copy timeit.Timer.timeit from CPython 3.4

we were relying on self.inner for %timeit,
which is an undocumented implementation detail
of Python, and no longer shared by PyPy.

This copies the CPython implementation,
so it is no longer an undocumented attribute.

It will likely not be optimal for some PyPy timings,
but at least it works.

IPython/core/magics/execution.py

@@ -1,25 +1,19 b''
 # -*- coding: utf-8 -*-
-"""Implementation of execution-related magic functions.
-"""
-from __future__ import print_function
-#-----------------------------------------------------------------------------
-#  Copyright (c) 2012 The IPython Development Team.
-#
-#  Distributed under the terms of the Modified BSD License.
-#
-#  The full license is in the file COPYING.txt, distributed with this software.
-#-----------------------------------------------------------------------------
+"""Implementation of execution-related magic functions."""
 
-#-----------------------------------------------------------------------------
-# Imports
-#-----------------------------------------------------------------------------
+# Copyright (c) IPython Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+from __future__ import print_function
 
-# Stdlib
 import ast
 import bdb
+import gc
+import itertools
 import os
 import sys
 import time
+import timeit
 from pdb import Restart
 
 # cProfile was added in Python2.5
@@ -33,7 +27,6 b' except ImportError:'
     except ImportError:
         profile = pstats = None
 
-# Our own packages
 from IPython.core import debugger, oinspect
 from IPython.core import magic_arguments
 from IPython.core import page
@@ -115,6 +108,34 b' class TimeitTemplateFiller(ast.NodeTransformer):'
         return node
 
 
+class Timer(timeit.Timer):
+    """Timer class that explicitly uses self.inner
+    
+    which is an undocumented implementation detail of CPython,
+    not shared by PyPy.
+    """
+    # Timer.timeit copied from CPython 3.4.2
+    def timeit(self, number=timeit.default_number):
+        """Time 'number' executions of the main statement.
+
+        To be precise, this executes the setup statement once, and
+        then returns the time it takes to execute the main statement
+        a number of times, as a float measured in seconds.  The
+        argument is the number of times through the loop, defaulting
+        to one million.  The main statement, the setup statement and
+        the timer function to be used are passed to the constructor.
+        """
+        it = itertools.repeat(None, number)
+        gcold = gc.isenabled()
+        gc.disable()
+        try:
+            timing = self.inner(it, self.timer)
+        finally:
+            if gcold:
+                gc.enable()
+        return timing
+
+
 @magics_class
 class ExecutionMagics(Magics):
     """Magics related to code execution, debugging, profiling, etc.
@@ -945,8 +966,6 b' python-profiler package from non-free.""")'
         does not matter as long as results from timeit.py are not mixed with
         those from %timeit."""
 
-        import timeit
-
         opts, stmt = self.parse_options(line,'n:r:tcp:qo',
                                         posix=False, strict=False)
         if stmt == "" and cell is None:
@@ -963,7 +982,7 b' python-profiler package from non-free.""")'
         if hasattr(opts, "c"):
             timefunc = clock
 
-        timer = timeit.Timer(timer=timefunc)
+        timer = Timer(timer=timefunc)
         # this code has tight coupling to the inner workings of timeit.Timer,
         # but is there a better way to achieve that the code stmt has access
         # to the shell namespace?