env: do not modify os.environ

Replace updates of os.environ with explcit passing of `env` to
subprocess calls.

Relates-to: #3199

poetry/utils/env.py

@@ -11,6 +11,7 @@ import sysconfig
 import textwrap
 
 from contextlib import contextmanager
+from copy import deepcopy
 from pathlib import Path
 from subprocess import CalledProcessError
 from typing import Any
@@ -1143,15 +1144,13 @@ class Env(object):
             return self.run_pip(*args, **kwargs)
 
         bin = self._bin(bin)
+        env = kwargs.pop("env", {k: v for k, v in os.environ.items()})
 
         if not self._is_windows:
             args = [bin] + list(args)
-            if "env" in kwargs:
-                return os.execvpe(bin, args, kwargs["env"])
+            return os.execvpe(bin, args, env=env)
         else:
-                return os.execvp(bin, args)
-        else:
-            exe = subprocess.Popen([bin] + list(args), **kwargs)
+            exe = subprocess.Popen([bin] + list(args), env=env, **kwargs)
             exe.communicate()
             return exe.returncode
 
@@ -1388,23 +1387,34 @@ class VirtualEnv(Env):
         return os.path.exists(self.python)
 
     def _run(self, cmd: List[str], **kwargs: Any) -> Optional[int]:
-        with self.temp_environ():
-            os.environ["PATH"] = self._updated_path()
-            os.environ["VIRTUAL_ENV"] = str(self._path)
+        kwargs["env"] = self.get_temp_environ(environ=kwargs.get("env"))
+        return super(VirtualEnv, self)._run(cmd, **kwargs)
 
-            self.unset_env("PYTHONHOME")
-            self.unset_env("__PYVENV_LAUNCHER__")
+    def get_temp_environ(
+        self,
+        environ: Optional[Dict[str, str]] = None,
+        exclude: Optional[List[str]] = None,
+        **kwargs: str,
+    ) -> Dict[str, str]:
+        exclude = exclude or []
+        exclude.extend(["PYTHONHOME", "__PYVENV_LAUNCHER__"])
+
+        if environ:
+            environ = deepcopy(environ)
+            for key in exclude:
+                environ.pop(key, None)
+        else:
+            environ = {k: v for k, v in os.environ.items() if k not in exclude}
 
-            return super(VirtualEnv, self)._run(cmd, **kwargs)
+        environ.update(kwargs)
 
-    def execute(self, bin: str, *args: str, **kwargs: Any) -> Optional[int]:
-        with self.temp_environ():
-            os.environ["PATH"] = self._updated_path()
-            os.environ["VIRTUAL_ENV"] = str(self._path)
+        environ["PATH"] = self._updated_path()
+        environ["VIRTUAL_ENV"] = str(self._path)
 
-            self.unset_env("PYTHONHOME")
-            self.unset_env("__PYVENV_LAUNCHER__")
+        return environ
 
+    def execute(self, bin: str, *args: str, **kwargs: Any) -> Optional[int]:
+        kwargs["env"] = self.get_temp_environ(environ=kwargs.get("env"))
         return super(VirtualEnv, self).execute(bin, *args, **kwargs)
 
     @contextmanager
@@ -1416,10 +1426,6 @@ class VirtualEnv(Env):
             os.environ.clear()
             os.environ.update(environ)
 
-    def unset_env(self, key: str) -> None:
-        if key in os.environ:
-            del os.environ[key]
-
     def _updated_path(self) -> str:
         return os.pathsep.join([str(self._bin_dir), os.environ.get("PATH", "")])