datetime.datetime.now()时间格式转化是碰到的问题

import datetime

print datetime.datetime.now()

# 2016-03-28 17:16:00.812000

a = ‘2016-03-28 17:16:00.812000’

timeArray = time.strptime(a, '%Y-%m-%d %H:%M:%S.%f')   ##注意： %f是microseconds的格式化符号。

print timeArray

#time.struct_time(tm_year=2016, tm_mon=3, tm_mday=28, tm_hour=17, tm_min=22, tm_sec=38, tm_wday=0, tm_yday=88, tm_isdst=-1)

a = datetime.datetime.now()
print a                                  ## a变量不指向一个字符串对象，而是指向一个datetime.datetime类型的对象
# 2016-03-28 17:24:52.571000
timeArray = time.strptime(a, '%Y-%m-%d %H:%M:%S.%f')    ## 所有这里报错。 time.striptime的第一个参数，应该是字符串对象

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "C:\Python27\lib\_strptime.py", line 467, in _strptime_time
    return _strptime(data_string, format)[0]
  File "C:\Python27\lib\_strptime.py", line 322, in _strptime
    found = format_regex.match(data_string)
TypeError: expected string or buffer