Merge pull request #6479 from MechCoder/timeit_warning

Warnings if there is a bias in timings due to some side-effects of the method

docs/source/whatsnew/pr/warning_timeit_cache.rst

@@ -0,0 +1,3 b''
+Using %timeit prints warnings if there is atleast a 4x difference in timings
+between the slowest and fastest runs, since this might meant that the multiple
+runs are not independent of one another.
  No newline at end of file

IPython/core/magics/execution.py

@@ -1006,16 +1006,31 b' python-profiler package from non-free.""")'
         exec(code, self.shell.user_ns, ns)
         timer.inner = ns["inner"]
 
+        # This is used to check if there is a huge difference between the
+        # best and worst timings.
+        # Issue: https://github.com/ipython/ipython/issues/6471
+        worst_tuning = 0
         if number == 0:
             # determine number so that 0.2 <= total time < 2.0
             number = 1
             for _ in range(1, 10):
-                if timer.timeit(number) >= 0.2:
+                time_number = timer.timeit(number)
+                worst_tuning = max(worst_tuning, time_number / number)
+                if time_number >= 0.2:
                     break
                 number *= 10
         all_runs = timer.repeat(repeat, number)
         best = min(all_runs) / number
         if not quiet :
+            worst = max(all_runs) / number
+            if worst_tuning:
+                worst = max(worst, worst_tuning)
+            # Check best timing is greater than zero to avoid a
+            # ZeroDivisionError.
+            if worst > 4 * best and best > 0:
+                print("The slowest run took %0.2f times longer than the "
+                      "fastest. This could mean that an intermediate result "
+                      "is being cached " % (worst / best))
             print(u"%d loops, best of %d: %s per loop" % (number, repeat,
                                                               _format_time(best, precision)))
             if tc > tc_min: