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Python what is the correct timefomat for: 2018-11-13T20:20:39+00:00

I need to get a now() timestamp like this following: 2018-11-13T20:20:39+00:00 What is the correct format string for this?

To get an isoformat() string with time zone offset (the +00:00 at the end of the string) you need to supply a tzinfo object when constructing the datetime. the easiest way to do this is with the pytz library - pytz.timezone("UTC") returns the tzinfo for UTC.
There's another issue though, which is that technically that string doesn't quite match default isoformat() output because it has no microseconds. So a full example for the output requested would be:
import datetime
import pytz
datetime.datetime.now(tz=pytz.timezone("UTC")).replace(microsecond=0).isoformat()

This appears to be the isoformat.
You could use
import datetime as dt
# Get current time in utc
# Because the datetime object is timezone aware the +00:00 will be printed
current_time = dt.datetime.now(dt.timezone.utc)
# timespec will round the solution upto seconds
iso_string = current_time.isoformat(timespec="seconds")
print(iso_string)
will print 2019-11-19T19:51:46+00:00.

converting between timezones (format same instance in time) in python 3

I have a list of timestamps which look like so:
time_list=['2016-10-01T00:00:00+01:00','2016-10-01T23:00:00+00:00','2016-10-01T22:00:00+02:00',..]
I would like to apply a magic function to this list which gets them all in +00:00 timezone - this should result in (all timestamps should correctly adjusted to the +00:00 format):
ret_list=['2016-10-01T23:00:00+00:00','2016-10-01T23:00:00+00:00','2016-10-01T23:00:00+00:00',..]

You have to convert your isoformat strings to datetime objects first, change timezones to UTC and then stringify back.
If you are on python 3.7, according to this, you can use fromisoformat method of datetime, but if you don't, like me, I think the best option involves the use of dateutil module (you have to install it) and pytz:
import datetime as dt
from dateutil import parser
import pytz
time_list = ['2016-10-01T00:00:00+01:00','2016-10-01T23:00:00+00:00','2016-10-01T22:00:00+02:00']
utc_time_list = [parser.parse(x).astimezone(pytz.utc).isoformat() for x in time_list]
print(utc_time_list)
['2016-09-30T23:00:00+00:00', '2016-10-01T23:00:00+00:00', '2016-10-01T20:00:00+00:00']

Python: strftime a UTC timestamp to local time format

I'd like to use the timestamp from a database result and convert it to my locale time format. The timestamp itself is saved in UTC format: 2015-03-30 07:19:06.746037+02. After calling print value.strftime(format) with the format %d.%m.%Y %H:%M %z the output will be 30.03.2015 07:19 +0200. This might be the correct way to display timestamps with timezone information but unfortunately users here are not accustomed to that. What I want to achieve is the following for the given timestamp: 30.03.2015 09:19. Right now I'm adding two hours via
is_dst = time.daylight and time.localtime().tm_isdst > 0
utc_offset = - (tine.altzone if is_dst else time.timezone)
value = value + timedelta(seconds=utc_offset)
I was wondering if there is a more intelligent solution to my problem. (timestamp.tzinfo has a offset value, can/should this be used instead? The solution needs to be DST aware too.)

In your question the timestamp is already in desired timezone, so you don't need to do anything.
If you want to convert it to some other timezone you should be able to use;
YourModel.datetime_column.op('AT TIME ZONE')('your timezone name')
or,
func.timezone('your timezone name', YourModel.datetime_column)
in SQLAlchemy level.
On python level, consider using pytz

You don't need to do conversions manually when you use time zone aware database timestamps unless the timezone you want to display is different from the system timezone.
When you read and write datetime objects to the database the timezone info of the datetime object is taken into account, this means what you get back is the time in the local time zone, in your case +0200.

This SO post answers how to get local time from a timezoned timestamp.
Basically, use tzlocal.
import time
from datetime import datetime
import pytz # $ pip install pytz
from tzlocal import get_localzone # $ pip install tzlocal
# get local timezone
local_tz = get_localzone()
# test it
# utc_now, now = datetime.utcnow(), datetime.now()
ts = time.time()
utc_now, now = datetime.utcfromtimestamp(ts), datetime.fromtimestamp(ts)
local_now = utc_now.replace(tzinfo=pytz.utc).astimezone(local_tz) # utc -> local
assert local_now.replace(tzinfo=None) == now

Converting datetime.time() intho the same format as time.time()

I'm using the following to get utc datetime:
import datetime
import time
from pytz import timezone
now_utc = datetime.datetime.now(timezone('UTC'))
now = time.time()
print now_utc.time(), now
>> 04:51:39.337515 1332823899.34
I need to convert the format of now_utc.time() to look like the output of time.time().
How do I do that? Or, how to I get time.time() in utc?

time.mktime(now_utc.timetuple())
Updating the answer, since it was completely wrong, but mistake I made can be useful for others.
time.mktime does not retain the timezone, using the tuple as a local time, so this:
time.mktime(datetime.now().timetuple())
would rather work.
But, to make it short, time.time(), as explicitly stated in the spec, returns an UTC timestamp, so just use it.
(Also, you don't need pytz to get an UTC datetime, datetime.utcnow() does the same)

try:
now_utc.time().strftime(....)
from here

Change datetime to Unix time stamp in Python

Please help me to change datetime object (for example: 2011-12-17 11:31:00-05:00) (including timezone) to Unix timestamp (like function time.time() in Python).

Another way is:
import calendar
from datetime import datetime
d = datetime.utcnow()
timestamp=calendar.timegm(d.utctimetuple())
Timestamp is the unix timestamp which shows the same date with datetime object d.

import time
import datetime
dtime = datetime.datetime.now()
ans_time = time.mktime(dtime.timetuple())

Incomplete answer (doesn't deal with timezones), but hopefully useful:
time.mktime(datetime_object.timetuple())
** Edited based on the following comment **
In my program, user enter datetime, select timezone. ... I created a timezone list (use pytz.all_timezones) and allow user to chose one timezone from that list.
Pytz module provides the necessary conversions. E.g. if dt is your datetime object, and user selected 'US/Eastern'
import pytz, calendar
tz = pytz.timezone('US/Eastern')
utc_dt = tz.localize(dt, is_dst=True).astimezone(pytz.utc)
print calendar.timegm(utc_dt.timetuple())
The argument is_dst=True is to resolve ambiguous times during the 1-hour intervals at the end of daylight savings (see here http://pytz.sourceforge.net/#problems-with-localtime).