Pop PIP_REQUIRE_VIRTUALENV from isolated environment

Fixes #90.

Author: adamchainz

src/build/env.py

@@ -86,6 +86,9 @@ def _replace_env(self, key, new):  # type: (str, Optional[str]) -> None
         self._env[key] = os.environ.get(key, None)
         os.environ[key] = new
 
+    def _pop_env(self, key):  # type: (str) -> None
+        self._env[key] = os.environ.pop(key, None)
+
     def _restore_env(self):  # type: () -> None
         for key, val in self._env.items():
             if val is None:
@@ -143,6 +146,7 @@ def __enter__(self):  # type: () -> IsolatedEnvironment
         self._replace_env('PATH', os.pathsep.join(exe_path))
         self._replace_env('PYTHONPATH', os.pathsep.join(sys_path))
         self._replace_env('PYTHONHOME', self.path)
+        self._pop_env('PIP_REQUIRE_VIRTUALENV')
 
         self._symlink_relative(sysconfig.get_path('include'))
         self._symlink_relative(sysconfig.get_path('platinclude'))

tests/test_env.py

@@ -10,7 +10,7 @@
 import build.env
 
 
-def test_isolated_environment_setup(mocker):
+def test_isolated_environment_setup():
     old_path = os.environ['PATH']
     with build.env.IsolatedEnvironment.for_current() as env:
         if os.name != 'nt':
@@ -53,6 +53,13 @@ def test_isolated_environment_setup(mocker):
             assert os.path.exists(path)
 
 
+def test_isolated_environment_setup_require_virtualenv(mocker):
+    mocker.patch.dict(os.environ, {"PIP_REQUIRE_VIRTUALENV": "true"})
+    with build.env.IsolatedEnvironment.for_current():
+        assert "PIP_REQUIRE_VIRTUALENV" not in os.environ
+    assert os.environ["PIP_REQUIRE_VIRTUALENV"] == "true"
+
+
 def test_isolated_environment_install(mocker):
     with build.env.IsolatedEnvironment.for_current() as env:
         mocker.patch('subprocess.check_call')