* Provide option to pass environment variables to Environment and CompiledSubprocess (subprocess.Popen)

* Extend this option to find_system_enviornments and get_system_environment without breaking API

jedi/inference/compiled/subprocess/__init__.py

@@ -156,11 +156,21 @@ class CompiledSubprocess(object):
     # Start with 2, gets set after _get_info.
     _pickle_protocol = 2
 
-    def __init__(self, executable):
+    def __init__(self, executable, env_vars={}):
         self._executable = executable
+        self._env_vars = dict(env_vars)
         self._inference_state_deletion_queue = queue.deque()
         self._cleanup_callable = lambda: None
 
+        # Use explicit envionment to ensure reliable results (#1540)
+        if os.name == 'nt':
+            # if SYSTEMROOT (or case variant) exists in environment,
+            # ensure it goes to subprocess
+            for k, v in os.environ.items():
+                if 'SYSTEMROOT' == k.upper():
+                    self._env_vars.update({k: os.environ[k]})
+                    break  # don't risk multiple entries
+
     def __repr__(self):
         pid = os.getpid()
         return '<%s _executable=%r, _pickle_protocol=%r, is_crashed=%r, pid=%r>' % (
@@ -181,15 +191,6 @@ class CompiledSubprocess(object):
             os.path.dirname(os.path.dirname(parso_path)),
             '.'.join(str(x) for x in sys.version_info[:3]),
         )
-        # Use explicit envionment to ensure reliable results (#1540)
-        env = {}
-        if os.name == 'nt':
-            # if SYSTEMROOT (or case variant) exists in environment,
-            # ensure it goes to subprocess
-            for k, v in os.environ.items():
-                if 'SYSTEMROOT' == k.upper():
-                    env.update({k: os.environ[k]})
-                    break  # don't risk multiple entries
         process = GeneralizedPopen(
             args,
             stdin=subprocess.PIPE,
@@ -198,7 +199,7 @@ class CompiledSubprocess(object):
             # Use system default buffering on Python 2 to improve performance
             # (this is already the case on Python 3).
             bufsize=-1,
-            env=env
+            env=self._env_vars
         )
         self._stderr_queue = Queue()
         self._stderr_thread = t = Thread(