Fix editable --user installs with build isolation

changelog.d/3151.breaking.rst

@@ -0,0 +1 @@
+Made ``setup.py develop --user`` install to the user site packages directory even if it is disabled in the current interpreter.

setuptools/command/easy_install.py

@@ -169,12 +169,8 @@ def initialize_options(self):
         self.install_data = None
         self.install_base = None
         self.install_platbase = None
-        if site.ENABLE_USER_SITE:
-            self.install_userbase = site.USER_BASE
-            self.install_usersite = site.USER_SITE
-        else:
-            self.install_userbase = None
-            self.install_usersite = None
+        self.install_userbase = site.USER_BASE
+        self.install_usersite = site.USER_SITE
         self.no_find_links = None
 
         # Options not specifiable via command line
@@ -253,11 +249,9 @@ def finalize_options(self):  # noqa: C901  # is too complex (25)  # FIXME
             getattr(sys, 'windir', '').replace('.', ''),
         )
 
-        if site.ENABLE_USER_SITE:
-            self.config_vars['userbase'] = self.install_userbase
-            self.config_vars['usersite'] = self.install_usersite
-
-        elif self.user:
+        self.config_vars['userbase'] = self.install_userbase
+        self.config_vars['usersite'] = self.install_usersite
+        if self.user and not site.ENABLE_USER_SITE:
             log.warn("WARNING: The user site-packages directory is disabled.")
 
         self._fix_install_dir_for_user_site()
@@ -373,7 +367,7 @@ def _fix_install_dir_for_user_site(self):
         """
         Fix the install_dir if "--user" was used.
         """
-        if not self.user or not site.ENABLE_USER_SITE:
+        if not self.user:
             return
 
         self.create_home_path()

setuptools/tests/test_easy_install.py

@@ -19,6 +19,7 @@
 import pathlib
 import warnings
 from collections import namedtuple
+from pathlib import Path
 
 import pytest
 from jaraco import path
@@ -1109,3 +1110,45 @@ def test_use_correct_python_version_string(tmpdir, tmpdir_cwd, monkeypatch):
     assert cmd.config_vars['py_version'] == '3.10.1'
     assert cmd.config_vars['py_version_short'] == '3.10'
     assert cmd.config_vars['py_version_nodot'] == '310'
+
+
+def test_editable_user_and_build_isolation(setup_context, monkeypatch, tmp_path):
+    ''' `setup.py develop` should honor `--user` even under build isolation'''
+
+    # == Arrange ==
+    # Pretend that build isolation was enabled
+    # e.g pip sets the environment varible PYTHONNOUSERSITE=1
+    monkeypatch.setattr('site.ENABLE_USER_SITE', False)
+
+    # Patching $HOME for 2 reasons:
+    # 1. setuptools/command/easy_install.py:create_home_path
+    #    tries creating directories in $HOME
+    # given `self.config_vars['DESTDIRS'] = "/home/user/.pyenv/versions/3.9.10 /home/user/.pyenv/versions/3.9.10/lib /home/user/.pyenv/versions/3.9.10/lib/python3.9 /home/user/.pyenv/versions/3.9.10/lib/python3.9/lib-dynload"``  # noqa: E501
+    # it will `makedirs("/home/user/.pyenv/versions/3.9.10 /home/user/.pyenv/versions/3.9.10/lib /home/user/.pyenv/versions/3.9.10/lib/python3.9 /home/user/.pyenv/versions/3.9.10/lib/python3.9/lib-dynload")``  # noqa: E501
+    # 2. We are going to force `site` to update site.USER_BASE and site.USER_SITE
+    #    To point inside our new home
+    monkeypatch.setenv('HOME', str(tmp_path / 'home'))
+    monkeypatch.setattr('site.USER_BASE', None)
+    monkeypatch.setattr('site.USER_SITE', None)
+    user_site = Path(site.getusersitepackages())
+    user_site.mkdir(parents=True, exist_ok=True)
+
+    sys_prefix = (tmp_path / 'sys_prefix')
+    sys_prefix.mkdir(parents=True, exist_ok=True)
+    monkeypatch.setattr('sys.prefix', str(sys_prefix))
+
+    # == Sanity check ==
+    assert list(sys_prefix.glob("*")) == []
+    assert list(user_site.glob("*")) == []
+
+    # == Act ==
+    run_setup('setup.py', ['develop', '--user'])
+
+    # == Assert ==
+    # Should not install to sys.prefix
+    assert list(sys_prefix.glob("*")) == []
+    # Should install to user site
+    installed = {f.name for f in user_site.glob("*")}
+    # sometimes easy-install.pth is created and sometimes not
+    installed = installed - {"easy-install.pth"}
+    assert installed == {'UNKNOWN.egg-link'}