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Handle single color in ContourSet #28786

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Sep 14, 2024
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Handle single color in ContourSet #28786

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21 changes: 21 additions & 0 deletions doc/users/next_whats_new/contour_color.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
Specifying a single color in ``contour`` and ``contourf``
---------------------------------------------------------

`~.Axes.contour` and `~.Axes.contourf` previously accepted a single color
provided it was expressed as a string. This restriction has now been removed
and a single color in any format described in the :ref:`colors_def` tutorial
may be passed.

.. plot::
:include-source: true
:alt: Two-panel example contour plots. The left panel has all transparent red contours. The right panel has all dark blue contours.

import matplotlib.pyplot as plt

fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(6, 3))
z = [[0, 1], [1, 2]]

ax1.contour(z, colors=('r', 0.4))
ax2.contour(z, colors=(0.1, 0.2, 0.5))

plt.show()
27 changes: 17 additions & 10 deletions lib/matplotlib/contour.py
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Expand Up @@ -702,6 +702,11 @@ def __init__(self, ax, *args,
self._extend_min = self.extend in ['min', 'both']
self._extend_max = self.extend in ['max', 'both']
if self.colors is not None:
if mcolors.is_color_like(self.colors):
color_sequence = [self.colors]
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👍 for using a new name and not modifying colors. This makes it clear it's a local name and prevents users from thinking its the argument colors, which also gets stored as self.colors.

else:
color_sequence = self.colors

ncolors = len(self.levels)
if self.filled:
ncolors -= 1
Expand All @@ -718,19 +723,19 @@ def __init__(self, ax, *args,
total_levels = (ncolors +
int(self._extend_min) +
int(self._extend_max))
if (len(self.colors) == total_levels and
if (len(color_sequence) == total_levels and
(self._extend_min or self._extend_max)):
use_set_under_over = True
if self._extend_min:
i0 = 1

cmap = mcolors.ListedColormap(self.colors[i0:None], N=ncolors)
cmap = mcolors.ListedColormap(color_sequence[i0:None], N=ncolors)

if use_set_under_over:
if self._extend_min:
cmap.set_under(self.colors[0])
cmap.set_under(color_sequence[0])
if self._extend_max:
cmap.set_over(self.colors[-1])
cmap.set_over(color_sequence[-1])

# label lists must be initialized here
self.labelTexts = []
Expand Down Expand Up @@ -1498,10 +1503,12 @@ def _initialize_x_y(self, z):
The sequence is cycled for the levels in ascending order. If the
sequence is shorter than the number of levels, it's repeated.

As a shortcut, single color strings may be used in place of
one-element lists, i.e. ``'red'`` instead of ``['red']`` to color
all levels with the same color. This shortcut does only work for
color strings, not for other ways of specifying colors.
As a shortcut, a single color may be used in place of one-element lists, i.e.
``'red'`` instead of ``['red']`` to color all levels with the same color.

.. versionchanged:: 3.10
Previously a single color had to be expressed as a string, but now any
valid color format may be passed.

By default (value *None*), the colormap specified by *cmap*
will be used.
Expand Down Expand Up @@ -1569,10 +1576,10 @@ def _initialize_x_y(self, z):

An existing `.QuadContourSet` does not get notified if
properties of its colormap are changed. Therefore, an explicit
call `.QuadContourSet.changed()` is needed after modifying the
call ``QuadContourSet.changed()`` is needed after modifying the
colormap. The explicit call can be left out, if a colorbar is
assigned to the `.QuadContourSet` because it internally calls
`.QuadContourSet.changed()`.
``QuadContourSet.changed()``.

Example::

Expand Down
14 changes: 14 additions & 0 deletions lib/matplotlib/tests/test_contour.py
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Expand Up @@ -171,6 +171,20 @@ def test_given_colors_levels_and_extends():
plt.colorbar(c, ax=ax)


@pytest.mark.parametrize('color, extend', [('darkred', 'neither'),
('darkred', 'both'),
(('r', 0.5), 'neither'),
((0.1, 0.2, 0.5, 0.3), 'neither')])
def test_single_color_and_extend(color, extend):
z = [[0, 1], [1, 2]]

_, ax = plt.subplots()
levels = [0.5, 0.75, 1, 1.25, 1.5]
cs = ax.contour(z, levels=levels, colors=color, extend=extend)
for c in cs.get_edgecolors():
assert same_color(c, color)


@image_comparison(['contour_log_locator.svg'], style='mpl20', remove_text=False)
def test_log_locator_levels():

Expand Down
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