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Fix contour behavior when Z is constant (zmin == zmax) #31534

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Fix contour behavior when Z is constant (zmin == zmax) #31534

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13 changes: 11 additions & 2 deletions lib/matplotlib/contour.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,7 +6,7 @@
import functools
import math
from numbers import Integral

import warnings
import numpy as np
from numpy import ma

Expand Down Expand Up @@ -998,15 +998,24 @@
one contour line, but two filled regions, and therefore
three levels to provide boundaries for both regions.
"""


if self.zmin == self.zmax:
warnings.warn(
"Z is constant; contour levels are not meaningful.",
stacklevel=2
)
return np.array([self.zmin])

lev = self.locator.tick_values(self.zmin, self.zmax)

try:
if self.locator._symmetric:

Check warning on line 1013 in lib/matplotlib/contour.py

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GitHub Actions / ruff 

[rdjson] reported by reviewdog 🐶
Blank line contains whitespace

Raw Output:
message:"Blank line contains whitespace"  location:{path:"/home/runner/work/matplotlib/matplotlib/lib/matplotlib/contour.py"  range:{start:{line:1013  column:1}  end:{line:1013  column:8}}}  severity:WARNING  source:{name:"ruff"  url:"https://docs.astral.sh/ruff"}  code:{value:"W293"  url:"https://docs.astral.sh/ruff/rules/blank-line-with-whitespace"}  suggestions:{range:{start:{line:1013  column:1}  end:{line:1013  column:8}}}
return lev
except AttributeError:
pass

# Trim excess levels the locator may have supplied.
# Trim excess levels the locator may have supplied.
under = np.nonzero(lev < self.zmin)[0]
i0 = under[-1] if len(under) else 0
over = np.nonzero(lev > self.zmax)[0]
Expand Down
13 changes: 13 additions & 0 deletions lib/matplotlib/tests/test_contour.py
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Original file line number Diff line number Diff line change
Expand Up @@ -876,3 +876,16 @@ def test_contour_aliases(fig_test, fig_ref):
def test_contour_singular_color():
with pytest.raises(TypeError):
plt.figure().add_subplot().contour([[0, 1], [2, 3]], color="r")

def test_contour_constant_z_warns_and_single_level():
Z = np.ones((10, 10))

fig, ax = plt.subplots()

# Expect a warning
with pytest.warns(UserWarning, match="Z is constant"):
cs = ax.contour(Z)

# Ensure only one level is created
assert len(cs.levels) == 1
assert cs.levels[0] == 1.0
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