You can replace the microseconds with 0 and use isoformat:

import pytz
from datetime import datetime
tz = pytz.timezone('Asia/Taipei')
dt = datetime.now()
loc_dt = tz.localize(dt).replace(microsecond=0)
print loc_dt.isoformat()
2015-09-17T19:12:33+08:00

If you want to keep loc_dt as is do the replacing when you output:

loc_dt = tz.localize(dt)
print loc_dt.replace(microsecond=0).isoformat()

As commented you would be better passing the tz to datetime.now:

 dt = datetime.now(tz)

The reasons are discussed in pep-0495, you might also want to add an assert to catch any bugs when doing the replace:

 ssert loc_dt.resolution >= timedelta(microsecond=0)

If you want to format a datetime object in a specific format that is different from the standard format, it's best to explicitly specify that format:

>>> import datetime
>>> datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
'2011-11-03 18:21:26'

See the documentation of datetime.strftime() for an explanation of the % directives.

Starting from Python 3.6, the isoformat() method is flexible enough to also produce this format:

datetime.datetime.now().isoformat(sep=" ", timespec="seconds")

Local to ISO 8601:

import datetime
datetime.datetime.now().isoformat()
>>> '2024-08-01T14:38:32.499588'

UTC to ISO 8601:

import datetime
datetime.datetime.now(datetime.timezone.utc).isoformat()
>>> '2024-08-01T04:38:47.731215+00:00'

Local to ISO 8601 without microsecond:

import datetime
datetime.datetime.now().replace(microsecond=0).isoformat()
>>> '2024-08-01T14:38:57'

UTC to ISO 8601 with timezone information (Python 3):

import datetime
datetime.datetime.now(datetime.timezone.utc).isoformat()
>>> '2024-08-01T04:39:06.274874+00:00'

Local to ISO 8601 with timezone information (Python 3):

import datetime
datetime.datetime.now().astimezone().isoformat()
>>> '2024-08-01T14:39:16.698776+10:00'

Local to ISO 8601 with local timezone information without microsecond (Python 3):

import datetime
datetime.datetime.now().astimezone().replace(microsecond=0).isoformat()
>>> '2024-08-01T14:39:28+10:00'

Notice there is a bug when using astimezone() on utcnow(). This gives an incorrect result:

datetime.datetime.utcnow().astimezone().isoformat() #Incorrect result, do not use.

.utcnow() is deprecated, use .now(datetime.timezome.utc) instead.

For Python 2, see and use pytz.

If timezone without colon is ok, you can use

d = datetime.datetime(2018, 10, 9, 8, 19, 16, 999578, 
                      tzinfo=dateutil.tz.tzoffset(None, 7200))
s = d.strftime('%Y-%m-%dT%H:%M:%S.%f')[:-3] + d.strftime('%z')
# '2018-10-09T08:19:16.999+0200'

For colon, you need to split the timezone and add it there yourself. %z does not produce Z either for UTC.

And Python 3.6 supports timespec='milliseconds' so you should shim this:

try:
    datetime.datetime.now().isoformat(timespec='milliseconds')
    def milliseconds_timestamp(d):
        return d.isoformat(timespec='milliseconds')

except TypeError:
    def milliseconds_timestamp(d):
        z = d.strftime('%z')
        z = z[:3] + ':' + z[3:]
        return d.strftime('%Y-%m-%dT%H:%M:%S.%f')[:-3] + z

Given the latter definition in Python 3.6,

>>> milliseconds_timestamp(d) == d.isoformat(timespec='milliseconds')
True

with

>>> milliseconds_timestamp(d)
'2018-10-09T08:19:16.999+02:00'

Set microsecond as 0:

t = timezone(settings.TIME_ZONE).localize(datetime.now()).replace(microsecond=0)
t.isoformat()

datetime.datetime.replace(...) will return a new datetime object with specified attribute modified.

If you're using Python 3.6+, you can pass the timespec parameter to isoformat()

The function ends up calling _format_time, where you can see the supported values for the parameter:

specs = {
    'hours': '{:02d}',
    'minutes': '{:02d}:{:02d}',
    'seconds': '{:02d}:{:02d}:{:02d}',
    'milliseconds': '{:02d}:{:02d}:{:02d}.{:03d}',
    'microseconds': '{:02d}:{:02d}:{:02d}.{:06d}'
}

This will force the value to be rendered even if the specific part is zero