add common factor for decay

README.rst

@@ -95,6 +95,20 @@ gets a non-falsey result could be defined like like this::
     def poll_for_message(queue)
         return queue.get()
 
+
+@backoff.factor
+---------------
+
+The ``factor`` decorator is used to ajust other wait generator. 
+
+    @backoff.on_predicate(backoff.factor(backoff.constant, 60), interval=1)
+    def poll_for_message(queue)
+        return queue.get()
+
+Here `backoff.factor(backoff.constant, 60)` make backoff to sleep 60 times 
+longer than original wait generator.
+
+
 Jitter
 ------
 

backoff.py

@@ -85,6 +85,22 @@ def constant(interval=1):
         yield interval
 
 
+def factor(wait_gen, f):
+    """Wrapped decay by factor.
+
+    Args:
+        wait_gen: A generator yielding successive wait times in
+            seconds.
+        factor: Factor to multiply the wait_gen by.
+    """
+    @functools.wraps(wait_gen)
+    def deco(*args, **kwargs):
+        wait = wait_gen(*args, **kwargs)
+        while True:
+            yield next(wait) * f
+    return deco
+
+
 def random_jitter(value):
     """Jitter the value a random number of milliseconds.
 

backoff_tests.py

@@ -218,6 +218,35 @@ def succeeder(*args, **kwargs):
         assert details['wait'] >= 0.5 * 2 ** i
 
 
+def test_decay_factor(monkeypatch):
+    monkeypatch.setattr('time.sleep', lambda x: None)
+
+    log, log_success, log_backoff, log_giveup = _log_hdlrs()
+
+    @backoff.on_exception(backoff.factor(backoff.expo, 0.5),
+                          Exception,
+                          on_success=log_success,
+                          on_backoff=log_backoff,
+                          on_giveup=log_giveup,
+                          jitter=backoff.random_jitter)
+    @_save_target
+    def succeeder(*args, **kwargs):
+        # succeed after we've backed off twice
+        if len(log['backoff']) < 2:
+            raise ValueError("catch me")
+
+    succeeder(1, 2, 3, foo=1, bar=2)
+
+    # we try 3 times, backing off twice before succeeding
+    assert len(log['success']) == 1
+    assert len(log['backoff']) == 2
+    assert len(log['giveup']) == 0
+
+    for i in range(2):
+        details = log['backoff'][i]
+        assert details['wait'] >= 0.5 * 2 ** i
+
+
 def test_on_exception_success_full_jitter(monkeypatch):
     monkeypatch.setattr('time.sleep', lambda x: None)