Fix clabel() for backends without canvas.get_renderer()

`tight_layout.get_renderer()` does the right dance to set up a
renderer for any canvas.  Perhaps it also should be moved somewhere
more easily findable and it should be better disambiguated with
`backend_bases._get_renderer()`, but that can be left for another
time...

Author: anntzer

lib/matplotlib/contour.py

@@ -250,7 +250,7 @@ def get_label_width(self, lev, fmt, fsize):
         fig = self.axes.figure
         width = (text.Text(0, 0, lev, figure=fig,
                            size=fsize, fontproperties=self.labelFontProps)
-                 .get_window_extent(fig.canvas.get_renderer()).width)
+                 .get_window_extent(mpl.tight_layout.get_renderer(fig)).width)
         width *= 72 / fig.dpi
         return width
 

lib/matplotlib/tests/test_contour.py

@@ -392,3 +392,9 @@ def test_contour_linewidth(
         X = np.arange(4*3).reshape(4, 3)
         cs = ax.contour(X, linewidths=call_linewidths)
         assert cs.tlinewidths[0][0] == expected
+
+
+@pytest.mark.backend("pdf")
+def test_label_nonagg():
+    # This should not crash even if the canvas doesn't have a get_renderer().
+    plt.clabel(plt.contour([[1, 2], [3, 4]]))