What I need to do

I have a timezone-unaware datetime object, to which I need to add a time zone in order to be able to compare it with other timezone-aware datetime objects. I do not want to convert my entire application to timezone unaware for this one legacy case.

What I’ve Tried

First, to demonstrate the problem:

Python 2.6.1 (r261:67515, Jun 24 2010, 21:47:49) 
[GCC 4.2.1 (Apple Inc. build 5646)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> import pytz
>>> unaware = datetime.datetime(2011,8,15,8,15,12,0)
>>> unaware
datetime.datetime(2011, 8, 15, 8, 15, 12)
>>> aware = datetime.datetime(2011,8,15,8,15,12,0,pytz.UTC)
>>> aware
datetime.datetime(2011, 8, 15, 8, 15, 12, tzinfo=<UTC>)
>>> aware == unaware
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes

First, I tried astimezone:

>>> unaware.astimezone(pytz.UTC)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: astimezone() cannot be applied to a naive datetime
>>>

It’s not terribly surprising this failed, since it’s actually trying to do a conversion. Replace seemed like a better choice (as per Python: How to get a value of datetime.today() that is “timezone aware”?):

>>> unaware.replace(tzinfo=pytz.UTC)
datetime.datetime(2011, 8, 15, 8, 15, 12, tzinfo=<UTC>)
>>> unaware == aware
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes
>>> 

But as you can see, replace seems to set the tzinfo, but not make the object aware. I’m getting ready to fall back to doctoring the input string to have a timezone before parsing it (I’m using dateutil for parsing, if that matters), but that seems incredibly kludgy.

Also, I’ve tried this in both python 2.6 and python 2.7, with the same results.

Context

I am writing a parser for some data files. There is an old format I need to support where the date string does not have a timezone indicator. I’ve already fixed the data source, but I still need to support the legacy data format. A one time conversion of the legacy data is not an option for various business BS reasons. While in general, I do not like the idea of hard-coding a default timezone, in this case it seems like the best option. I know with reasonable confidence that all the legacy data in question is in UTC, so I’m prepared to accept the risk of defaulting to that in this case.

In general, to make a naive datetime timezone-aware, use the localize method:

import datetime
import pytz

unaware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0)
aware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0, pytz.UTC)

now_aware = pytz.utc.localize(unaware)
assert aware == now_aware

For the UTC timezone, it is not really necessary to use localize since there is no daylight savings time calculation to handle:

now_aware = unaware.replace(tzinfo=pytz.UTC)

works. (.replace returns a new datetime; it does not modify unaware.)

All of these examples use an external module, but you can achieve the same result using just the datetime module, as also presented in this SO answer:

from datetime import datetime
from datetime import timezone

dt = datetime.now()
dt.replace(tzinfo=timezone.utc)

print(dt.replace(tzinfo=timezone.utc).isoformat())
'2017-01-12T22:11:31+00:00'

Fewer dependencies and no pytz issues.

NOTE: If you wish to use this with python3 and python2, you can use this as well for the timezone import (hardcoded for UTC):

try:
    from datetime import timezone
    utc = timezone.utc
except ImportError:
    #Hi there python2 user
    class UTC(tzinfo):
        def utcoffset(self, dt):
            return timedelta(0)
        def tzname(self, dt):
            return "UTC"
        def dst(self, dt):
            return timedelta(0)
    utc = UTC()

I had use from dt_aware to dt_unaware

dt_unaware = dt_aware.replace(tzinfo=None)

and dt_unware to dt_aware

from pytz import timezone
localtz = timezone('Europe/Lisbon')
dt_aware = localtz.localize(dt_unware)

but answer before is also a good solution.

I use this statement in Django to convert an unaware time to an aware:

from django.utils import timezone

dt_aware = timezone.make_aware(dt_unaware, timezone.get_current_timezone())

I agree with the previous answers, and is fine if you are ok to start in UTC. But I think it is also a common scenario for people to work with a tz aware value that has a datetime that has a non UTC local timezone.

