python time(milli seconds) calculation

How to calculate milliseconds,from the code below.

a = datetime.datetime.now()
b = datetime.datetime.now()
c = b - a

&g开发者_如何学JAVAt;>> c

>>> c.days
0
>>> c.seconds
4
>>> c.microseconds

---

milliseconds = (c.days * 24 * 60 * 60 + c.seconds) * 1000 + c.microseconds / 1000.0

---

Or, new since 2.7:

c.total_seconds()*1000

(https://docs.python.org/2/library/datetime.html)