Python pytz timezone function returns a timezone that is off by 9 minutes
Answer based on the answer by Carl Meyer in Google Groups Answer
The reason for this difference, is that this is NOT the right way of converting a timezone agnostic datetime object to a timezone aware object.
The explanation being:
"A pytz timezone class does not represent a single offset from UTC, it represents a geographical area which, over the course of history, has probably gone through several different UTC offsets. The oldest offset for a given zone, representing the offset from before time zones were standardized (in the late 1800s, most places) is usually called "LMT" (Local Mean Time), and it is often offset from UTC by an odd number of minutes."
(quote from the cited answer in Google Groups)
Basically, you should do:
from datetime import datetimeimport pytzmy_datetime = datetime(2015, 6, 11, 13, 30)my_tz = pytz.timezone('America/Chicago') good_dt = my_tz.localize(my_datetime)print(good_dt)out: 2015-06-11 13:30:00-05:00
Unless your local timezone has a fixed UTC offset then it is pointless to talk about its specific value without providing a specific date/time.
If you provide the time e.g., the current time then you'll see that pytz produces the expected UTC offset:
>>> from datetime import datetime>>> import pytz>>> datetime.now(pytz.timezone('America/Chicago')).strftime('%Z%z')'CST-0600'See
If you don't provide a specific date/time then pytz may return an arbitrary utc offset from the set of available utc offsets for the given timezone. The recent pytz versions return utc offsets that correspond to the earliest time (LMT as a rule) but you should not rely on it. You and your friend may use different pytz versions that may explain the difference in results.
Just because my curiosity wasn't exactly satisfied, I did a little more digging into this problem recently.
Initially, it seemed that the difference stemmed from different versions of pytz. However, after downgrading my version of pytz to a version where I had confirmed that I got a different result from that on my machine, I found that this wasn't the root of the issue: even with the same version of pytz my machine seemed to be using a UTC offset based on LMT, while the other machines were using one based off CDT or CST.
Based on my conversation with @J.F.Sebastian, I assumed that the only other likely possibility was a system level difference. I dug into the pytz source code a little bit more, and found that the file where pytz gets at least some of it's timezone information from is in /usr/share/zoneinfo/. So I looked at the file /usr/share/zoneinfo/America/Chicago and although it is a binary file, part of it is readable. Half way through the file there is a list of timezones: LMTCDTCSTESTCWTCPT. As you can see, LMT is the first name in the list, and as @J.F.Sebastian suggested, that seems to be the one that pytz uses in the situation described in my original question.
That is how the list looks in Ubuntu 15.10. However, in earlier versions of Ubuntu (e.g., Trusty and Precise) where I was getting the result -600 instead of -609 result, the same list is CDTCSTESTCWTCPT.
I will admit that this comes from a lot of blind exploring and half understanding, but it seems like this is what accounts for the differences I was seeing across machines. As far as why the zoneinfo files differ across versions, and what these differences mean for Ubuntu, I have no idea, but I thought I would share my findings for those who are similarly curious, and to potentially receive insightful corrections/supplemental information from the community.