fix

docs/changelog/1239.bugfix.rst

@@ -0,0 +1,2 @@
+the isolated build env now ignores :conf:`sitepackages`, :conf:`deps` and :conf:`description` as these do not make
+sense - by :user:`gaborbernat`

src/tox/config/__init__.py

@@ -1135,6 +1135,8 @@ def parse_build_isolation(self, config, reader):
             section_name = "testenv:{}".format(name)
             if section_name not in self._cfg.sections:
                 self._cfg.sections[section_name] = {}
+            self._cfg.sections[section_name]["deps"] = ""
+            self._cfg.sections[section_name]["sitepackages"] = "False"
             self._cfg.sections[section_name]["description"] = "isolated packaging environment"
             config.envconfigs[name] = self.make_envconfig(
                 name, "{}{}".format(testenvprefix, name), reader._subs, config

tests/unit/config/test_config.py

@@ -2946,6 +2946,27 @@ def test_isolated_build_overrides(newconfig, capsys):
     assert deps == []
 
 
+@pytest.mark.parametrize(
+    "key, set_value, default", [("deps", "crazy", []), ("sitepackages", "True", False)]
+)
+def test_isolated_build_ignores(newconfig, capsys, key, set_value, default):
+    config = newconfig(
+        [],
+        """
+            [tox]
+            isolated_build = True
+
+            [testenv]
+            {} = {}
+        """.format(
+            key, set_value
+        ),
+    )
+    package_env = config.envconfigs.get(".package")
+    value = getattr(package_env, key)
+    assert value == default
+
+
 def test_config_via_pyproject_legacy(initproj):
     initproj(
         "config_via_pyproject_legacy-0.5",