Merge pull request matplotlib#21542 from vfdev-5/fix-21378

[ENH]: Use new style format strings for colorbar ticks

Author: jklymak

lib/matplotlib/colorbar.py

@@ -352,6 +352,8 @@ class Colorbar:
     ticks : `~matplotlib.ticker.Locator` or array-like of float
 
     format : str or `~matplotlib.ticker.Formatter`
+        If string, it supports '%' operator and `str.format` formats:
+        e.g. ``"%4.2e"`` or ``"{x:.2e}"``.
 
     drawedges : bool
 
@@ -487,7 +489,12 @@ def __init__(self, ax, mappable=None, *, cmap=None,
             self.locator = ticks    # Handle default in _ticker()
 
         if isinstance(format, str):
-            self.formatter = ticker.FormatStrFormatter(format)
+            # Check format between FormatStrFormatter and StrMethodFormatter
+            try:
+                self.formatter = ticker.FormatStrFormatter(format)
+                _ = self.formatter(0)
+            except TypeError:
+                self.formatter = ticker.StrMethodFormatter(format)
         else:
             self.formatter = format  # Assume it is a Formatter or None
         self.draw_all()

lib/matplotlib/tests/test_colorbar.py

@@ -543,14 +543,15 @@ def test_colorbar_renorm():
     assert np.isclose(cbar.vmax, z.max() * 1000)
 
 
-def test_colorbar_format():
+@pytest.mark.parametrize('fmt', ['%4.2e', '{x:.2e}'])
+def test_colorbar_format(fmt):
     # make sure that format is passed properly
     x, y = np.ogrid[-4:4:31j, -4:4:31j]
     z = 120000*np.exp(-x**2 - y**2)
 
     fig, ax = plt.subplots()
     im = ax.imshow(z)
-    cbar = fig.colorbar(im, format='%4.2e')
+    cbar = fig.colorbar(im, format=fmt)
     fig.canvas.draw()
     assert cbar.ax.yaxis.get_ticklabels()[4].get_text() == '8.00e+04'