If you were to just go by name, one would probably infer replace() will be applicable and produce the right datetime aware object. This is not the case.

the replace( tzinfo=… ) seems to be random in its behaviour. It is therefore useless. Do not use this!

localize is the correct function to use. Example:

localdatetime_aware = tz.localize(datetime_nonaware)

Or a more complete example:

import pytz
from datetime import datetime
pytz.timezone('Australia/Melbourne').localize(datetime.now())

gives me a timezone aware datetime value of the current local time:

datetime.datetime(2017, 11, 3, 7, 44, 51, 908574, tzinfo=<DstTzInfo 'Australia/Melbourne' AEDT+11:00:00 DST>)

Use dateutil.tz.tzlocal() to get the timezone in your usage of datetime.datetime.now() and datetime.datetime.astimezone():

from datetime import datetime
from dateutil import tz

unlocalisedDatetime = datetime.now()

localisedDatetime1 = datetime.now(tz = tz.tzlocal())
localisedDatetime2 = datetime(2017, 6, 24, 12, 24, 36, tz.tzlocal())
localisedDatetime3 = unlocalisedDatetime.astimezone(tz = tz.tzlocal())
localisedDatetime4 = unlocalisedDatetime.replace(tzinfo = tz.tzlocal())

Note that datetime.astimezone will first convert your datetime object to UTC then into the timezone, which is the same as calling datetime.replace with the original timezone information being None.

This codifies @Sérgio and @unutbu’s answers. It will “just work” with either a pytz.timezone object or an IANA Time Zone string.

def make_tz_aware(dt, tz='UTC', is_dst=None):
    """Add timezone information to a datetime object, only if it is naive."""
    tz = dt.tzinfo or tz
    try:
        tz = pytz.timezone(tz)
    except AttributeError:
        pass
    return tz.localize(dt, is_dst=is_dst) 

This seems like what datetime.localize() (or .inform() or .awarify()) should do, accept both strings and timezone objects for the tz argument and default to UTC if no time zone is specified.

Python 3.9 adds the zoneinfo module so now only the the standard library is needed!

from zoneinfo import ZoneInfo
from datetime import datetime
unaware = datetime(2020, 10, 31, 12)

Attach a timezone:

>>> unaware.replace(tzinfo=ZoneInfo('Asia/Tokyo'))
datetime.datetime(2020, 10, 31, 12, 0, tzinfo=zoneinfo.ZoneInfo(key='Asia/Tokyo'))
>>> str(_)
'2020-10-31 12:00:00+09:00'

Attach the system’s local timezone:

>>> unaware.replace(tzinfo=ZoneInfo('localtime'))
datetime.datetime(2020, 10, 31, 12, 0, tzinfo=zoneinfo.ZoneInfo(key='localtime'))
>>> str(_)
'2020-10-31 12:00:00+01:00'

Subsequently it is properly converted to other timezones:

>>> unaware.replace(tzinfo=ZoneInfo('localtime')).astimezone(ZoneInfo('Asia/Tokyo'))
datetime.datetime(2020, 10, 31, 20, 0, tzinfo=backports.zoneinfo.ZoneInfo(key='Asia/Tokyo'))
>>> str(_)
'2020-10-31 20:00:00+09:00'

Wikipedia list of available time zones

There is a backport to allow use in Python 3.6 to 3.8:

sudo pip install backports.zoneinfo

Then:

from backports.zoneinfo import ZoneInfo

In the format of unutbu’s answer; I made a utility module that handles things like this, with more intuitive syntax. Can be installed with pip.

import datetime
import saturn

unaware = datetime.datetime(2011, 8, 15, 8, 15, 12, 0)
now_aware = saturn.fix_naive(unaware)

now_aware_madrid = saturn.fix_naive(unaware, 'Europe/Madrid')

for those that just want to make a timezone aware datetime

import datetime
import pytz

datetime.datetime(2019, 12, 7, tzinfo=pytz.UTC)

quite new to Python and I encountered the same issue. I find this solution quite simple and for me it works fine (Python 3.6):

unaware=parser.parse("2020-05-01 0:00:00")
aware=unaware.replace(tzinfo=tz.tzlocal()).astimezone(tz.tzlocal())

Changing between timezones

import pytz
from datetime import datetime

other_tz = pytz.timezone('Europe/Madrid')

# From random aware datetime...
aware_datetime = datetime.utcnow().astimezone(other_tz)
>> 2020-05-21 08:28:26.984948+02:00

# 1. Change aware datetime to UTC and remove tzinfo to obtain an unaware datetime
unaware_datetime = aware_datetime.astimezone(pytz.UTC).replace(tzinfo=None)
>> 2020-05-21 06:28:26.984948

# 2. Set tzinfo to UTC directly on an unaware datetime to obtain an utc aware datetime
aware_datetime_utc = unaware_datetime.replace(tzinfo=pytz.UTC)
>> 2020-05-21 06:28:26.984948+00:00

# 3. Convert the aware utc datetime into another timezone
reconverted_aware_datetime = aware_datetime_utc.astimezone(other_tz)
>> 2020-05-21 08:28:26.984948+02:00

# Initial Aware Datetime and Reconverted Aware Datetime are equal
print(aware_datetime1 == aware_datetime2)
>> True