, Values
 from typing import List, Optional
 
@@ -91,11 +92,7 @@ def add_options(self) -> None:
             dest="target_dir",
             metavar="dir",
             default=None,
-            help=(
-                "Install packages into <dir>."
-                "The directory should also be in python's path to aviod "
-                "un-intential upgrading of packages."
-            ),
+            help=("Install packages into <dir>."),
         )
         cmdoptions.add_target_python_options(self.cmd_opts)
 
@@ -313,6 +310,9 @@ def run(self, options: Values, args: List[str]) -> int:
 
         global_options = options.global_options or []
 
+        if options.target_dir is not None and options.target_dir not in sys.path:
+            sys.path.append(options.target_dir)
+
         session = self.get_default_session(options)
 
         target_python = make_target_python(options)
@@ -365,7 +365,6 @@ def run(self, options: Values, args: List[str]) -> int:
             )
 
             self.trace_basic_info(finder)
-
             requirement_set = resolver.resolve(
                 reqs, check_supported_wheels=not options.target_dir
             )

tests/functional/test_install.py

@@ -1107,18 +1107,8 @@ def test_install_package_with_target(script: PipTestEnvironment) -> None:
     result = script.pip_install_local("-t", target_dir, "simple==1.0")
     result.did_create(Path("scratch") / "target" / "simple")
 
-    # When the target location is not in the path packages are installed over
-    # eachother.
-    result = script.pip_install_local(
-        "-t",
-        target_dir,
-        "simple==1.0",
-    )
-    result.did_update(Path("scratch") / "target" / "simple")
-
-    # When target directory is in python path repeated call without --upgrade,
+    # When using target directory repeated call without --upgrade,
     # no files should have changed
-    script.environ["PYTHONPATH"] = target_dir
     result = script.pip_install_local(
         "-t",
         target_dir,