Timezones in Python datetime objects

I’ve learned something useful about Python timezone handling in datetime objects.

Let’s suppose I got a timestamp from an external source. It’s in a string variable now.


<span>>>></span> <span>timestamp_str</span> <span>=</span> <span>"2023-07-19T18:15:10-07:00"</span>
<span>>>></span> <span>timestamp_str</span> <span>=</span> <span>"2023-07-19T18:15:10-07:00"</span>
>>> timestamp_str = "2023-07-19T18:15:10-07:00"

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Cool, it’s in ISO format, let’s parse it!


<span>>>></span> <span>from</span> <span>datetime</span> <span>import</span> <span>datetime</span>
<span>>>></span> <span>timestamp</span> <span>=</span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>)</span>
<span>>>></span> <span>from</span> <span>datetime</span> <span>import</span> <span>datetime</span>
<span>>>></span> <span>timestamp</span> <span>=</span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>)</span>
>>> from datetime import datetime
>>> timestamp = datetime.fromisoformat(timestamp_str)

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Is it in the past?


<span>>>></span> <span>timestamp</span> <span><</span> <span>datetime</span><span>.</span><span>now</span><span>()</span>
<span>Traceback</span> <span>(</span><span>most</span> <span>recent</span> <span>call</span> <span>last</span><span>):</span>
  <span>File</span> <span>"<stdin>"</span><span>,</span> <span>line</span> <span>1</span><span>,</span> <span>in</span> <span><</span><span>module</span><span>></span>
<span>TypeError</span><span>:</span> <span>can</span><span>'t compare offset-naive and offset-aware datetimes </span>
<span>>>></span> <span>timestamp</span> <span><</span> <span>datetime</span><span>.</span><span>now</span><span>()</span>
<span>Traceback</span> <span>(</span><span>most</span> <span>recent</span> <span>call</span> <span>last</span><span>):</span>
  <span>File</span> <span>"<stdin>"</span><span>,</span> <span>line</span> <span>1</span><span>,</span> <span>in</span> <span><</span><span>module</span><span>></span>
<span>TypeError</span><span>:</span> <span>can</span><span>'t compare offset-naive and offset-aware datetimes </span>
>>> timestamp < datetime.now()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't compare offset-naive and offset-aware datetimes

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What? One of these datetime objects contain timezone information, the other one doesn’t. They don’t play with each other nicely.


<span>>>></span> <span>timestamp</span><span>.</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:15:10-07:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:24:10.978546'</span>
<span>>>></span> <span>timestamp</span><span>.</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:15:10-07:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:24:10.978546'</span>
>>> timestamp.isoformat()
'2023-07-19T18:15:10-07:00'
>>> datetime.now().isoformat()
'2023-07-19T18:24:10.978546'

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OK, how can I add timezone to now? I’m in Central European Summer Time (CEST) at the moment. But as I learned, it’s a bit complicated. I will get back to this later, let’s work with UTC, it’s easier. datetime.now accepts a timezone parameter.


<span>>>></span> <span>from</span> <span>datetime</span> <span>import</span> <span>timezone</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:25:38.4567+00:00'</span>
<span>>>></span> <span>timestamp</span> <span><</span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>)</span>
<span>False</span>
<span>>>></span> <span>from</span> <span>datetime</span> <span>import</span> <span>timezone</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:25:38.4567+00:00'</span>
<span>>>></span> <span>timestamp</span> <span><</span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>)</span>
<span>False</span>
>>> from datetime import timezone
>>> datetime.now(timezone.utc).isoformat()
'2023-07-19T18:25:38.4567+00:00'
>>> timestamp < datetime.now(timezone.utc)
False

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That’s better. But can I use my own timezone here? I can, if I know what my timezone is. CEST is UTC +2. The timezone class is here to help.


