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dopplershift opened this issue Jul 18, 2017 · 3 comments
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Weird behavior with pint.Quantity + masked array #8908

dopplershift opened this issue Jul 18, 2017 · 3 comments
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@dopplershift
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topic: units and array ducktypes
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v2.1

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@dopplershift
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From my SciPy 2017 talk, the combination of pint.Quantity for unit support with masked arrays produces problems not seen with either individually:

%matplotlib inline

import matplotlib.pyplot as plt
import numpy as np
import pint

units = pint.UnitRegistry()

data = np.random.randn(30,)
data_units = data * units.meters
data_masked = np.ma.array(data, mask=data<0.)
data_masked_units = data_masked * units.meters

fig, axes = plt.subplots(1, 3, sharex=True, sharey=True, figsize=(8, 4))
axes[0].plot(data_units)
axes[1].plot(data_masked)
axes[2].plot(data_masked_units)

produces:

bug
@dopplershift dopplershift self-assigned this Jul 18, 2017
@dopplershift dopplershift added this to the 2.1 (next point release) milestone Jul 18, 2017
@dopplershift
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It's possible the problem lies outside matplotlib, but need to track down what matplotlib is doing exactly to evoke this behavior.

@dopplershift
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So the behavior in the leftmost panel is just reflecting the behavior of:

>>> np.asarray(data_masked_units)
array([  9.01813386e-02,   1.00000000e+00,   1.00000000e+00,
         1.00000000e+00,   7.39062957e-01,   1.00000000e+00,
         1.00000000e+00,   2.99023902e-01,   7.01311229e-04,
         1.05263214e+00,   1.88309697e+00,   1.75411405e-01,
         1.12874122e+00,   3.08208814e-01,   1.00000000e+00,
         1.00000000e+00,   1.00000000e+00,   7.67253492e-01,
         1.43523599e-01,   1.00000000e+00,   1.00000000e+00,
         1.00000000e+00,   1.00000000e+00,   1.00000000e+00,
         1.86541911e+00,   1.00000000e+00,   1.00000000e+00,
         1.00000000e+00,   1.08489593e+00,   1.00000000e+00])

The reason this line runs is because there's an isintance check, which fails with a Quantity that holds a masked array, here:

matplotlib/lib/matplotlib/lines.py

Lines 648 to 655 in 5fd199c

def recache(self, always=False):
if always or self._invalidx:
xconv = self.convert_xunits(self._xorig)
if isinstance(self._xorig, np.ma.MaskedArray):
x = np.ma.asarray(xconv, float).filled(np.nan)
else:
x = np.asarray(xconv, float)
x = x.ravel()

So, how would we feel about replacing the isinstance(self._xorig, np.ma.MaskedArray) with something more duck-type-y, like hasattr(self._xorig, 'mask')? Other suggestions?

@dopplershift
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The more I stare at that, I wonder at the wisdom of using np.ma.asarray(xconv, float).filled(np.nan) rather than xconv.astype(float).filled(np.nan), since implicitly xconv has to already be some kind of masked array, since that's where the mask to be filled comes from. If we use the latter form, we could replace isinstance with a try...except AttributeError block.

dopplershift added a commit to dopplershift/matplotlib that referenced this issue Aug 17, 2017
@dopplershift
BUG: Fix weird behavior with mask and units (Fixes matplotlib#8908)
3d9af3d 
Refactor many places that check for masked arrays, and fill the masked
values with nans, with a helper to accomplish that. In the helper,
replace the isinstance check with a attribute check for 'mask'. This
allows libraries, like pint, that wrap mask arrays to pass the check and
be converted appropriately.

Also fix one spot using atleast_1d with _check_1d.
dopplershift added a commit to dopplershift/matplotlib that referenced this issue Aug 17, 2017
@dopplershift
BUG: Fix weird behavior with mask and units (Fixes matplotlib#8908)
a921eb6 
Refactor many places that check for masked arrays, and fill the masked
values with nans, with a helper to accomplish that. In the helper,
replace the isinstance check with a attribute check for 'mask'. This
allows libraries, like pint, that wrap mask arrays to pass the check and
be converted appropriately.

Also fix one spot using atleast_1d with _check_1d.
dstansby added a commit that referenced this issue Aug 29, 2017
@dstansby
Merge pull request #9049 from dopplershift/fix-8908
e3a34c5 
BUG: Fix weird behavior with mask and units (Fixes #8908)
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