avoid calling __init__ outside of constructor

src/tox/_pytestplugin.py

@@ -250,7 +250,7 @@ def logging_levels(self, quiet, verbose):
 def newmocksession(mocksession, newconfig):
     def newmocksession_(args, source, plugins=()):
         config = newconfig(args, source, plugins=plugins)
-        mocksession.__init__(config)
+        mocksession._reset(config)
         return mocksession
 
     return newmocksession_

src/tox/package/builder/isolated.py

@@ -1,4 +1,5 @@
 from __future__ import unicode_literals
+
 import json
 from collections import namedtuple
 

src/tox/session/__init__.py

@@ -89,6 +89,9 @@ class Session(object):
     """The session object that ties together configuration, reporting, venv creation, testing."""
 
     def __init__(self, config, popen=subprocess.Popen):
+        self._reset(config, popen)
+
+    def _reset(self, config, popen=subprocess.Popen):
         self.config = config
         self.popen = popen
         self.resultlog = ResultLog()