<span>>>></span> <span>from</span> <span>datetime</span> <span>import</span> <span>timedelta</span>
<span>>>></span> <span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>))</span>
<span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>seconds</span><span>=</span><span>7200</span><span>))</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>))).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:31:355.529775+02:00'</span>
<span>>>></span> <span>timestamp</span> <span><</span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>)))</span>
<span>False</span>
<span>>>></span> <span>from</span> <span>datetime</span> <span>import</span> <span>timedelta</span>
<span>>>></span> <span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>))</span>
<span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>seconds</span><span>=</span><span>7200</span><span>))</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>))).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:31:355.529775+02:00'</span>
<span>>>></span> <span>timestamp</span> <span><</span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>)))</span>
<span>False</span>
>>> from datetime import timedelta
>>> timezone(timedelta(hours=2))
datetime.timezone(datetime.timedelta(seconds=7200))
>>> datetime.now(timezone(timedelta(hours=2))).isoformat()
'2023-07-19T18:31:355.529775+02:00'
>>> timestamp < datetime.now(timezone(timedelta(hours=2)))
False

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But what if I don’t know my timezone? How can I get the local timezone? If it’s a remote host, it can be anywhere. What I found is that the datetime.astimezone() method can help here. If it gets a target timezone parameter, it converts the datetime to that timezone. If it gets nothing (or None) it uses the system local timezone.


<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:37:47.447466'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T16:37:55.417677+00:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>(</span><span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>))).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:38:32.881836+02:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:39:25.116842+02:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:37:47.447466'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T16:37:55.417677+00:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>(</span><span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=</span><span>2</span><span>))).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:38:32.881836+02:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:39:25.116842+02:00'</span>
>>> datetime.now().isoformat()
'2023-07-19T18:37:47.447466'
>>> datetime.now(timezone.utc).isoformat()
'2023-07-19T16:37:55.417677+00:00'
>>> datetime.now(timezone.utc).astimezone(timezone(timedelta(hours=2))).isoformat()
'2023-07-19T18:38:32.881836+02:00'
>>> datetime.now(timezone.utc).astimezone().isoformat()
'2023-07-19T18:39:25.116842+02:00'

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Yep, I’m really in CEST. I can get that info in the tzinfo attribute.


<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>().</span><span>tzinfo</span>
<span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>seconds</span><span>=</span><span>7200</span><span>),</span> <span>'CEST'</span><span>)</span>
<span>>>></span> <span>local_timezone</span> <span>=</span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>().</span><span>tzinfo</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:40:56.4701'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>local_timezone</span><span>).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:41:02.30460+02:00'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>().</span><span>tzinfo</span>
<span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>seconds</span><span>=</span><span>7200</span><span>),</span> <span>'CEST'</span><span>)</span>
<span>>>></span> <span>local_timezone</span> <span>=</span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>timezone</span><span>.</span><span>utc</span><span>).</span><span>astimezone</span><span>().</span><span>tzinfo</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>().</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:40:56.4701'</span>
<span>>>></span> <span>datetime</span><span>.</span><span>now</span><span>(</span><span>local_timezone</span><span>).</span><span>isoformat</span><span>()</span>
<span>'2023-07-19T18:41:02.30460+02:00'</span>
>>> datetime.now(timezone.utc).astimezone().tzinfo
datetime.timezone(datetime.timedelta(seconds=7200), 'CEST')
>>> local_timezone = datetime.now(timezone.utc).astimezone().tzinfo
>>> datetime.now().isoformat()
'2023-07-19T18:40:56.4701'
>>> datetime.now(local_timezone).isoformat()
'2023-07-19T18:41:02.30460+02:00'

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So astimezone works fine with no parameters, but I find it hard to read. I’m still looking for the really readable solution.

But here comes the next challenge, if you are still not dizzy. You read the following timestamp from a textfile.


<span>>>></span> <span>timestamp_str</span> <span>=</span> <span>"2023-07-19T18:39:25"</span>
<span>>>></span> <span>timestamp_str</span> <span>=</span> <span>"2023-07-19T18:39:25"</span>
>>> timestamp_str = "2023-07-19T18:39:25"

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It doesn’t contain timezone information, but you know that it’s in Pacific Daylight Time. How can you add that timezone information to the datetime object?

Can datetime.astimezone help here? It not just simply adds the timezone information to the datetime, it adjusts date and time accordingly.


