00:06 $ ipython
Python 3.5.2 |Continuum Analytics, Inc.| (default, Jul  2 2016, 17:53:06) 
Type "copyright", "credits" or "license" for more information.

IPython 5.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: import matplotlib.pyplot as plt

In [2]: plt.ion()

In [3]: plt.contour( [[1]], [[1]], [[1]])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-b799b0b3bd2d> in <module>()
----> 1 plt.contour( [[1]], [[1]], [[1]])

/home/tcaswell/src/p/matplotlib/lib/matplotlib/pyplot.py in contour(*args, **kwargs)
   2777         ax.hold(hold)
   2778     try:
-> 2779         ret = ax.contour(*args, **kwargs)
   2780     finally:
   2781         ax.hold(washold)

/home/tcaswell/src/p/matplotlib/lib/matplotlib/__init__.py in inner(ax, *args, **kwargs)
   1878                     warnings.warn(msg % (label_namer, func.__name__),
   1879                                   RuntimeWarning, stacklevel=2)
-> 1880             return func(ax, *args, **kwargs)
   1881         pre_doc = inner.__doc__
   1882         if pre_doc is None:

/home/tcaswell/src/p/matplotlib/lib/matplotlib/axes/_axes.py in contour(self, *args, **kwargs)
   5810             self.cla()
   5811         kwargs['filled'] = False
-> 5812         contours = mcontour.QuadContourSet(self, *args, **kwargs)
   5813         self.autoscale_view()
   5814         return contours

/home/tcaswell/src/p/matplotlib/lib/matplotlib/contour.py in __init__(self, ax, *args, **kwargs)
   1419         are described in QuadContourSet.contour_doc.
   1420         """
-> 1421         ContourSet.__init__(self, ax, *args, **kwargs)
   1422 
   1423     def _process_args(self, *args, **kwargs):

/home/tcaswell/src/p/matplotlib/lib/matplotlib/contour.py in __init__(self, ax, *args, **kwargs)
    862 
    863         self._process_args(*args, **kwargs)
--> 864         self._process_levels()
    865 
    866         if self.colors is not None:

/home/tcaswell/src/p/matplotlib/lib/matplotlib/contour.py in _process_levels(self)
   1202         # The following attributes are no longer needed, and
   1203         # should be deprecated and removed to reduce confusion.
-> 1204         self.vmin = np.amin(self.levels)
   1205         self.vmax = np.amax(self.levels)
   1206 

/home/tcaswell/mc3/envs/dd35/lib/python3.5/site-packages/numpy/core/fromnumeric.py in amin(a, axis, out, keepdims)
   2395     else:
   2396         return _methods._amin(a, axis=axis,
-> 2397                             out=out, **kwargs)
   2398 
   2399 

/home/tcaswell/mc3/envs/dd35/lib/python3.5/site-packages/numpy/core/_methods.py in _amin(a, axis, out, keepdims)
     27 
     28 def _amin(a, axis=None, out=None, keepdims=False):
---> 29     return umr_minimum(a, axis, None, out, keepdims)
     30 
     31 def _sum(a, axis=None, dtype=None, out=None, keepdims=False):

ValueError: zero-size array to reduction operation minimum which has no identity

In [4]: %debug
> /home/tcaswell/mc3/envs/dd35/lib/python3.5/site-packages/numpy/core/_methods.py(29)_amin()
     27 
     28 def _amin(a, axis=None, out=None, keepdims=False):
---> 29     return umr_minimum(a, axis, None, out, keepdims)
     30 
     31 def _sum(a, axis=None, dtype=None, out=None, keepdims=False):

ipdb> u
> /home/tcaswell/mc3/envs/dd35/lib/python3.5/site-packages/numpy/core/fromnumeric.py(2397)amin()
   2395     else:
   2396         return _methods._amin(a, axis=axis,
-> 2397                             out=out, **kwargs)
   2398 
   2399 

ipdb> u
> /home/tcaswell/src/p/matplotlib/lib/matplotlib/contour.py(1204)_process_levels()
   1202         # The following attributes are no longer needed, and
   1203         # should be deprecated and removed to reduce confusion.
-> 1204         self.vmin = np.amin(self.levels)
   1205         self.vmax = np.amax(self.levels)
   1206 

ipdb> self.levels
array([], dtype=float64)
ipdb>

I do not get it to kill python, but get the above exception (with numpy 1.11, might be different from OP).

The issues here seems to be that when we try to find the levels we prune them to levels inside of the data range via

# For line contours, drop levels outside the data range.
        return lev[(lev > zmin) & (lev < zmax)]

in mpl.contour.CountourSet._autolev

which if the data is singular results in no levels and hence the issue.

The specific work here is to:

adapt _autolev to do something 'sensible' in the singular case. Probably we should return 2 levels using our standard expand_non_singular method?

This is a very limited change (so new-contributor friendly), but might require careful thought about what it should do in the singular case, hence medium difficulty.

Uh oh!

Contour kills Python #7486

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