python time 字符串转UTC

今天遇到一个一个问题，是将字符串类型的时间转化为UTC时间。例如time_str = "2009/11/09 12:23:23" 转化为UTC int型。
找了一些资料，发现time模块就能完成这个转换。
import time
time_str = "2009/11/09 12:23:23"
time_s = time.strptime(time_str,"%Y/%m/%d %H:%M:%S")
utc_f = time.mktime(time_s)
utc_i = int(time.mktime(time_s))
print time_s
(2009, 11, 9, 12, 23, 23, 0, 313, -1)
print utc_f
1257740603.0
print utc_t
1257740603

在linux中输入“date  +%s” 就能输出当前utc时间。
附录：
http://www.opengroup.org/onlinepubs/009695399/functions/strptime.html
Directive:
%a - abbreviated weekday name
%A - full weekday name
%b - abbreviated month name
%B - full month name
%c - preferred date and time representation
%C - century number (the year divided by 100, range 00 to 99)
%d - day of the month (01 to 31)
%D - same as %m/%d/%y
%e - day of the month (1 to 31)
%g - like %G, but without the century
%G - 4-digit year corresponding to the ISO week number (see %V).
%h - same as %b
%H - hour, using a 24-hour clock (00 to 23)
%I - hour, using a 12-hour clock (01 to 12)
%j - day of the year (001 to 366)
%m - month (01 to 12)
%M - minute
%n - newline character
%p - either am or pm according to the given time value
%r - time in a.m. and p.m. notation
%R - time in 24 hour notation
%S - second
%t - tab character
%T - current time, equal to %H:%M:%S
%u - weekday as a number (1 to 7), Monday=1. Warning: In Sun Solaris Sunday=1
%U - week number of the current year, starting with the first Sunday as the first day of the first week
%V - The ISO 8601 week number of the current year (01 to 53), where week 1 is the first week that has at least 4 days in the current year, and with Monday as the first day of the week
%W - week number of the current year, starting with the first Monday as the first day of the first week
%w - day of the week as a decimal, Sunday=0
%x - preferred date representation without the time
%X -