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

Fixes #21378

lib/matplotlib/colorbar.py

@@ -352,6 +352,7 @@ class Colorbar:
     ticks : `~matplotlib.ticker.Locator` or array-like of float
 
     format : str or `~matplotlib.ticker.Formatter`
+        If string, it supports only new-style format, e.g. ``"{x:.2e}"``.
 
     drawedges : bool
 
@@ -471,7 +472,6 @@ def __init__(self, ax, mappable=None, *, cmap=None,
 
         self.locator = None
         self.minorlocator = None
-        self.formatter = None
         self.__scale = None  # linear, log10 for now.  Hopefully more?
 
         if ticklocation == 'auto':
@@ -486,10 +486,7 @@ def __init__(self, ax, mappable=None, *, cmap=None,
         else:
             self.locator = ticks    # Handle default in _ticker()
 
-        if isinstance(format, str):
-            self.formatter = ticker.FormatStrFormatter(format)
-        else:
-            self.formatter = format  # Assume it is a Formatter or None
+        self.formatter = format
         self.draw_all()
 
         if isinstance(mappable, contour.ContourSet) and not mappable.filled:
@@ -510,6 +507,15 @@ def __init__(self, ax, mappable=None, *, cmap=None,
         # Set the cla function to the cbar's method to override it
         self.ax.cla = self._cbar_cla
 
+    @property
+    def formatter(self):
+        return self._long_axis().get_major_formatter()
+
+    @formatter.setter
+    def formatter(self, fmt):
+        if fmt is not None:
+            self._long_axis().set_major_formatter(fmt)
+
     def _cbar_cla(self):
         """Function to clear the interactive colorbar state."""
         for x in self._interactive_funcs:
@@ -798,7 +804,6 @@ def update_ticks(self):
         self._get_ticker_locator_formatter()
         self._long_axis().set_major_locator(self.locator)
         self._long_axis().set_minor_locator(self.minorlocator)
-        self._long_axis().set_major_formatter(self.formatter)
 
     def _get_ticker_locator_formatter(self):
         """
@@ -811,7 +816,6 @@ def _get_ticker_locator_formatter(self):
         Called by update_ticks...
         """
         locator = self.locator
-        formatter = self.formatter
         minorlocator = self.minorlocator
         if isinstance(self.norm, colors.BoundaryNorm):
             b = self.norm.boundaries
@@ -837,11 +841,7 @@ def _get_ticker_locator_formatter(self):
         if minorlocator is None:
             minorlocator = ticker.NullLocator()
 
-        if formatter is None:
-            formatter = self._long_axis().get_major_formatter()
-
         self.locator = locator
-        self.formatter = formatter
         self.minorlocator = minorlocator
         _log.debug('locator: %r', locator)
 
@@ -1162,6 +1162,7 @@ def _reset_locator_formatter_scale(self):
         self._process_values()
         self.locator = None
         self.minorlocator = None
+        # TODO: what is the impact of that ?
         self.formatter = None
         if (self.boundaries is not None or
                 isinstance(self.norm, colors.BoundaryNorm)):

lib/matplotlib/tests/test_colorbar.py

@@ -550,7 +550,7 @@ def test_colorbar_format():
 
     fig, ax = plt.subplots()
     im = ax.imshow(z)
-    cbar = fig.colorbar(im, format='%4.2e')
+    cbar = fig.colorbar(im, format='{x:.2e}')
     fig.canvas.draw()
     assert cbar.ax.yaxis.get_ticklabels()[4].get_text() == '8.00e+04'