refactor internal finalization mechanics such that all fixture arguments

in a test invocation will have a corresponding FixtureDef instance.
also fixes issue246 (again).

simplify parametrized fixture teardown by making it work lazy:
during the setup of a parametrized fixture instance any previously
setup instance which was setup with a different param is torn down
before setting up the new one.

Author: hpk42

CHANGELOG

@@ -1,6 +1,19 @@
 Unreleased
 -----------------------------------
 
+- simplified and fixed implementation for calling finalizers when
+  parametrized fixtures or function arguments are involved.  finalization 
+  is now performed lazily at setup time instead of in the "teardown phase".
+  While this might sound odd at first, it helps to ensure that we are 
+  correctly handling setup/teardown even in complex code.  User-level code
+  should not be affected unless it's implementing the pytest_runtest_teardown
+  hook and expecting certain fixture instances are torn down within (very
+  unlikely and would have been unreliable anyway).
+
+- fix issue246 (again) fix finalizer order to be LIFO on independent fixtures
+  depending on a parametrized higher-than-function scoped fixture. 
+  (fix quite some effort so please bear with the complexity of this sentence :)
+ 
 - fix issue244 by implementing special index for parameters to only use
   indices for paramentrized test ids
 

_pytest/main.py

@@ -230,6 +230,8 @@ def __init__(self, name, parent=None, config=None, session=None):
         #: allow adding of extra keywords to use for matching
         self.extra_keyword_matches = set()
 
+        # used for storing artificial fixturedefs for direct parametrization
+        self._name2pseudofixturedef = {}
         #self.extrainit()
 
     @property
@@ -365,6 +367,8 @@ def addfinalizer(self, fin):
         self.session._setupstate.addfinalizer(fin, self)
 
     def getparent(self, cls):
+        """ get the next parent node (including ourself)
+        which is an instance of the given class"""
         current = self
         while current and not isinstance(current, cls):
             current = current.parent