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         title="Permalink to Idiosyncrasies in Python's timezone handling">Idiosyncrasies in Python's timezone handling</a></h2>

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        Published on <abbr class="published" title="2013-02-23T04:13:00"> Sat 23 February 2013 </abbr> under
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    <p>As I work to fix timezone logic in <a href="https://github.com/codingjourney/upsheet">upsheet</a>, I've become somewhat frustrated with date and time handling in Python. Coming from a Java background, I know this stuff is awfully difficult to get right. Python has a large community driven by sophisticated users, however, so I expected things to be pretty smooth. I was wrong:</p>
<ul>
<li><strong><em>datetime</em> vs. <em>time</em></strong>
Two modules with overlapping functionality and no clear separation of scope.</li>
<li><strong><em>datetime.datetime</em></strong>
A class with the same name as its module. This is probably common in the Python library but it makes for weird expressions such as <em>datetime.datetime.strptime(...)</em>. Kind of nudges you to drop the module prefix with <em>from datetime import *</em>.</li>
<li><strong>timezone handling in <em>datetime</em> is a red-headed stepchild</strong>
The <em>datetime</em> module tactfully avoids the gritty details of coming up with the local system's current timezone. You can supply a timezone but you have to provide your own <em>tzinfo</em> subclass to do it.</li>
<li><strong>timezone handling in <em>time</em> is a red-headed stepchild</strong>
You get <em>timezone</em> for the non-DST system timezone, <em>altzone</em> for the DST system timezone, <em>daylight</em> to see if <em>altzone</em> is even applicable and <em>localtime().tm_isdst</em> to decide (provided <em>altzone</em> is applicable) whether <em>timezone</em> or <em>altzone</em> is the local system's current timezone. Whew! I mean, it's wonderfully precise and flexible but kind of all over the place.</li>
<li><strong>timezone offsets are represented inconsistently</strong>
While <em>datetime.tzinfo.utcoffset()</em> must return its result in minutes, both <em>time.timezone</em> and <em>time.altzone</em> provide the offset in seconds. Trivial yet irritating.</li>
<li><strong><em>datetime.time</em> vs. <em>time</em></strong>
A class in one module with the same name as a related module. When you use <em>datetime</em> without the module prefix, this forces you to either also use <em>time</em> without the prefix - further polluting the naming scope - or alias it using <em>import time as sillytime</em>.</li>
<li><strong>inconsistent timezone offsets</strong>
While <em>time.timezone</em> and <em>time.altzone</em> specify timezone offsets in seconds west of UTC, <em>tzinfo.utcoffset()</em> must return minutes east of UTC.</li>
<li><strong>mischievous documentation</strong>
One example: the description of <em>struct_time.tm_isdst</em> says <em>"0, 1 or -1; see below"</em>. Of course, there's no further mention of <em>tm_isdst</em> anywhere in the document. A sentence buried in the next paragraph does say <em>"A -1 argument as the daylight savings flag, passed to mktime() will usually result in the correct daylight savings state to be filled in."</em>. Which means the -1 is utterly irrelevant except for one specific use-case.</li>
</ul>
<p>Having figured all of this out, I can now construct a <em>datetime.datetime</em> instance equipped with the correct <em>tzinfo</em> for the current local timezone. This goes to show how hard it is to design a really good API.</p>
<p>I do hope it all works the same way under Windows...</p>
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