add common factor for decay

[mapix]

@bgreen-litl pls review

• Add factor decorator to control sleep time unit.

>>> x = backoff.on_exception(backoff.fibo, Exception, max_tries=4, jitter=None)(t)
>>> x()
Backing off t() 1.0s (Exception: a)
Backing off t() 1.0s (Exception: a)
Backing off t() 2.0s (Exception: a)
Giving up t() after 4 tries (Exception: a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vagrant/backoff/backoff.py", line 302, in retry
    ret = target(*args, **kwargs)
  File "<stdin>", line 2, in t
Exception: a
>>> x = backoff.on_exception(backoff.factor(backoff.fibo, 0.1), Exception, max_tries=4, jitter=None)(t)
>>> x()
Backing off t() 0.1s (Exception: a)
Backing off t() 0.1s (Exception: a)
Backing off t() 0.2s (Exception: a)
Giving up t() after 4 tries (Exception: a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/vagrant/backoff/backoff.py", line 302, in retry
    ret = target(*args, **kwargs)
  File "<stdin>", line 2, in t
Exception: a

[coveralls]

Coverage remained the same at 100.0% when pulling d654c21 on mapix:master into ffa29fd on litl:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 83847ac on mapix:master into ffa29fd on litl:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 83847ac on mapix:master into ffa29fd on litl:master.

[coveralls]

Coverage remained the same at 100.0% when pulling eab391d on mapix:master into ffa29fd on litl:master.

[bgreen-litl]

Thank you for the PR.

As it turns out, we currently support a factor keyword argument on the backoff.expo wait generator. Your PR concerns making it available to the backoff.fibo wait generator as well. This seems like a good idea.

The way backoff handles parameterization of the wait generators is that any extra keyword arguments supplied in the decorator function declaration are passed to the wait generator at invocation time. For consistency with backoff.expo's support of factor, as well as in the interest of keeping the API surface as small as possible, I think this feature could be most cleanly implemented as an additional keyword arg to backoff.fibo.

Untested, but I think the implementation would be:

def fibo(max_value=None, factor=1):
    a = 1
    b = 1
    while True:
        if max_value is None or a * factor < max_value:
            yield a * factor
            a, b = b, a + b
        else:
            yield max_value

And to use it you'd pass factor as a keyword arg:

@backoff.on_exception(backoff.fibo, Exception, max_tries=4, factor=0.1, jitter=None)
# ...