Prettier formatting

autotime.py

@@ -4,24 +4,36 @@
 
 
 class LineWatcher(object):
-    def __init__(self, ip):
-        self.shell = ip
+    def __init__(self):
         self.start_time = 0.0
 
     def start(self):
         self.start_time = time.time()
 
     def stop(self):
-        print('time: %.6f s' % (time.time() - self.start_time))
+        diff = time.time() - self.start_time
+        assert diff > 0
+        print('time: %s' % format_delta(diff))
+
+
+def format_delta(num, suffix='s', threshold=1000):
+    base_unit = 1000000000
+    num *= base_unit  # smallest unit is 1 / 1000000000 of the biggest unit
+    for unit in ['n', 'u', 'm', '']:
+        if num < threshold:
+            return '%3.2f %s%s' % (num, unit, suffix)
+        num /= threshold
+    return '%.2f %s' % (num, suffix)
+
+
+timer = LineWatcher()
 
 
 def load_ipython_extension(ip):
-    timer = LineWatcher(ip)
     ip.events.register('pre_run_cell', timer.start)
     ip.events.register('post_run_cell', timer.stop)
 
 
 def unload_ipython_extension(ip):
-    timer = LineWatcher(ip)
     ip.events.unregister('pre_run_cell', timer.start)
     ip.events.unregister('post_run_cell', timer.stop)