Avoid using pyplot for check_figures_equal (#31076)

ayshih · web-flow · commit c224d484330c · 2026-02-25T13:02:47.000+01:00
* Avoid using pyplot for check_figures_equal

* Fix tests

* Add a check for both figures being empty

* Fix tests for empty-figure check

* Add suggested comment

* Revise comment about cairo backend
diff --git a/lib/matplotlib/testing/decorators.py b/lib/matplotlib/testing/decorators.py
@@ -15,6 +15,7 @@
 import matplotlib.units
 import matplotlib.testing
 from matplotlib import _pylab_helpers, cbook, ft2font, pyplot as plt, ticker
+from matplotlib.figure import Figure
 from .compare import comparable_formats, compare_images, make_test_filename
 from .exceptions import ImageComparisonFailure
 
@@ -410,27 +411,21 @@ def wrapper(*args, ext, request, **kwargs):
 
             file_name = "".join(c for c in request.node.name
                                 if c in ALLOWED_CHARS)
-            try:
-                fig_test = plt.figure("test")
-                fig_ref = plt.figure("reference")
-                with _collect_new_figures() as figs:
-                    func(*args, fig_test=fig_test, fig_ref=fig_ref, **kwargs)
-                if figs:
-                    raise RuntimeError('Number of open figures changed during '
-                                       'test. Make sure you are plotting to '
-                                       'fig_test or fig_ref, or if this is '
-                                       'deliberate explicitly close the '
-                                       'new figure(s) inside the test.')
-                test_image_path = result_dir / (file_name + "." + ext)
-                ref_image_path = result_dir / (file_name + "-expected." + ext)
-                fig_test.savefig(test_image_path)
-                fig_ref.savefig(ref_image_path)
-                _raise_on_image_difference(
-                    ref_image_path, test_image_path, tol=tol
-                )
-            finally:
-                plt.close(fig_test)
-                plt.close(fig_ref)
+            fig_test = Figure()
+            fig_ref = Figure()
+            func(*args, fig_test=fig_test, fig_ref=fig_ref, **kwargs)
+            if len(fig_test.get_children()) == 1 and len(fig_ref.get_children()) == 1:
+                # no artists have been added. The only child is fig.patch.
+                raise RuntimeError("Both figures are empty.  Make sure you are "
+                                   "plotting to fig_test or fig_ref.")
+
+            test_image_path = result_dir / (file_name + "." + ext)
+            ref_image_path = result_dir / (file_name + "-expected." + ext)
+            fig_test.savefig(test_image_path)
+            fig_ref.savefig(ref_image_path)
+            _raise_on_image_difference(
+                ref_image_path, test_image_path, tol=tol
+            )
 
         parameters = [
             param
diff --git a/lib/matplotlib/tests/test_backend_cairo.py b/lib/matplotlib/tests/test_backend_cairo.py
@@ -2,6 +2,7 @@
 
 import pytest
 
+import matplotlib.pyplot as plt
 from matplotlib.testing.decorators import check_figures_equal
 from matplotlib import (
     collections as mcollections, patches as mpatches, path as mpath)
@@ -46,3 +47,7 @@ def test_patch_alpha_coloring(fig_test, fig_ref):
                                       facecolor=(1, 0, 0, 0.5),
                                       edgecolor=(0, 0, 1, 0.75))
     ax.add_collection(col)
+
+    # Have pyplot manage the figures to ensure the cairo backend is used
+    plt.figure(fig_ref)
+    plt.figure(fig_test)
diff --git a/lib/matplotlib/tests/test_colorbar.py b/lib/matplotlib/tests/test_colorbar.py
@@ -895,11 +895,12 @@ def test_twoslope_colorbar():
     fig.colorbar(pc)
 
 
-@check_figures_equal()
-def test_remove_cb_whose_mappable_has_no_figure(fig_ref, fig_test):
-    ax = fig_test.add_subplot()
-    cb = fig_test.colorbar(cm.ScalarMappable(), cax=ax)
+def test_remove_cb_whose_mappable_has_no_figure():
+    fig, ax = plt.subplots()
+    assert fig.get_axes() != []
+    cb = fig.colorbar(cm.ScalarMappable(), cax=ax)
     cb.remove()
+    assert fig.get_axes() == []
 
 
 def test_aspects():
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -1602,7 +1602,9 @@ def test_large_image(fig_test, fig_ref, dim, size, msg, origin, high_memory):
     with pytest.warns(UserWarning,
                       match=f'Data with more than {msg} cannot be '
                       'accurately displayed.'):
-        fig_test.canvas.draw()
+        with io.BytesIO() as buffer:
+            # Write to a buffer to trigger the warning
+            fig_test.savefig(buffer)
 
     array = np.zeros((1, size // 2 + 1))
     array[:, array.size // 2:] = 1
diff --git a/lib/matplotlib/tests/test_polar.py b/lib/matplotlib/tests/test_polar.py
@@ -117,7 +117,7 @@ def test_polar_units_1(fig_test, fig_ref):
     xs = [30.0, 45.0, 60.0, 90.0]
     ys = [1.0, 2.0, 3.0, 4.0]
 
-    plt.figure(fig_test.number)
+    plt.figure(fig_test)
     plt.polar([x * units.deg for x in xs], ys)
 
     ax = fig_ref.add_subplot(projection="polar")
@@ -134,7 +134,7 @@ def test_polar_units_2(fig_test, fig_ref):
     ys = [1.0, 2.0, 3.0, 4.0]
     ys_km = [y * units.km for y in ys]
 
-    plt.figure(fig_test.number)
+    plt.figure(fig_test)
     # test {theta,r}units.
     plt.polar(xs_deg, ys_km, thetaunits="rad", runits="km")
     assert isinstance(plt.gca().xaxis.get_major_formatter(),
diff --git a/lib/matplotlib/tests/test_testing.py b/lib/matplotlib/tests/test_testing.py
@@ -2,7 +2,6 @@
 
 import pytest
 
-import matplotlib.pyplot as plt
 from matplotlib.testing.decorators import check_figures_equal
 
 
@@ -17,6 +16,8 @@ def test_warn_to_fail():
 @check_figures_equal()
 @pytest.mark.parametrize("b", [1])
 def test_parametrize_with_check_figure_equal(a, fig_ref, b, fig_test):
+    fig_ref.add_subplot()
+    fig_test.add_subplot()
     assert a == b
 
 
@@ -28,14 +29,7 @@ def should_fail(test, ref):
 
 
 @pytest.mark.xfail(raises=RuntimeError, strict=True,
-                   reason='Test for check_figures_equal test creating '
-                          'new figures')
+                   reason="Both figures are empty")
 @check_figures_equal()
-def test_check_figures_equal_extra_fig(fig_test, fig_ref):
-    plt.figure()
-
-
-@check_figures_equal()
-def test_check_figures_equal_closed_fig(fig_test, fig_ref):
-    fig = plt.figure()
-    plt.close(fig)
+def test_check_figures_equal_empty_figs(fig_test, fig_ref):
+    pass
