{,Range}Slider: accept callable valfmt arguments

Co-authored-by: Tim Hoffmann <[email protected]>

Author: doronbehar

lib/matplotlib/widgets.py

@@ -364,8 +364,9 @@ def __init__(self, ax, label, valmin, valmax, *, valinit=0.5, valfmt=None,
             The slider initial position.
 
         valfmt : str, default: None
-            %-format string used to format the slider value.  If None, a
-            `.ScalarFormatter` is used instead.
+            The way to format the slider value. If a string, it must be in %-format.
+            If a callable, it must have the signature ``valfmt(val: float) -> str``.
+            If None, a `.ScalarFormatter` is used.
 
         closedmin : bool, default: True
             Whether the slider interval is closed on the bottom.
@@ -547,7 +548,10 @@ def _update(self, event):
     def _format(self, val):
         """Pretty-print *val*."""
         if self.valfmt is not None:
-            return self.valfmt % val
+            if callable(self.valfmt):
+                return self.valfmt(val)
+            else:
+                return self.valfmt % val
         else:
             _, s, _ = self._fmt.format_ticks([self.valmin, val, self.valmax])
             # fmt.get_offset is actually the multiplicative factor, if any.
@@ -644,9 +648,11 @@ def __init__(
             The initial positions of the slider. If None the initial positions
             will be at the 25th and 75th percentiles of the range.
 
-        valfmt : str, default: None
-            %-format string used to format the slider values.  If None, a
-            `.ScalarFormatter` is used instead.
+        valfmt : str or callable, default: None
+            The way to format the range's minimal and maximal values. If a
+            string, it must be in %-format. If a callable, it must have the
+            signature ``valfmt(val: float) -> str``. If None, a
+            `.ScalarFormatter` is used.
 
         closedmin : bool, default: True
             Whether the slider interval is closed on the bottom.
@@ -890,7 +896,10 @@ def _update(self, event):
     def _format(self, val):
         """Pretty-print *val*."""
         if self.valfmt is not None:
-            return f"({self.valfmt % val[0]}, {self.valfmt % val[1]})"
+            if callable(self.valfmt):
+                return f"({self.valfmt(val[0])}, {self.valfmt(val[1])})"
+            else:
+                return f"({self.valfmt % val[0]}, {self.valfmt % val[1]})"
         else:
             _, s1, s2, _ = self._fmt.format_ticks(
                 [self.valmin, *val, self.valmax]

lib/matplotlib/widgets.pyi

@@ -64,7 +64,7 @@ class SliderBase(AxesWidget):
     valmax: float
     valstep: float | ArrayLike | None
     drag_active: bool
-    valfmt: str | None
+    valfmt: str | Callable[[float], str] | None
     def __init__(
         self,
         ax: Axes,
@@ -73,7 +73,7 @@ class SliderBase(AxesWidget):
         closedmax: bool,
         valmin: float,
         valmax: float,
-        valfmt: str | None,
+        valfmt: str | Callable[[float], str] | None,
         dragging: Slider | None,
         valstep: float | ArrayLike | None,
     ) -> None: ...
@@ -130,7 +130,7 @@ class RangeSlider(SliderBase):
         valmax: float,
         *,
         valinit: tuple[float, float] | None = ...,
-        valfmt: str | None = ...,
+        valfmt: str | Callable[[float], str] | None = ...,
         closedmin: bool = ...,
         closedmax: bool = ...,
         dragging: bool = ...,