<span>>>></span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>)</span>
<span>datetime</span><span>.</span><span>datetime</span><span>(</span><span>2023</span><span>,</span> <span>7</span><span>,</span> <span>19</span><span>,</span> <span>18</span><span>,</span> <span>39</span><span>,</span> <span>25</span><span>)</span>
<span>>>></span> <span>pdt_timezone</span> <span>=</span> <span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=-</span><span>7</span><span>))</span>
<span>>>></span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>).</span><span>astimezone</span><span>(</span><span>pdt_timezone</span><span>)</span>
<span>datetime</span><span>.</span><span>datetime</span><span>(</span><span>2023</span><span>,</span> <span>7</span><span>,</span> <span>19</span><span>,</span> <span>9</span><span>,</span> <span>39</span><span>,</span> <span>25</span><span>,</span> <span>tzinfo</span><span>=</span><span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>days</span><span>=-</span><span>1</span><span>,</span> <span>seconds</span><span>=</span><span>61200</span><span>)))</span>
<span>>>></span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>)</span>
<span>datetime</span><span>.</span><span>datetime</span><span>(</span><span>2023</span><span>,</span> <span>7</span><span>,</span> <span>19</span><span>,</span> <span>18</span><span>,</span> <span>39</span><span>,</span> <span>25</span><span>)</span>
<span>>>></span> <span>pdt_timezone</span> <span>=</span> <span>timezone</span><span>(</span><span>timedelta</span><span>(</span><span>hours</span><span>=-</span><span>7</span><span>))</span>
<span>>>></span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>).</span><span>astimezone</span><span>(</span><span>pdt_timezone</span><span>)</span>
<span>datetime</span><span>.</span><span>datetime</span><span>(</span><span>2023</span><span>,</span> <span>7</span><span>,</span> <span>19</span><span>,</span> <span>9</span><span>,</span> <span>39</span><span>,</span> <span>25</span><span>,</span> <span>tzinfo</span><span>=</span><span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>days</span><span>=-</span><span>1</span><span>,</span> <span>seconds</span><span>=</span><span>61200</span><span>)))</span>
>>> datetime.fromisoformat(timestamp_str)
datetime.datetime(2023, 7, 19, 18, 39, 25)
>>> pdt_timezone = timezone(timedelta(hours=-7))
>>> datetime.fromisoformat(timestamp_str).astimezone(pdt_timezone)
datetime.datetime(2023, 7, 19, 9, 39, 25, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))

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This is not exactly what I wanted. It turned out that the solution is simply replacing tzinfo in the datetime object.


<span>>>></span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>).</span><span>replace</span><span>(</span><span>tzinfo</span><span>=</span><span>pdt_timezone</span><span>)</span>
<span>datetime</span><span>.</span><span>datetime</span><span>(</span><span>2023</span><span>,</span> <span>7</span><span>,</span> <span>19</span><span>,</span> <span>18</span><span>,</span> <span>39</span><span>,</span> <span>25</span><span>,</span> <span>tzinfo</span><span>=</span><span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>days</span><span>=-</span><span>1</span><span>,</span> <span>seconds</span><span>=</span><span>61200</span><span>)))</span>
<span>>>></span> <span>datetime</span><span>.</span><span>fromisoformat</span><span>(</span><span>timestamp_str</span><span>).</span><span>replace</span><span>(</span><span>tzinfo</span><span>=</span><span>pdt_timezone</span><span>)</span>
<span>datetime</span><span>.</span><span>datetime</span><span>(</span><span>2023</span><span>,</span> <span>7</span><span>,</span> <span>19</span><span>,</span> <span>18</span><span>,</span> <span>39</span><span>,</span> <span>25</span><span>,</span> <span>tzinfo</span><span>=</span><span>datetime</span><span>.</span><span>timezone</span><span>(</span><span>datetime</span><span>.</span><span>timedelta</span><span>(</span><span>days</span><span>=-</span><span>1</span><span>,</span> <span>seconds</span><span>=</span><span>61200</span><span>)))</span>
>>> datetime.fromisoformat(timestamp_str).replace(tzinfo=pdt_timezone)
datetime.datetime(2023, 7, 19, 18, 39, 25, tzinfo=datetime.timezone(datetime.timedelta(days=-1, seconds=61200)))

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Much better. Happy? Kind of. Can we just store every timestamp with timezone info added? Or everything it UTC? Thanks.

原文链接：Timezones in Python datetime objects

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