CodeBreak 1/9/2021

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Code Breaks

  1/9/2021   Summary of topics:

  • How to organise plotting large numbers of plots from heterogenous models in python
  • Layout of subplots with matplotlib in python
  • Fast percentile calculation in python

 

Organising workflows in python

cf-xarray is a useful library for writing general code for a heterogenous set of models that might not share the same names for things like dimensions (e.g. lat/latitude/LAT/) and variables. cf-xarray has code for inferring this information and allows you to refer to them in a general way. It is available in the conda environments and the documentation is here:

https://cf-xarray.readthedocs.io/en/latest/

The python3 pathlib library is an object oriented path manipulation library that makes working with paths a lot simpler and cleaner, e.g. when opening data files, and saving processed output or plots to disk:

https://docs.python.org/3/library/pathlib.html

As far as how to organise plotting of a large number of different plots from a range of models, there are a range of data structures that might suit this purpose, and it comes down to the specifics of what needs to be done and personal preference, but some options are:  

  For more information this is a pretty comprehensive write up of some of the commonly used data structures in python   https://realpython.com/python-data-structures/  

Subplots in matplotlib

  Scott wrote a blog showing a sophisticated use of subplot, but also has some tips for organising the plots by saving references to each in a dictionary named for the plot type:   https://climate-cms.org/2018/04/27/subplots.html
Unrelated to the original topics, but some of the attendees didn' t know it was possible to connect a jupyter notebook directly to gadi compute nodes, which is useful for anyone who must access data on /scratch, or have workloads that are currently too onerous for VDI or ood.nci.org.au (though ood is designed to cater for larger jobs than VDI).  This is all covered on our wiki   http://climate-cms.wikis.unsw.edu.au/Running_Jupyter_Notebook#On_Gadi   which also covers creating symbolic links to access files in other locations on the file system, e.g. /g/data  

Fast percentile calculation in python

  Calculating a percentile climatology where for each day in the year, that day plus 15 days either side are gathered together over all years to make a (dayofyear, lat, lon) dataset of 90th percentiles from each sample   Rolling will get a 31 day sample centred around the day of interest (at the start and end of the dataset the values will be NAN)       tas.rolling(time=31, center=True)   Construct will take the rolling samples and convert them to a new dimension, so the data now has dimensions (time, lat, lon, window)       tas.rolling(time=31, center=True).construct(time='window')   Groupby will collect all the equivalent days of the year (remember the 'time' axis is the centre of the sample)       (tas.rolling(time=31, center=True)

  .construct(time='window')

  .groupby(time='dayofyear'))   Normally you can just add a reduction operation (e.g. mean) to the end here, but that doesn't work with percentiles in this case. Instead do a loop:       doy_pct = []

    for doy, sample in (tas

.rolling(time=31,center=True)

.construct(time='window')

.groupby('time.dayofyear')):

        print(doy)

        doy_pct.append(sample.load().quantile(0.9, dim=['time', 'window']))       xarray.concat(doy_pct, dim='dayofyear')   See that we've called `.load()` inside the loop, which avoids a Dask chunking error when doing a percentile.   Try it with just a single point to help understand how this is working   Note the use of the list to gather the percentiles arrays for each day, then concatenate along a new dimension 'dayofyear'.