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Re: [matplotlib-devel] Date overhaul?
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From: Paul G. <pga...@gm...> - 2015-01-09 19:54:54
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Thanks for the responses. If no one minds, I'm take a look at how to best implement datetime64 this weekend and the degree to which it might be useful, then maybe I can start an MEP for it. I agree with Chris's sentiment that it's likely not a bad idea to start on it now, since there will almost certainly be a significant lag in raising the Numpy dependency version anyway, so if it can be implemented in some reasonable way now, we might as well, otherwise it may be some years before we get to it. Let's say we want a time zone aware date time converter. The basic goal is to convert some input type (datetime) to the MPL internal type (float days past Jan 0, 0001). We also need to tell MPL how to format the axis (default formatter, locator, limits, label). - The convert() method takes the input type (datetime) and the xunits (or yunits) keyword argument and converts it to the MPL type. The axis input can be used to change the results depending on the plot type (polar plots always require radians for example). For the TZ converter, would take the input value (datetime), convert it to the time zone requested by the units input, then convert that to a float using dates.date2num(). Note that the input can be a sequence or a single value so the converter has to handle both cases. - The axisinfo() method is used to provide the default axis locator and formatter objects when the user creates a plot with this type. The axis input is useful here to handle the result differently for a polar plot. For the TZ converter, this would be exactly the same as the web docs - return the default locator and formatter for dates. Most of the time this method can just return standard MPL formatters and locators (for either dates or numbers). - The default_units() method provides a default value for the xunits or yunits keyword argument if one isn't specified. The default in this case might be "UTC". Hope that helps some, if you have more specific questions feel free to send them to me. Ted ------------------------------ *From:* Thomas Caswell [tca...@gm...] *Sent:* Thursday, January 08, 2015 11:14 AM *To:* Drain, Theodore R (392P); mat...@li... *Subject:* Re: [matplotlib-devel] Date overhaul? I was hoping for something a bit more extensive of the intenals. I have tried to understand the units code a couple of times now and always hit a brick wall. On Thu Jan 08 2015 at 2:07:02 PM Drain, Theodore R (392P) < the...@jp...> wrote: > Google search "matplotlib units" yields: > http://matplotlib.org/api/units_api.html > > > > So it sounds like the update is to make MPL's internal date system higher > resolution which would be great. We would just need to update our > converters to convert to that format instead of the current floating point > format. Our custom classes are not public (and can't really be made > public) but they aren't very complicated so we can certainly talk about the > implementation if that helps. > > > ------------------------------ > *From:* Thomas Caswell [tca...@gm...] > *Sent:* Thursday, January 08, 2015 10:57 AM > *To:* Drain, Theodore R (392P); mat...@li... > > *Subject:* Re: [matplotlib-devel] Date overhaul? > One of the other driving factors to over-haul the default date > handling is that floats do not have enough precision to deal with > nano-second resolution data (which is what I think drove pandas to use > datetime64). > > It sounds like the correct solution > > Is the unit framework documented anywhere and are those custom classes > public? > > Tom > > On Thu Jan 08 2015 at 12:59:17 PM Drain, Theodore R (392P) < > the...@jp...> wrote: > >> I agree w/ the original poster that it would help to have a MEP to >> clearly define what the goals of the overhaul are >> >> >> >> Something else to keep in mind: we at least don't normally plot dates in >> "earth" based time systems. ~10 years ago we contracted with John Hunter >> to add the arbitrary unit system to MPL. This allows users to plot in >> their own data types and define a converter to handle the conversion to MPL >> types and labeling. We have our own "date time" like class which handles >> relativistic time scales (TDB, TT, TAI, GPS, Mars time, etc) at extremely >> high precision. We register a unit converter w/ MPL which allows our users >> to plot these types natively and use the xunits keyword (or yunits) to >> control how the plot looks. So we can do this: >> >> >> >> plot( x, y, xunits="GPS", yunits="km/s" ) >> >> plot( x, y, xunits="PST", yunits="mph" ) >> >> >> >> It would also be pretty easy to add a time zone aware unit converter with >> the existing MPL code which would allow you to do things w/ datetime like >> this: >> >> >> >> plot( x, y, xunits="UTC+8" ) >> >> plot( x, y, xunits="EST" ) >> >> >> >> I guess the point of this is to remind folks that not everyone plots >> dates in time zone based systems... >> >> >> >> Ted >> >> >> ------------------------------ >> *From:* Chris Barker [chr...@no...] >> *Sent:* Thursday, January 08, 2015 9:00 AM >> *To:* mat...@li... >> *Subject:* Re: [matplotlib-devel] Date overhaul? >> >> On Thu, Jan 8, 2015 at 7:04 AM, Skip Montanaro <sk...@po...> >> wrote: >> >>> I'm real naive about this stuff, but I have always wondered why >>> matplotlib didn't just use datetime objects, or at least use >>> timezone-aware datetime objects as an "interchange" format to get the >>> timezone stuff right. >>> >> >> Time zone handling is a pain in the %}€* >> >> And the definitions keep changing. >> >> So you need a complex DB and library that needs frequent updating. >> >> This is why neither the standard library nor numpy support time zone >> handling out of the box. >> >> But the datetime object does support a hook to add timezone info. >> >> The numpy datetime64 may implementation _may_ provide a similar hook >> in the future. >> >> There is the pytz package, which MPL could choose to depend on. >> >> But even that is a bit ugly--e.g. from the pytz docs: >> >> """Unfortunately using the tzinfo argument of the standard datetime >> constructors ‘’does not work’’ with pytz for many timezones.""" >> >> So my suggestion is that MPL punts, and stick with leaving time zone >> handling up to the user, I.e only use datetimes that are timezone "naive". >> What this means is that MPL would always a assume all datetimes interacting >> with each other are in the same time zone (including same DST status). >> >> Anyway, I'm being a bit lazy here, so I may be wrong, but I think the >> issue at hand is that MPL currently uses a float array to store and >> manipulate datetimes, and the thought is that it may be better to use >> numpy datetime64 arrays -- that would give us more consistent precision, >> and less code to convert to/from various datetime formats. >> I'm a bit on the fence about whether it's time to do it, as datetime64 >> does have issues with the locale timezone, but as any implementation would >> want to work with not-just-the-latest numpy anyway, it may make sense to >> start now. >> >> -Chris >> >> >> >> >> >> >> -- >> >> Christopher Barker, Ph.D. >> Oceanographer >> >> Emergency Response Division >> NOAA/NOS/OR&R (206) 526-6959 voice >> 7600 Sand Point Way NE (206) 526-6329 fax >> Seattle, WA 98115 (206) 526-6317 main reception >> >> Chr...@no... >> ------------------------------------------------------------ >> ------------------ >> Dive into the World of Parallel Programming! 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