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From: Peter Pearson <pkpearson@nowhere.invalid>
Newsgroups: comp.lang.python
Subject: Re: Matplotlib X-axis timezone trouble [SOLVED]
Date: 4 Jul 2015 17:37:07 GMT
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On Sat, 04 Jul 2015 07:29:45 +0300, Akira Li <4kir4.1i@gmail.com> wrote:
> Peter Pearson <pkpearson@nowhere.invalid> writes:
>
>> The following code produces a plot with a line running from (9:30, 0) to
>> (10:30, 1), not from (8:30, 0) to (9:30, 1) as I desire.
>>
>> If I use timezone None instead of pacific, the plot is as desired, but
>> of course that doesn't solve the general problem of which this is a
>> much-reduced example.
>>
>> If I use timezone US/Central, I get the same (bad) plot.
>>
>> import matplotlib.pyplot as plt
>> import datetime
>> import pytz
>> pacific = pytz.timezone("US/Pacific")
>> fig = plt.figure()
>> plt.plot([datetime.datetime(2014, 10, 7, 8, 30, tzinfo=pacific),
>>           datetime.datetime(2014, 10, 7, 9, 30, tzinfo=pacific)],
>>          [0,1], marker="o", color="green")
>> fig.autofmt_xdate()
>> plt.show()
>>
>> Does anybody know why this shift is occurring?  Is Matplotlib
>> confused about what timezone to use in labeling the axis?  How
>> would I tell it what timezone to use (preferably explicitly in
>> the code, not in matplotlibrc)?
>>
>
> Your pytz usage is incorrect.
>
> Don't pass a pytz tzinfo object to the datetime construtor directly, use
> `.localize()` method instead. Read the note at the very beginning of
> pytz docs http://pytz.sourceforge.net/

Exactly.  Thank you.

For newcomers, the denouement of this thread is this:

 * Matplotlib had nothing to do with this problem, it was correctly
   displaying bad datetime.datetime values.

 * Python's datetime.datetime(..., tzinfo=timezone) is unreliable if
   timezone has daylight-saving time.

 * pytz's creators provide the localize() method specifically to
   remedy this problem.  If you want to create a datetime object
   that has a timezone that might have daylight-saving time,
   use localize().

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