Improve barbs() error message #7407

dopplershift · 2016-11-04T20:25:49Z

So for the following code with badly sized (x,y with 3, u,v with 4) arrays:

x = np.arange(3)
y = np.arange(3)
u = np.ma.array(15. * np.ones((4,)), mask=[False, True, False, False])
v = np.ma.array(15. * np.ones_like(u), mask=[False, True, False, False])
plt.barbs(x, y, u, v)

The error would end up with something inscrutable (I lost over an hour figuring out what was wrong):

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

<SNIP>

/Users/rmay/repos/matplotlib/lib/matplotlib/quiver.py in __init__(self, ax, *args, **kw)
    942         self.set_transform(transforms.IdentityTransform())
    943 
--> 944         self.set_UVC(u, v, c)
    945 
    946     def _find_tails(self, mag, rounding=True, half=5, full=10, flag=50):

/Users/rmay/repos/matplotlib/lib/matplotlib/quiver.py in set_UVC(self, U, V, C)
   1138         plot_barbs = self._make_barbs(u, v, flags, barbs, halves, empty,
   1139                                       self._length, self._pivot, self.sizes,
-> 1140                                       self.fill_empty, self.flip)
   1141         self.set_verts(plot_barbs)
   1142 

/Users/rmay/repos/matplotlib/lib/matplotlib/quiver.py in _make_barbs(self, u, v, nflags, nbarbs, half_barb, empty_flag, length, pivot, sizes, fill_empty, flip)
   1071 
   1072             # Add vertices for each flag
-> 1073             for i in range(nflags[index]):
   1074                 # The spacing that works for the barbs is a little to much for
   1075                 # the flags, but this only occurs when we have more than 1

TypeError: only integer arrays with one element can be converted to an index

It also only happens when masked arrays are involved--otherwise it silently succeeds, which seems poor as well. With this change now the error is easy to identify:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

<SNIP>

/Users/rmay/repos/matplotlib/lib/matplotlib/quiver.py in __init__(self, ax, *args, **kw)
    948         self.set_transform(transforms.IdentityTransform())
    949 
--> 950         self.set_UVC(u, v, c)
    951 
    952     def _find_tails(self, mag, rounding=True, half=5, full=10, flag=50):

/Users/rmay/repos/matplotlib/lib/matplotlib/quiver.py in set_UVC(self, U, V, C)
   1135             x, y, u, v = delete_masked_points(self.x.ravel(), self.y.ravel(),
   1136                                               self.u, self.v)
-> 1137             check_array_shapes(x, y, u, v)
   1138 
   1139         magnitude = np.hypot(u, v)

/Users/rmay/repos/matplotlib/lib/matplotlib/quiver.py in check_array_shapes(*arrays)
    396     all_shapes = set(a.shape for a in arrays)
    397     if len(all_shapes) != 1:
--> 398         raise ValueError('The shapes of the passed in arrays do not match.')
    399 
    400 

ValueError: The shapes of the passed in arrays do not match.

NelleV

The error message looks good, but I've got a couple of comments on the API.

NelleV · 2016-11-04T20:34:24Z

lib/matplotlib/quiver.py

@@ -392,6 +392,12 @@ def _parse_args(*args):
    return X, Y, U, V, C


+def check_array_shapes(*arrays):


Can you make this function private? I don't think it should be part of our public API.

I also think the name of the function is not very explicit on what it does. How about check_consistent_shape?

NelleV · 2016-11-04T20:35:32Z

lib/matplotlib/quiver.py

+def check_array_shapes(*arrays):
+    all_shapes = set(a.shape for a in arrays)
+    if len(all_shapes) != 1:
+        raise ValueError('The shapes of the passed in arrays do not match.')


With this change, when given masked arrays with incompatible sizes, barbs() will raise a ValueError with a message stating as much. Previously, it would error with a TypeError that did not make it clear at all the source of the problem.

dopplershift · 2016-11-04T20:48:39Z

Changed the name of the helper to _check_consistent_shapes.

NelleV · 2016-11-04T20:50:46Z

👍 Thanks for the patch!
I recently spend a couple of hours understanding why scatter wasn't happy because of a cryptic error message so this patch makes me very happy :)

dopplershift · 2016-11-04T20:53:09Z

Well considering I wrote barbs() and it still cost me time, I'd say we need to do better. 😬

NelleV · 2016-11-04T22:17:19Z

lib/matplotlib/quiver.py

+def _check_consistent_shapes(*arrays):
+    all_shapes = set(a.shape for a in arrays)
+    if len(all_shapes) != 1:
+        raise ValueError('The shapes of the passed in arrays do not match.')


I think that line is too long (pep8 is complaining somewhere).

Nope, even worse--it was the docstring typo I fixed. I rewrapped and pushed.

phobson

I don't want to hold this up, but I would prefer if we left nose out of the test

phobson · 2016-11-04T22:49:58Z

lib/matplotlib/tests/test_quiver.py

@@ -133,6 +134,20 @@ def test_barbs():
             cmap='viridis')


+@cleanup
+@raises(ValueError)


Just a thought: we could avoid falling back to nose with pytest like this:

@cleanup def test_bad_masked_sizes(): 'Test error handling when given differing sized masked arrays' x = np.arange(3) y = np.arange(3) u = np.ma.array(15. * np.ones((4,))) v = np.ma.array(15. * np.ones_like(u)) u[1] = np.ma.masked v[1] = np.ma.masked fig, ax = plt.subplots() with pytest.raises(ValueError): ax.barbs(x, y, u, v)

If we're allowed to explicitly use pytest now, I will absolutely do that. I just tried to be consistent with what's already around.

I'm not seeing anywhere that we're explicitly using pytest yet...I sense this shouldn't be the first. 😞

oh, I thought it was official!

I personally don't care… I much prefer nose's API than pytests, so I have a tendency to use naturally pytest. I'd be interested in knowing whether we should write everything pytest-like.

Ah...on master.

Oops. Sorry for the noise. I missed the destination for this.

NelleV · 2016-11-04T23:35:03Z

Thanks @dopplershift !

dopplershift modified the milestones: 2.0 (style change major release), 2.0.1 (next bug fix release) Nov 4, 2016

NelleV requested changes Nov 4, 2016

View reviewed changes

dopplershift force-pushed the barbs-error branch from 9ad1c9a to 11c9aeb Compare November 4, 2016 20:47

NelleV approved these changes Nov 4, 2016

View reviewed changes

NelleV changed the title ~~Improve barbs() error message~~ [MRG+1] Improve barbs() error message Nov 4, 2016

NelleV reviewed Nov 4, 2016

View reviewed changes

MNT: Fix docstring typo.

65d818d

dopplershift force-pushed the barbs-error branch from 11c9aeb to 65d818d Compare November 4, 2016 22:29

phobson reviewed Nov 4, 2016

View reviewed changes

NelleV merged commit dc13688 into matplotlib:v2.x Nov 4, 2016

dopplershift deleted the barbs-error branch November 4, 2016 23:50

QuLogic changed the title ~~[MRG+1] Improve barbs() error message~~ Improve barbs() error message Nov 5, 2016

QuLogic modified the milestones: 2.0.1 (next bug fix release), 2.0 (style change major release) Dec 7, 2016

		@@ -392,6 +392,12 @@ def _parse_args(*args):
		return X, Y, U, V, C


		def check_array_shapes(*arrays):

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Improve barbs() error message #7407

Improve barbs() error message #7407

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dopplershift commented Nov 4, 2016

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dopplershift commented Nov 4, 2016

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NelleV commented Nov 4, 2016

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NelleV commented Nov 4, 2016

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