When I input the simple code:

import datetime

, I was given error message:

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
AttributeError: 'module' object has no attribute 'utcnow'

But python’s document of utcnow is just here: https://docs.python.org/library/datetime.html#datetime.datetime.utcnow. Why does utcnow not work in my computer? Thank you!

You are confusing the module with the type.

Use either:

import datetime


or use:

from datetime import datetime


e.g. either reference the datetime type in the datetime module, or import that type into your namespace from the module. If you use the latter form and need other types from that module, don’t forget to import those too:

from datetime import date, datetime, timedelta

Demo of the first form:

>>> import datetime
>>> datetime
<module 'datetime' from '/Users/mj/Development/venvs/stackoverflow-2.7/lib/python2.7/lib-dynload/datetime.so'>
>>> datetime.datetime
<type 'datetime.datetime'>
>>> datetime.datetime.utcnow()
datetime.datetime(2013, 10, 4, 23, 27, 14, 678151)