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julien-h2 opened this issue Dec 14, 2018 · 4 comments
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Unclear error message for plt.xticks(names) #12990

julien-h2 opened this issue Dec 14, 2018 · 4 comments
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v3.1.0

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@julien-h2
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julien-h2 commented Dec 14, 2018 •
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I suggest to improve the error message when an argument of xticks is missing. This would considerably ease debugging.

from matplotlib import pyplot as plt

xs = [1, 2, 3]
ys = [50, 10, 20]
ticks = ['banana', 'apple', 'pear']
plt.bar(xs, ys)
plt.xticks(ticks)

Here the problem is that I forgot the xs argument for plt.xticks:

plt.xticks(xs, ticks)

The error message should state so, but here's what it currently prints:

AttributeError: 'NoneType' object has no attribute 'update'

And the traceback:

------------------------------------------------------------------
AttributeError                   Traceback (most recent call last)
<ipython-input-80-3ea0d1225cd3> in <module>
     11 plt.bar(xs-width/2, neg_ys, width=width)
     12 plt.bar(xs+width/2, pos_ys, width=width)
---> 13 plt.xticks(ticks)

/usr/local/lib/python3.6/dist-packages/matplotlib/pyplot.py in xticks(ticks, labels, **kwargs)
   1547         labels = ax.get_xticklabels()
   1548     elif labels is None:
-> 1549         locs = ax.set_xticks(ticks)
   1550         labels = ax.get_xticklabels()
   1551     else:

/usr/local/lib/python3.6/dist-packages/matplotlib/axes/_base.py in set_xticks(self, ticks, minor)
   3335             Default is ``False``.
   3336         """
-> 3337         ret = self.xaxis.set_ticks(ticks, minor=minor)
   3338         self.stale = True
   3339         return ret

/usr/local/lib/python3.6/dist-packages/matplotlib/axis.py in set_ticks(self, ticks, minor)
   1702         """
   1703         # XXX if the user changes units, the information will be lost here
-> 1704         ticks = self.convert_units(ticks)
   1705         if len(ticks) > 1:
   1706             xleft, xright = self.get_view_interval()

/usr/local/lib/python3.6/dist-packages/matplotlib/axis.py in convert_units(self, x)
   1528             return x
   1529 
-> 1530         ret = self.converter.convert(x, self.units, self)
   1531         return ret
   1532 

/usr/local/lib/python3.6/dist-packages/matplotlib/category.py in convert(value, unit, axis)
     51 
     52         # force an update so it also does type checking
---> 53         unit.update(values)
     54 
     55         str2idx = np.vectorize(unit._mapping.__getitem__,

AttributeError: 'NoneType' object has no attribute 'update'
@anntzer
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anntzer commented Dec 14, 2018

For anyone who wants to work on this, note that it is legal to pass strings as positions to xticks(), in case the axis is categorical (e.g. bar(["foo", "bar"], [1, 2]); xticks(["foo"]))...

@timhoffm
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timhoffm commented Dec 15, 2018 •
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@anntzer is the categorial stuff explicitly supported for xticks/set_xticks? Neither pyplot.xticks nor Axes.set_xticks mention it and xticks(["a", "b"]) in your example messes up the label positions:

image

@anntzer
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anntzer commented Dec 15, 2018

I think categorical support should be assumed unless a function explicitly says it doesn't support categoricals (for example neither plot() nor bar() mention they support categoricals either).

In the example you give, this is "expected": the categories "a" and "b" don't exist yet, so they get created at x = 2 and x = 3, to the right of "foo" and "bar".

@timhoffm timhoffm mentioned this issue Dec 15, 2018
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@timhoffm
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@anntzer moved the discussion on categoricals to #12993. This is rather a fundamental question what plt.xticks(['a', 'b']) is supposed to do and thus beyond improving the error message for the given issue (even though the result of the discussion will affect how to properly handle this issue here).

@timhoffm timhoffm mentioned this issue Dec 17, 2018
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@QuLogic QuLogic modified the milestones: v3.0.3, v3.1 Jan 13, 2019
@QuLogic QuLogic mentioned this issue May 2, 2023
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