Fix editable --user installs with build isolation

pypa#3019

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

@@ -1146,8 +1146,6 @@ def test_editable_user_and_build_isolation(setup_context, monkeypatch, tmpdir):
 
     # == Assert ==
     # Should not install to sys.prefix
-    with pytest.raises(AssertionError):
-        assert sys_prefix.listdir() == []
+    assert sys_prefix.listdir() == []
     # Should install to user site
-    with pytest.raises(AssertionError):
-        assert {f.basename for f in user_site.listdir()} == {'UNKNOWN.egg-link'}
+    assert {f.basename for f in user_site.listdir()} == {'UNKNOWN.egg-link'}