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Implementation of polyfit and polyval #3733

Merged
merged 23 commits into from
Mar 25, 2020
Merged

Implementation of polyfit and polyval #3733

merged 23 commits into from
Mar 25, 2020

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aulemahal
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@aulemahal aulemahal commented Jan 30, 2020 •
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  • Closes Implement polyfit? #3349
  • Tests added
  • Passes isort -rc . && black . && mypy . && flake8
  • Fully documented, including whats-new.rst for all changes and api.rst for new API

Following discussions in #3349, I suggest here an implementation of polyfit and polyval for xarray. However, this is still work in progress, a lot of testing is missing, all docstrings are missing. But, mainly, I have questions on how to properly conduct this.

My implementation mostly duplicates the code of np.polyfit, but making use of dask.array.linalg.lstsq and dask.array.apply_along_axis for dask arrays. The same method as in xscale.signal.fitting.polyfit, but I add NaN-awareness in a 1-D manner.
The version with numpy is also slightly different of np.polyfit because of the NaN skipping, but I wanted the function to replicate its behaviour. It returns a variable number of DataArrays, depending on the keyword arguments (coefficients, [ residuals, matrix rank, singular values ] / [covariance matrix]).
Thus giving a medium-length function that has a lot of duplicated code from numpy.polyfit. I thought of simply using a xr.apply_ufunc, but that makes chunking along the fitted dimension forbidden and difficult to return the ancillary results (residuals, rank, covariance matrix...).

Questions:
1 ) Are the functions where they should go?
2 ) Should xarray's implementation really replicate the behaviour of numpy's? A lot of extra code could be removed if we'd say we only want to compute and return the residuals and the coefficients. All the other variables are a few lines of code away for the user that really wants them, and they don't need the power of xarray and dask anyway.

@dcherian
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Just a quick comment:

xr.apply_ufunc, but that makes chunking along the fitted dimension forbidden

dask="allowed" should work and is appropriate here since you're using a dask function.

difficult to return the ancillary results

yes we really should fix this. One trick is to stack the returned information in to one array and split that array up into multiple dataarrays after the apply_ufunc call.

@aulemahal
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aulemahal commented Jan 30, 2020 •
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Oh dask="allowed", that's true!
For the coeficients and residuals, I can easily do this stacking and unstacking and return only one variable. So, apply_ufunc could be used. However, it would not be very useful right now, considering they way I implemented the least-squares fit directly. Meaning: the left hand side computation and scaling and the not-fitted-dimensions stacking would be done on each call of numpy.polyfit if I did call it directly. Right now, we save some of this overhead by doing it once before iterating on 1d axes (which is anyway needed with skipna=True).

huard
huard reviewed Feb 3, 2020
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TomNicholas
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Co-Authored-By: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>
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This looks really excellent. Thanks @aulemahal !

Any thoughts @dcherian @TomNicholas before we merge?

@pep8speaks
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pep8speaks commented Mar 25, 2020 •
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Hello @aulemahal! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-03-25 14:38:12 UTC

@max-sixty
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Thanks @aulemahal . Sorry I missed this: I don't think I get notifications when code is pushed, only on a comment. Ping on green and I'll merge.

@aulemahal
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Ping. It be green.

@dcherian
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I can take a look later this week if needed

@max-sixty
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Great, I'm merging, but post back here for further feedback and we can iterate.

Thank you very much @aulemahal !

@max-sixty max-sixty merged commit ec215da into pydata:master Mar 25, 2020
@aulemahal
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Yay! Many thanks, this will greatly improve our projects.

@kmuehlbauer kmuehlbauer mentioned this pull request Mar 27, 2020
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dcherian added a commit to dcherian/xarray that referenced this pull request Mar 28, 2020
@dcherian
Merge remote-tracking branch 'upstream/master' into map-blocks-schema
4a355e6 
* upstream/master: (54 commits)
  Limit repr of arrays containing long strings (pydata#3900)
  expose a few zarr backend functions as semi-public api (pydata#3897)
  Use drawstyle instead of linestyle in plot.step. (pydata#3274)
  Implementation of polyfit and polyval (pydata#3733)
  misplaced quote in whatsnew (pydata#3889)
  Rename ordered_dict_intersection -> compat_dict_intersection (pydata#3887)
  Control attrs of result in `merge()`, `concat()`, `combine_by_coords()` and `combine_nested()` (pydata#3877)
  xfail test_uamiv_format_write (pydata#3885)
  Use `fixes` in PR template (pydata#3886)
  Tweaks to "how_to_release" (pydata#3882)
  whatsnew section for 0.16.0
  Release v0.15.1
  whatsnew for 0.15.1 (pydata#3879)
  update panel documentation (pydata#3880)
  reword the whats-new entry for unit support (pydata#3878)
  Raise error when assigning to IndexVariable.values & IndexVariable.data (pydata#3862)
  Re-enable tests xfailed in pydata#3808 and fix new CFTimeIndex failures due to upstream changes (pydata#3874)
  add spacing in the versions section of the issue report (pydata#3876)
  map_blocks: allow user function to add new unindexed dimension. (pydata#3817)
  Delete associated indexes when deleting coordinate variables. (pydata#3840)
  ...
dcherian added a commit to MeraX/xarray that referenced this pull request Mar 29, 2020
@dcherian
Merge remote-tracking branch 'upstream/master' into patch-1
068aac7 
* upstream/master: (75 commits)
  Implement idxmax and idxmin functions (pydata#3871)
  Update pre-commit-config.yaml (pydata#3911)
  Revert "Use `fixes` in PR template (pydata#3886)" (pydata#3912)
  update the docstring of diff (pydata#3909)
  Un-xfail test_dayofyear_after_cftime_range (pydata#3907)
  Limit repr of arrays containing long strings (pydata#3900)
  expose a few zarr backend functions as semi-public api (pydata#3897)
  Use drawstyle instead of linestyle in plot.step. (pydata#3274)
  Implementation of polyfit and polyval (pydata#3733)
  misplaced quote in whatsnew (pydata#3889)
  Rename ordered_dict_intersection -> compat_dict_intersection (pydata#3887)
  Control attrs of result in `merge()`, `concat()`, `combine_by_coords()` and `combine_nested()` (pydata#3877)
  xfail test_uamiv_format_write (pydata#3885)
  Use `fixes` in PR template (pydata#3886)
  Tweaks to "how_to_release" (pydata#3882)
  whatsnew section for 0.16.0
  Release v0.15.1
  whatsnew for 0.15.1 (pydata#3879)
  update panel documentation (pydata#3880)
  reword the whats-new entry for unit support (pydata#3878)
  ...
@pilotchute pilotchute mentioned this pull request Apr 8, 2020
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@aulemahal aulemahal mentioned this pull request May 27, 2020
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@kuchaale kuchaale mentioned this pull request Jun 12, 2020
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Implement polyfit?
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@aulemahal @dcherian @max-sixty @pep8speaks @huard @TomNicholas