Add cbook function for str formatting.

Author: stefmolin

lib/matplotlib/axes/_axes.py

@@ -2623,8 +2623,9 @@ def bar_label(self, container, labels=None, *, fmt="%g", label_type="edge",
             label texts will be the data values formatted with *fmt*.
 
         fmt : str or callable, default: '%g'
-            A format string for the label or a function to call with the value
-            as the first argument.
+            An unnamed %-style or {}-style format string for the label or a
+            function to call with the value as the first argument.
+            When *fmt* is a string, %-style takes precedence over {}-style.
 
         label_type : {'edge', 'center'}, default: 'edge'
             The label type. Possible values:
@@ -2747,21 +2748,11 @@ def sign(x):
                 lbl = ''
 
             if lbl is None:
-                if callable(fmt):
-                    formatted_value = fmt(value)
-                elif isinstance(fmt, str):
-                    if fmt.count('{:') == fmt.count('}') == 1:
-                        formatted_value = fmt.format(value)
-                    else:
-                        formatted_value = fmt % value
-                else:
-                    raise ValueError(
-                        'fmt expected to be a callable or a format string. '
-                        f'Got "{fmt}".'
-                    )
-            else:
-                formatted_value = lbl
-            annotation = self.annotate(formatted_value,
+                try:
+                    lbl = fmt(value)
+                except TypeError:
+                    lbl = cbook._auto_format_str(fmt, value)
+            annotation = self.annotate(lbl,
                                        xy, xytext, textcoords="offset points",
                                        ha=ha, va=va, **kwargs)
             annotations.append(annotation)

lib/matplotlib/cbook/__init__.py

@@ -2312,3 +2312,35 @@ def _unpack_to_numpy(x):
         if isinstance(xtmp, np.ndarray):
             return xtmp
     return x
+
+
+def _auto_format_str(fmt, value):
+    """
+    Apply *value* to the format string *fmt*.
+
+    This works both with unnamed %-style formatting and
+    unnamed {}-style formatting. %-style formatting has priority.
+    If *fmt* is %-style formattable that will be used. Otherwise,
+    {}-formatting is applied. Strings without formatting placeholders
+    are passed through as is.
+
+    Examples
+    --------
+    >>> _auto_format_str('%.2f m', 0.2)
+    '0.20 m'
+    >>> _auto_format_str('{} m', 0.2)
+    '0.2 m'
+    >>> _auto_format_str('const', 0.2)
+    'const'
+    >>> _auto_format_str('%d or {}', 0.2)
+    '0 or {}'
+    """
+    try:
+        lbl = fmt % value
+        # when used in `Axes.bar_label()`` this doesn't always raise an error
+        # when the {}-style formatting should be used instead of %-style
+        if lbl == fmt:
+            raise TypeError
+        return lbl
+    except (TypeError, ValueError):
+        return fmt.format(value)

lib/matplotlib/tests/test_axes.py

@@ -7762,10 +7762,13 @@ def test_bar_label_location_errorbars():
     assert labels[1].get_va() == 'top'
 
 
-def test_bar_label_fmt():
+@pytest.mark.parametrize('fmt', [
+    '%.2f', '{:.2f}', '{:.2f}'.format
+])
+def test_bar_label_fmt(fmt):
     ax = plt.gca()
     rects = ax.bar([1, 2], [3, -4])
-    labels = ax.bar_label(rects, fmt='%.2f')
+    labels = ax.bar_label(rects, fmt=fmt)
     assert labels[0].get_text() == '3.00'
     assert labels[1].get_text() == '-4.00'
 

lib/matplotlib/tests/test_cbook.py

@@ -895,3 +895,18 @@ def test_safe_first_element_with_none():
     datetime_lst[0] = None
     actual = cbook._safe_first_non_none(datetime_lst)
     assert actual is not None and actual == datetime_lst[1]
+
+
+@pytest.mark.parametrize('fmt, value, result', [
+    ('%.2f m', 0.2, '0.20 m'),
+    ('{:.2f} m', 0.2, '0.20 m'),
+    ('{} m', 0.2, '0.2 m'),
+    ('const', 0.2, 'const'),
+    ('%d or {}', 0.2, '0 or {}'),
+    ('{{{:,.0f}}}', 2e5, '{200,000}'),
+    ('{:.2%}', 2/3, '66.67%'),
+    ('$%g', 2.54, '$2.54'),
+])
+def test_auto_format_str(fmt, value, result):
+    """Apply *value* to the format string *fmt*."""
+    assert cbook._auto_format_str(fmt, value) == result