Don't load py.test.config inside py._code._assertionnew

Loading py.test.config triggers py.test initialisation while py.code
should stay independent of py.test.  By adding the hook as an
attribute to py.test AssertionError py.code can get access to the
hooks only when py.test is loaded already.

--HG--
branch : trunk

py/_code/_assertionnew.py

@@ -108,10 +108,16 @@ unary_map = {
 
 
 class DebugInterpreter(ast.NodeVisitor):
-    """Interpret AST nodes to gleam useful debugging information."""
+    """Interpret AST nodes to gleam useful debugging information.
+
+    The _pytesthook attribute is used to detect if the py.test
+    pytest_assertion plugin is loaded and if so call it's hooks.
+    """
 
     def __init__(self, frame):
         self.frame = frame
+        self._pytesthook = getattr(py.builtin.builtins.AssertionError,
+                                   "_pytesthook")
 
     def generic_visit(self, node):
         # Fallback when we don't have a special implementation.
@@ -177,13 +183,14 @@ class DebugInterpreter(ast.NodeVisitor):
             if not result:
                 break
             left_explanation, left_result = next_explanation, next_result
-        hook_result = py.test.config.hook.pytest_assert_compare(
-            op=op_symbol, left=left_result, right=next_result)
-        if hook_result:
-            for new_expl in hook_result:
-                if new_expl:
-                    explanation = '\n~'.join(new_expl)
-                    break
+        if self._pytesthook:
+            hook_result = self._pytesthook.pytest_assert_compare(
+                op=op_symbol, left=left_result, right=next_result)
+            if hook_result:
+                for new_expl in hook_result:
+                    if new_expl:
+                        explanation = '\n~'.join(new_expl)
+                        break
         return explanation, result
 
     def visit_BoolOp(self, boolop):