Format a datetime into a string with milliseconds

@Cabbi raised the issue that on some systems (Windows with Python 2.7), the microseconds format %f may incorrectly give "0", so it's not portable to simply trim the last three characters. Such systems do not follow the behavior specified by the documentation:

The following code carefully formats a timestamp with milliseconds:

>>> from datetime import datetime
>>> (dt, micro) = datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S.%f').split('.')
>>> "%s.%03d" % (dt, int(micro) / 1000)
'2016-02-26 04:37:53.133'

To get the exact output that the OP wanted, we have to strip punctuation characters:

>>> from datetime import datetime
>>> (dt, micro) = datetime.utcnow().strftime('%Y%m%d%H%M%S.%f').split('.')
>>> "%s%03d" % (dt, int(micro) / 1000)
'20160226043839901'

To get a date string with milliseconds, use [:-3] to trim the last three digits of %f (microseconds):

>>> from datetime import datetime
>>> datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S.%f')[:-3]
'2020-05-04 10:18:32.926'

With Python 3.6+, you can set isoformat's timespec:

>>> from datetime import datetime
>>> datetime.utcnow().isoformat(sep=' ', timespec='milliseconds')
'2019-05-10 09:08:53.155'